AN68 - LT1510 Design Manual · 2018-03-19 · order to design a reliable battery charger. A fast battery charger must quickly recharge a battery to full charge. Fast charging batteries

Application Note 68

AN68-1

December 1996

Applications Engineering Staff

LT1510 Design Manual

INTRODUCTION

The ever-growing popularity of portable equipment inrecent years has pushed battery technologists to searchfor battery types that store more energy in a smallervolume, weigh less and are safer. Also, the power sourceselection for charging the batteries has diversified. Forexample, a notebook computer can be connected to acar battery, a power adapter, a docking station or even tosolar cells.

The variety of input voltages, coupled with the need forhigh efficiency and the need for accurate constant voltageand constant current, as in the case of Li-Ion batteries (seebelow), have led to the introduction of a switching typeconstant-voltage, constant-current battery charger IC, theLT®1510.

Being a switching regulator, the LT1510 can operate overa large range of input voltages, up to 28V, with efficiencyin the 90% range. Because the LT1510 operates in currentmode, its output performance is not affected by inputvoltage changes.

An important feature of the LT1510 is its constant-currentoutput (see Figure 1). Although other switching regulatorsoffer current limiting, the LT1510 offers constant currentwith 5% accuracy. In addition, the transition from con-stant current to constant voltage and back is very smooth.

Only a few basic calculations are required to design withthe LT1510. As the reader will see later, a voltage divider(two resistors) and a current programming resistor needto be selected for the constant voltage and constantcurrent, respectively.

In constant-current/constant-voltage operation, theLT1510 can charge lithium-ion (Li-Ion) and sealed-lead-acid (SLA) batteries. In constant-current only operation,the LT1510 can charge nickel-metal-hydride (NiMH) andnickel-cadmium (NiCd) batteries.

TABLE OF CONTENTS

ON BATTERIES AND CHARGERS ............................................................................................................................................. AN68-2LT1510 OPERATION/BLOCK DIAGRAM.................................................................................................................................... AN68-5COMPONENT SELECTION ........................................................................................................................................................ AN68-8INTEGRATING THE LT1510 INTO A SYSTEM ......................................................................................................................... AN68-13CHARGING BATTERIES/TERMINATION METHODS ................................................................................................................ AN68-16APPENDIX A: TEST RESULTS ................................................................................................................................................ AN68-28APPENDIX B: AUXILIARY CIRCUITS FOR TESTING BATTERIES AND CHARGERS................................................................. AN68-35

, LTC and LT are registered trademarks of Linear Technology Corporation.

Application Note 68

AN68-2

ADAPTER

VCC

GND PROGOVP BATTERY

LT1510

CONSTANT CURRENT

OUTPUT/ BATTERY VOLTAGE

AN68 F01

OUTPUT/CHARGE CURRENT

VCONST = 2.465 (1 + R1/R2)

ICONST = 2.465 (2000/R3)

CONSTANT VOLTAGE

R1

R2R3

ADAPTER

BAT

+

+

Other advantages of designing with the LT1510 include:

• Efficiency in the 90% range

• High constant-voltage and constant-current accuracy

• An internal sense resistor that can be connected to eitherterminal of the battery

• Wide range of battery voltages (2V to above 20V)

• Small inductor

• Internal power switch

• Low drain in sleep mode

• Soft start

• Shutdown control

• Up to 1.5A charge current

This application note will show how to design with theLT1510 and will describe various methods of terminatingthe charge. Test results and test methods are included inthe appendices. The circuits in this application note areconstant-current and constant-current/constant-voltage

battery charger circuits developed by Linear TechnologyCorporation. These examples are intended to serve asstarting points in your design process. Most of them canbe adapted to your specific application and battery chem-istry needs.

Another product in the Linear Technology Corporation lineof constant-current /constant-voltage battery charger ICsis the LT1511. The LT1511 offers higher charge current(3A) and total system current control. The LT1511 can beconfigured so that when the system current requirementincreases, the charge current decreases and the totalsystem current matches the power adapter’s maximumcurrent.

ON BATTERIES AND CHARGERS

Developing a stand-alone or embedded battery charger fortoday’s portable products, incorporating the latest batterytechnology, requires careful consideration of how thesystem elements, including battery, charger, system con-troller and system load, work together.

An understanding of the charging characteristics of thebattery and the application’s requirements is necessary inorder to design a reliable battery charger. A fast batterycharger must quickly recharge a battery to full charge. Fastcharging batteries requires accurate charge terminationwhen the battery is fully charged in order to preventdamage or reduced battery life. Similarly, excessive dis-charging can damage any battery type.

Battery Charger Checklist

In order to organize your approach, start with the followingchecklist of design considerations:

• Select an appropriate battery for your application’sneeds

• Know your battery charging needs

Charging current

Charging voltage and its temperature dependence

Primary charge termination method for the battery

Secondary (safety) charge termination method

Figure 1. Constant-Current and Constant-Voltage Modesof the LT1510 with Simplified Schematic (Above)

Application Note 68

AN68-3

Can you include a temperature sensor in your batterypack?

Recommended charging method to achieve longestbattery life

Effect of cell temperature on charging method andcharge level

• Stand-alone or embedded charger?

• What charger power source is available?

• Single or multiple battery pack charging capability?

• Environmental operating conditions

• Charging interval: standard (overnight) or fast(< 4 hours)

• Will your application draw current during batterycharging?

• What kind of control do you need over your batterycharger? (shutdown, fault signals, charge level signals,charge completion signals)

• What will your charger do when the battery is removed?

• What happens when a fully charged battery is “hotplugged” or “cold plugged” into the charger?

• What happens when the adapter is plugged live into thesystem?

Comparing Four Rechargeable Battery Chemistries

The major rechargeable batteries readily available todayare nickel-cadmium (NiCd), nickel-metal-hydride (NiMH),sealed-lead-acid (SLA) and lithium-ion (Li-Ion). Thesebatteries serve the common function of supplying renew-able energy to user applications, but not every battery typeis appropriate for every application. Different battery tech-nologies have distinctive characteristics that determinetheir suitability for a particular use. These characteristicsinclude energy density, cell voltage, battery internal resis-tance, maximum charge rate, discharge profile, life (num-ber of charge/discharge cycles), self-discharge rate anddischarge rate. Table 1 shows typical characteristics ofbatteries of each chemistry.

Table 1. Battery Type Characteristics

SLA NiCd NiMH Li-Ion

Energy Density (W-Hr/kg) 30 40 60 90

Energy Density (W-Hr/ l) 60 100 140 210

Operating Cell Voltage (V) 2.0 1.2 1.2 3.6**

Discharge Profile Sloping Flat Flat Sloping

Life in Cycles* 500 1000 800 1000

Self-Discharge 3%/mo 15%/mo 20%/mo 6%/mo

Internal Resistance Low Lowest Moderate Highest

Discharge Rate <5C <10C <3C <2C

* Until only 80% of initial charge capacity is achievable upon recharge.** The operating cell voltage drops during discharge. This is an average voltage.

Understand the Charging Requirements for Your Bat-tery: Different battery chemistries have different chargerequirements. The descriptions below indicate the mostcommon charging methods for these battery types. Addi-tional methods, such as pulse charging, are not coveredhere.

System reliability may require a primary and a secondarytermination method for preventing overcharge.

Battery capacity is described by the bold letter C, whichrepresents the capacity in ampere-hours. Charging at Crate means charging at a current of C amps (for instance,charging a 1.3AH battery at 1.3A rate). Batteries are not100% efficient in converting charge current into storedcharge. It therefore takes longer than one hour to chargea battery to full capacity when charging at the C rate.Consult your battery manufacturers for their recommendedcharging rates and methods.

• Nickel-Cadmium: NiCd batteries are charged with aconstant-current profile. NiCd batteries can be continu-ously charged at the standard C/10 trickle rate indefi-nitely without excessive temperature rise or damage.

Fast charging NiCd batteries requires a charge termina-tion method. Primary termination can be based upon∆T/∆t (rate of temperature rise) or – ∆V (cell voltagedecrease at full charge) sensing. It is recommended thatthe secondary termination be a ∆TCO (temperature riseover ambient) termination. Many manufacturers’ NiCdbatteries can be charged at significantly greater than theC/1 rate, reducing the charge time to as little as fifteenminutes.

Application Note 68

AN68-4

• Nickel-Metal-Hydride: NiMH batteries are charged witha constant-current profile. The standard C/10 rate charg-ing works well for overnight charges. NiMH batteries aremore susceptible to damage from overcharging thanNiCd batteries; the charge must therefore be reduced toC/40 or switched to a pulsed trickle charge after 16hours. This can be implemented with a timer.

Fast charging NiMH cells also require charge termina-tion. NiMH cells exhibit a slower increase in cell voltageduring charge than NiCd cells and a flatter peak. Primarytermination can be based on sensing the zero dV/dtcondition of the battery voltage characteristic (voltagepeak) or ∆TCO. Following this, the charging circuitshould reduce the current to a maintenance charge ofC/40 or a pulsed trickle charge to counteract the batter-ies’ self-discharge characteristic. Secondary termina-tion can be temperature related or controlled by a timer.

• Sealed-Lead-Acid: SLA batteries can be charged with aconstant-voltage, current-limited supply or with a con-stant-current supply. For standby power applications,constant-voltage (float) charging is the traditionalmethod. Charge is delivered at the current limit until the“float” voltage across the battery is reached and thevoltage is then held constant while the current into thebattery decreases naturally as the cells reach full charge.The float voltage of approximately 2.25V per cell can bemaintained indefinitely. For longer battery life, the floatvoltage should change by – 1mV to – 5mV per °C per cellto match the cell voltage temperature dependence.

Fast charging with the constant-voltage method isachieved by increasing the charging voltage to approxi-mately 2.45V per cell, which extends the charging timeat the current limit and reduces total charge time. Whenthe battery voltage reaches the constant charging volt-age, the current decreases naturally as the cells reachfull charge. After a minimum charge-current level isreached, the charging voltage is reduced to a nominalfloat voltage or stopped.

Fast charging with the constant-current method re-quires monitoring the battery voltage. Consult manufac-turers’ data sheets for battery characteristic details.

• Lithium-Ion: Li-Ion batteries are charged with a con-stant-voltage, current-limited supply. These batteriesrequire special attention due to their susceptibility todamage in overcharge, deep-discharge and short-cir-cuit conditions. Constant current is supplied until thecell voltage reaches 4.1V or 4.2V per cell (depending onthe manufacturer), followed by constant-voltage charg-ing with the required accuracy of ±50mV per cell. Thecharging current then tapers down naturally. Increasedbattery cycle life may be achieved by terminating thecharge 30 to 90 minutes after the charging current dropsbelow some current threshold.

Several manufacturers include fault-sensing and cur-rent-balancing circuits within the battery pack. Thefault-sensing circuit will open a series connection withinthe battery pack in the event of excessive cell voltage,discharge current or temperature, or in the event of anundervoltage condition. The use of battery packs con-taining appropriate cell monitor/control devices is rec-ommended. The current-balancing circuit diverts chargecurrent from fully charged cells to partially chargedcells.

Charge Termination Techniques: To prevent battery dam-age and extend battery cycle life with fast charging, it isnecessary to terminate charging after NiCd and NiMH cellshave reached full charge. In addition, it is a recommendedpractice to provide a secondary charge termination methodas a safety measure. Some charge algorithms include“top-off” charge stages before completing the charge.

A number of methods have been used to detect the fullycharged condition of a battery and terminate the charge.Several reliable termination methods are based on thethermal release of energy in the battery near full charge.The voltage characteristic of a battery during constant-current charge is also an indicator of the electrochemicalprocess of battery cell recharging, and several of thefollowing methods are based on battery voltage. Not alltermination methods are good for all battery types.

The most common termination techniques are discussedbelow. The best suited battery for the termination is givenin parenthesis.

Application Note 68

AN68-5

• IMIN: After the charger has reached a constant-voltagestate, the charge current tapers off. Termination istriggered when the current drops below a set currentthreshold. (Li-Ion, SLA)

• dT/dt detects the rate of change in temperature withtime. Termination can be based on a maximum dT/dt,such as 1°C/min for NiCd. (NiCd)

• ∆TCO (delta temperature cutoff) detects temperaturerise over ambient temperature. Terminates on presetthreshold temperature differential. (NiCd, SLA)

• TCO (temperature cutoff) represents an absolute batterytemperature at which the charge is terminated.(NiCd, NiMH)

• –∆V (negative delta V): At constant current, NiCd andNiMH batteries exhibit a temperature rise toward the endof charge. Since they have a negative temperaturecoefficient, their voltage drops. Termination is activatedwhen a decrease in battery voltage is detected. (NiCd,NiMH)

• dV/dt (slope of voltage time curve) detects the rate ofchange of battery voltage with time at constant-currentcharge. (NiCd)

• Zero dV/dt (zero voltage change) detects the actual peakvoltage at constant-current charge. (NiMH)

• Slope inflection method (using the second derivative ofVBAT versus time) detects the negative going, zero-crossing rate of change in slope of the voltage/timecurve just before charge completion. (NiCd)

• Threshold voltage detection terminates charge or re-duces charge current significantly when the batteryreaches a certain maximum voltage.

• Timer sets the maximum charging time limit and termi-nates when the limit is reached. (Li-Ion, NiCd, NiMH,SLA)

Table 2 summarizes the standard charge and fast chargeinformation.

Table 2. Battery Charging Characteristics

SLA NiCd NiMH Li-IonCurrent Constant Constant CurrentLimit Current Current LimitFloat Constant

Voltage Voltage

Standard Charge

Constant Current (A) 0.25C 0.1C 0.1C 0.1C

Constant Voltage 2.25 1.50 1.50 4.1V or(V/Cell) 4.2V±50mV

Time (hours) 24 16 16

Temperature Range 0°/45°C 5°/40°C 5°/40°C 5°/40°C

Termination None None Timer Timer

Fast Charge

Constant Current >1.5C >1C >1C 1C

Constant Voltage 2.45 1.50 1.50 4.1 or(V/Cell) 4.2V±50mV

Typical Time (hours) <1.5 <3 <3 <2.5

Temperature Range 0°/30°C 15°/40°C 15°/40°C 10°/40°C

Primary Termination IMIN* dT/dt Zero dV/dt IMIN*+ Timer

∆TCO –∆V –∆V dT/dt

Slope ∆TCOInflection

∆TCO

Secondary Termination Timer TCO TCO TCO

∆TCO Timer Timer Timer

*IMIN is minimum current threshold termination.

LT1510 OPERATION/BLOCK DIAGRAM

The LT1510 is a current mode, PWM (pulse-width modu-lated), positive buck switcher that operates in one of twostates: constant current or constant voltage. Figure 2shows a block diagram of the LT1510 along with a typicalcharger circuit. The main functions in the diagram can bedivided into three major groups: the PWM switcher func-tion, the constant-current function and the constant-voltage function. Only constant-current or constant-volt-age components can be active at any given time. The PWMswitcher function is active as long as the charger is notdisabled; it includes a 200kHz oscillator, set-reset flip-flop, switch QSW, PWM comparator C1, buffer B1 andcurrent amplifier CA2. The constant-current componentsinclude RS1 and CA1. The constant-voltage componentsinclude RS1, CA1 and voltage amplifier VA.

Application Note 68

AN68-6

–

+

–

+

–

+

–

+

– +

VSW

0.7V

1.5V

VBAT

VREF

VC

GND

SLOPE COMPENSATION

R2

R3

C1PWM

B1

CA2

–

+

–

+

CA1

VA

+

+

VREF 2.465V

SHUTDOWN

200kHz OSCILLATOR

S

R

R

R1 1k

RS1

DB

IBAT ICHRG

IPROG

VCC

VIN

VCC

BOOST

SW

SENSE

BAT

OVP*

AN68 F02PROG

RPROG

RC

RAVE

CAVE

60k

IPROG IBAT

= 500µA/A

CB

D2

L1

CIN

D1

CC

QSW

gm = 0.64Ω

+

*NOT AVAILABLE IN 8-PIN SO PACKAGE

RV2

RV1COUT

B

Figure 2. LT1510 Block Diagram Showing Basic Charger Circuit

Since the current gain of RS1 and CA1 is 2000, RPROG’svalue prescribes the constant-current current level. Thecharge current level can be calculated from:

IRCONST

PROG=

2 465

2000. (01)

In constant-voltage operation, the 200kHz loop operatessimilarly to that in constant-current, but the voltage regu-lation is achieved by overriding RS1 and CA1 with voltagedivider RV1, RV2 and voltage amplifier VA. The loop regu-lates the voltage at the OVP pin to equal VREF. The voltageat the BAT pin in a constant-voltage state is:

In constant-current operation, the 200kHz oscillator setsthe set-reset flip-flop (SR-FF), which turns QSW on. Thecurrent rise through L1, RS1 and B is amplified by CA1,converted from current to voltage by R1, buffered by B1and compared to the steady-state voltage at the VC pin byC1. When the VC level is reached at the positive input of C1,the SR-FF resets and waits for the next 200kHz oscillatorset signal. Current regulation is achieved by a slow loopcontaining RAVE, CAVE, CA2, CC and RC. Since CA2 and the60k resistor constitute a high gain voltage amplifier, thevoltages at its negative input (or the PROG pin) and itspositive input are equal. In other words, in current modethe voltage at the PROG pin is equal to VREF (2.465V).

Application Note 68

AN68-7

VR R

RCONSTV V

V= +

2 465 1 2

2. (02)

RAVE and CAVE smooth the output of CA1 in the constant-current state or VA in the constant-voltage state. CC and RCcompensate the regulation loop.

CIN bypasses the input and COUT smoothes the chargecurrent into the battery B.

Pin Descriptions

GND: This is the ground pin. There are two kinds of groundpins for the LT1510: electrical and fused. The function ofthe electrical ground pin is to serve as the analog groundreference for the LT1510. For best regulation in constant-voltage operation, connect the bottom side of RV2 asclose as possible to this pin or run a separate trace fromthe resistor to this pin. The four pins at the corners of the16-pin package are fused to the internal die for heatsinking. Connect these pins to expanded printed circuitboard copper areas for proper heat removal.

SW: The LT1510 topology is positive buck. The NPNswitch (QSW) in the positive buck topology is connectedbetween the input supply and the inductor/catch diodenode. Inside the LT1510, the bipolar switch is connectedbetween the VCC and SW pins. Keep the trace from the SWpin to D2 short and wide. To minimize generated electro-magnetic interference (EMI), keep the trace from the SWpin to L1 as short and wide as possible.

BOOST: The monolithic NPN transistor selected for theswitch QSW is superior to a PNP in terms of speed andcollector resistance. However, its saturation voltage islimited by its base-emitter drop. The LT1510’s BOOST pinprovides a means of bootstrapping the drive to QSW,thereby allowing it to saturate against the input rail.Capacitor CB, which is charged and refreshed during theoff time through diode DB and SENSE pin, acts as abootstrap. CB delivers base drive to QSW through theBOOST pin. For best switch performance and, conse-quently, best efficiency and highest switching duty cycle,connect the anode of DB to a source of 3V to 6V that isactive when the charger is.

OVP: The OVP (overvoltage protection) pin is used toprogram the output voltage in the constant-voltage state.

The output voltage is sensed through a voltage dividercomprising RV1 and RV2 and fed back to the OVP pin. TheOVP feedback sense voltage is 2.465V.

SENSE: Inductor current and average charging currentare controlled by monitoring an on-chip 0.08Ω senseresistor RS1. This resistor is internally connected betweenthe SENSE pin and the BAT pin. Inductor current informa-tion is used to control the buck regulator and the averagecurrent information is used to control the battery chargingcurrent. In most applications the sense resistor appears inseries with the output side of the buck regulator inductor,but it is also possible to sense current at ground in thenegative terminal of the battery. Do not bypass the SENSEpin. Note that the total pin-to-pin resistance is higher(0.2Ω) than the value of the sense resistor itself.

BAT: The “downstream” side of the sense resistor (seeSENSE pin description) is connected to the BAT pin. Inmost applications the BAT pin is connected directly to thepositive terminal of the battery. The BAT pin constitutesthe output of the buck regulator and must therefore bebypassed. If the sense resistor is used to measure currentin the negative terminal of the battery, the BAT pin can begrounded.

VC: The VC pin is used for frequency compensation of thebuck regulator in both constant-current and constant-voltage operation. The VC pin is also used for soft start andshutdown. A CC capacitor of at least 0.1µF filters out noiseand controls the rate of soft start. Switching starts at 0.7V;higher VC corresponds to higher charging current innormal operation. For switching shutdown pull VC toground with a transistor.

PROG: The PROG pin is used to program the constantcurrent by selecting the RPROG value. CAVE and RAVE mustbe connected to the PROG pin. CAVE averages the currentmonitored through RS1 so the constant-current controlloop regulates the average current into the battery. Duringnormal operation, the PROG pin voltage stays close to2.465V. If the PROG pin is shorted to ground, the switch-ing will stop. When a current sinking device is connectedto PROG pin for the purpose of changing the constantcurrent charge, the device has to be able to sink current ata compliance of up to 2.465V.

VCC: This is the input supply to the LT1510. Short VCC1 andVCC2 together when using the 16-pin package. The

Application Note 68

AN68-8

operating voltage range is 8V to 28V. Below 8V theundervoltage lockout may be activated and switching maystop; 28V is the absolute maximum value. VCC must be atleast 2V above the highest battery voltage. VCC should notbe forced to > 0.7V below the SW pin because there is aparasitic diode connected from the SW pin to the VCC pin.For good bypassing, a low ESR capacitor of 10µF or higheris required. The trace from the bypass capacitor to the VCCpin should be as short as possible.

COMPONENT SELECTION

This section provides the user with the criteria for select-ing the components of a typical circuit with 2-level con-stant-current (ICONST) charge, constant-voltage (VCONST)charge and charge disable, as shown in Figure 3.

When selecting components, keep the following points inmind:

• The topology of the LT1510 is positive buck.

• The average output current is regulated in the constant-current state.

• The average output voltage is regulated in the constant-voltage state.

• The switching frequency is 200kHz.

• The recommended parts will operate over the full inputand output ranges at a constant current of up to 1.3A.

The critical parameters for parts selection are discussed inthe following paragraphs. The designer must apply safetymargins as necessary for the system.

CA1 (internal to the LT1510) has 700µA of input biascurrent typically. (This current flows into the BAT pin orSENSE pin.) This current is supplied by the step-downconverter (through the SENSE pin) when the charger isactive. When the charger is in shutdown mode (VC < 0.3V),there is an inherent BAT pin source current of 375µAmaximum that flows out of the BAT pin. When the batteryis present, it will absorb this current regardless of thevalues of R1 and R2. However, if the battery is removedand the charger is in shutdown mode, the output voltageat BAT pin can rise to a voltage as high as:

[(R1 + R2)(375µA)] volts

If the output voltage must stay below battery voltage whenthe battery is removed, the divider current must be at least375µA. Adding a switch transistor, Q1, as shown in Figure3, takes care of the increased battery drain. When VIN is

Figure 3. A Typical LT1510 Based Li-Ion Charger Circuit

+–

+

B1** Li-Ion+

+

GND*

SW

BOOST

GND

OVP

SENSE

GND*

GND*

GND*

VCC2

VCC1

PROG

VC

BAT

GND*

GND*

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

C1 0.22µF

C3 0.1µF

C4 1µF

C5† 10µF 25V

L1*** 33µH

CR2 1N914

CR1 1N5819

CR3 1N5819VIN

11V TO 25V

C2 22µF 25V TANT

R1 11.3k 0.25%

R2 4.87k

0.25%R3

100kR4 1k

R5 300Ω

R6 37.9k

R7 4.22k

NO_CHARGE HI_CHARGE

AN68 F03

Q1 VN2222

Q2 VN2222

Q3 VN2222

* SOLDER TO GROUND PLANE FOR HEAT DISSIPATION ** MOLI ENERGY ICR-18650 *** COILTRONICS CTX33-2 † TOKIN OR MARCOM CERAMIC SURFACE MOUNT

4.1V

4.1V

U1 LT1510

+

R8 100k

Application Note 68

AN68-9

removed, Q1 is off and battery drain is prevented. If thereare high frequency components at VIN, they may passthrough the gate to source capacitance of Q1 to the OVPpin. An added 220k resistor between VIN and the gate ofQ1 attenuates the high frequency signal.

For better efficiency, it is recommended that the anode ofCR2 be connected to a voltage source between 3V and 6V.A 10µF ceramic bypass capacitor should be connected tothe anode with a short lead.

Care must be taken when selecting input and outputcapacitors. They should be rated at 200kHz and haveadequate ripple current rating. Poor choice of input oroutput capacitors may be detected too late. The inputcapacitor should be able to withstand the high surgecurrent that occurs when a power adaptor is hot-pluggedto the charger. The output capacitor should be able towithstand the high surge current that occurs when abattery is replaced. Tantalum capacitors have a knownfailure mechanism when subjected to high surge currents.The surge current ratings of tantalum capacitors areproportional to their voltage ratings. Consult the manufac-turer. The actual capacitance is not critical in most cases.

High capacity ceramic capacitors by Tokin or UnitedChemi-Con/MARCOM and electrolytic OS-CON capacitorsby Sanyo are recommended for the input. Solid tantalumcapacitors such as AVX TPS and Sprague 593D arerecommended for the output.

Boost Capacitor (C1) Selection

A 0.22µF ceramic capacitor will work under all conditions.The voltage across C1 is the battery voltage. An AVX12065C224MAT2A surface mount capacitor performswell.

Output Capacitor (C2) Selection

C2 removes the high frequency components of (smoothes)the battery charge current.

The highest transient current through C2 occurs when thebattery is plugged in and the power adapter is not live. Thetransient current magnitude depends on circuit construc-tion and the components in the power path (traces, leads,solder joints, etc.). The AVX TPSD226M025R0200 (22µF,

25V, tantalum, SMT) withstands transient current equal tothe operating voltage in amps (in other words, 25A for 25Voperating voltage).

The RMS ripple current can be calculated from:

IV V V

L V fRIPPLE

CC BAT BAT

CC=

−( )( )( )( )( )( )

0 29.(03)

where:

VCC is the maximum voltage at the VCC pin

VBAT is the voltage at BAT pin

f is the switching frequency (200kHz)

L is the minimum inductance of L1

For example, if the input voltage is 12V to 20V, theinductor, L1, is 30µH ±8% and drops 35% maximum atthe charge current of 1A and the battery is a 3-cell NiCd, thefollowing values should be plugged into the equationabove:

VBAT = (1.6)(3) = 4.8V

VCC = 20V

L = (30)(0.92)(0.65) = 17.9µH

After plugging the numbers in, the calculated maximumripple current is 0.3A RMS.

Compensation Capacitor (C3) Selection

A 0.1µF ceramic surface mount capacitor, such as the AVX12065C104MAT2A, performs well and gives a soft startduration of 3ms. For a longer soft start duration, a largervalue is required.

PROG Pin Capacitor (C4) Selection

This capacitor averages the output of CA1 (PROG pin) forthe constant-current loop. A 1µF ceramic capacitor isrecommended. The AVX 12063G105ZAT2A surface mountcapacitor performs well. A 300Ω resistor (see PROG pinresistor) is required in series with C4.

Application Note 68

AN68-10

SWITCH CURRENT (A)0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.8 2.01.6

BOOS

T CU

RREN

T (m

A)

50

45

40

35

30

25

20

15

10

5

0

AN68 F04

VCC = 16V

VBOOST = 38V 28V 18V

Figure 4. Switch Current vs Boost Current vs Boost Voltage

Input Diode (CR3) Selection

CR3 provides polarity protection, prevents battery drainand eliminates current transient spikes. When the poweradapter is removed without the input diode, there could bebattery drain through the following path; positive batteryterminal, BAT pin to SENSE pin through the internal senseresistor, L1 and the SW pin to the VCC pin through theinternal parasitic diode.

CR3 also eliminates large current transient spikes that canoccur when a power adapter with a large output capacitoris cold-plugged into the system. The current transient maycompromise the input capacitor (C5) and the connectorcontacts unless CR3 is installed.

A low forward voltage rectifier will increase charger effi-ciency. The reverse voltage is the maximum battery volt-age. The average forward current is:

II V

VCR AVECHRG BAT

IN3 ( ) =

( )( )(05)

CR3 should be able to withstand the current transient thatoccurs when the power adapter is “hot-plugged” to thecharger. A Motorola 1N5819 (leaded) or MBRS140LT3(SMT) performs well.

Switching Inductor (L1) Selection

L1 is an essential part of the positive buck topology asshown in Figure 3. The average current that flows through

Input Capacitor (C5) Selection

C5 bypasses the input supply for the LT1510. It is as-sumed that C5 conducts all input AC switching current.The maximum RMS ripple current through C5 is approxi-mately half the DC charging current. A high transientcurrent flows through the input capacitor when a poweradapter is “hot plugged” to the charger. A Tokin1E106Z5YU-C304F-T (10µF, 25V, ceramic, SMT) per-forms well. Other alternatives are ceramic capacitors byUnited Chemi-Con/MARCON, OS-CON capacitors by Sanyoand high ripple current electrolytics. Caution must beexercised when using tantalum capacitor for bypassing.Only a few tantalum capacitor types, such as AVX TPS andSprague 593D series, have the ripple and transient currentratings required. Consult the manufacturer.

The trace from C5 to the VCC pin of the LT1510 and to theground plane should be as short and wide as possible.

Catch Diode (CR1) Selection

CR1 serves as the catch diode in the schematic of Figure3. The reverse voltage across it is VIN and the maximumaverage forward current is:

IIV

V VCR AVEOUT

ININ OUT1( ) =

−( ) (04)

where:

IOUT is the maximum output current,

VIN is the maximum input voltage and

VOUT is the lowest output voltage.

A high speed Schottky power rectifier is recommended. AMotorola 1N5819 (leaded) or MBRS140LT3 (SMT) per-forms well.

Boost Diode (CR2) Selection

The maximum reverse voltage of CR2 is VIN. The averagecurrent is the BOOST pin current, which can be found inthe “Switch Current vs Boost Current vs Boost Voltage”graph in Figure 4. The peak current is higher. A switchingdiode, such as the Motorola 1N914 (leaded) orMMBD914LT1 (SMT), performs well here.

Application Note 68

AN68-11

Constant-Voltage Programming Resistors (R1, R2)Selection

The programming resistors R1 and R2 divide the batteryvoltage (constant voltage) down to the threshold level of2.465V. It is recommended that the voltage divider resis-tance R1 + R2 have a high value so that the battery drainis kept to minimum and the need for Q1 is eliminated.There is, however, a restriction: shutdown output voltage(see Page 8). If there is a limit to the output voltage whenthe battery is removed and the charger is in shutdownmode, then maximum allowed branch resistance is,VBAT(MAX)/375µA.

If output voltage with battery removed is not an issue,divider current can be much lower. As an example, withVBAT = 8.2V, and drain current of 25µA:

RV

Ak2

2 46525

98 6≅ =. .µ (07)

R2 is selected as 100k.

RV V

R nAk

BAT REF1

2 465 2 50232 2≅

−( )( ) +

=. /

. (08)


Another example for a 2-cell Li-Ion battery that requires8.2V ±100mV. The maximum output voltage in shutdownmode (with battery removed) is 8.5V.

The maximum value for R2 is:

RA

k22 465375

6 57≤ =. .µ (08.1)

R2 is selected as 4.87k. R1 is calculated as:

R

knA

k18 2 2 465

2 4654 87

5011 33≅

−( )+

=. .

.

.

.(09)

R1 is selected as 11.3k.

The battery drain current for an inactive charger is8.4V/(4.87kΩ + 11.3kΩ) = 0.52mA. Q1 can be added toeliminate this drain current.

The recommended tolerance for R1 and R2 is 0.25%.

L1 is the charge current. A 30µH inductor is acceptable formost applications. A Coiltronics CTX33-2 with windings inparallel or a Coiltronics CTX8-1 with windings in seriesperforms well.

L1’s maximum peak current can be calculated from:

I IV V V

V L fL PEAK CHRG

IN OUT OUT

IN1

0 5( ) = +

( ) −( )( )( )( )( )

.(06)

where:

VIN is the maximum input voltage,

VOUT is the lowest output (battery) voltage,

f is the switching frequency (200kHz),

L is the minimum inductance (H) of L1 and

ICHRG is the maximum charge current.

Battery Drain Inhibit MOSFET (Q1) Selection

Q1’s drain current is VBAT/(R1 + R2), when the charger isactive, or 375µA when the charger is in shutdown mode.If the VIN line has high frequency noise that can penetratethrough the gate-to-source capacitance of Q1, it is recom-mended that a 220k resistor be added in series with thegate. A VN2222 or 2N7002 are typical choices for leadedor SMT parts.

Shutdown MOSFET (Q2) Selection

If the manufacturer does not recommend a float voltage(indefinite constant voltage across the battery), as in thecase of Li-Ion, the charger must shut down at the end ofthe charge regimen. This can be achieved by forcing the VCpin low with MOSFET Q2. The drain current is 1mA. AVN2222 or 2N7002 are typical choices for leaded or SMTparts.

High Charge MOSFET (Q3) Selection

When Q3 is on, the equivalent value of R6 and R7 in parallelbecomes the constant-current programming resistor (seeR6, R7 selection). The off-state drain leakage current of Q3should be significantly lower than the programming cur-rent through R6. A VN2222 or 2N7002 are typical choicesfor leaded or SMT parts.

Application Note 68

AN68-12

Compensation Resistor (R4) Selection

This resistor is part of the compensation loop. A 1k, 5%resistor is recommended for most cases. The stability ofthe current loop can be tested by forcing a voltage stepacross the output while in a constant-current state.

PROG Pin Resistor (R5) Selection

This resistor is part of the compensation loop. A 300Ω, 5%is recommended for most cases. The stability of thevoltage loop can be tested by forcing a current step acrossthe output while in a constant-voltage state.

Constant-Current Programming Resistors (R6, R7)Selection

The values of R6 and R7 can be found from the followingformula:

RIEQUCONST

=( )( )2 465 2000.

(10)

where REQU equals R6 for the low constant-current chargeand R6 in parallel with R7 for high constant-currentcharge. As an example, a 1.3AH battery has to be chargedat C and trickle charged at C/10 (0.13A).

R k62 465 2000

0 1337 9≅

( )( )=

.

.. (11)


R kEQU ≅( )( )

=2 465 2000

1 33 79

.

.. (12)

17

13 79

138 3

7 4 206R

R k≅ − ≅. .

; . (13)


The LT1510 (U1) Package Selection

An 8-pin package can be used if only constant-currentoperation is required, but a 16-pin package must be usedfor constant-current and constant-voltage operation andfor better heat dissipation. The four corner pins of theSO-16 package are fused (connected to the die internally)and it is recommended that these pins be soldered to theground plane. With these pins soldered to expanded PClands, the thermal resistance of the SO-16 package is50°C/W. The thermal resistance of the DIP package is75°C/W.

The plots in Figure 5 can be used for selecting an SO-8 orSO-16 package based on input voltage, output voltage andoutput current. These curves can be used to a maximumambient temperature of 60°C. Refer to the data sheet formore accurate thermal calculations.

INPUT VOLTAGE (V)0

MAX

IMUM

CHA

RGIN

G CU

RREN

T (A

)

1.3

1.1

0.9

0.7

0.5

0.320

THERMALLY LIMITED MAXIMUM CHARGING CURRENT 8-PIN SO

THERMALLY LIMITED MAXIMUM CHARGING CURRENT 16-PIN SO

5 10 15 25

16V BATTERY

12V BATTERY

8V BATTERY

4V BATTERY

(θJA=125°C/W) TAMAX=60°C TJMAX=125°C

INPUT VOLTAGE (V)0

MAX

IMUM

CHA

RGIN

G CU

RREN

T (A

)

1.5

1.3

1.1

0.9

0.7

0.520

AN68 F05

5 10 15 25

(θJA=50°C/W) TAMAX=60°C TJMAX=125°C

16V BATTERY

12V BATTERY

8V BATTERY4V BATTERY

Figure 5. Comparing SO-8 and SO-16 Packages Thermal Limits

Application Note 68

AN68-13

INTEGRATING THE LT1510 INTO A SYSTEM

When an LT1510 based charger is integrated into a systemwith a power adapter or power supply as a source, and abattery and a switching regulator as a load, some issuesneed to considered. The next paragraphs describe a fewof them.

Minimum Operating Input Voltage

The minimum operating input voltage of an LT1510 basedbattery charger has to satisfy three LT1510 requirements:undervoltage lockout (VIN1), minimum VCC – VBAT voltagedifference (VIN2) and maximum duty cycle (VIN3). Seeequations below. Other factors that affect the minimumoperating input voltage are maximum output voltage,input diode forward voltage, resistance along the powerpath (including sense resistor, switch on resistance, traceresistance, solder joint resistance and connector resis-tance) coupled with maximum charge current.

The undervoltage lockout is 8V. The input-to-output volt-age difference (VCC – VBAT) is 2V and is defined by twoparameters: “maximum VBAT with switch on” and “inputcommon mode limit (high),” as found in the data sheet.The maximum duty cycle is 0.93. The following equationscan be used for calculating the minimum input voltage VIN.

V VV V V

IN DIN

IN DIN BAT

1

2

82

= += + + (14)

V VV I

DIN DINBAT CONST

30 7

= ++ ( )( ).

where:VIN is the charger minimum input voltage. Use the highestof VIN1, VIN2 or VIN3.

VDIN is the forward voltage of the input diode. The inputdiode can be replaced by a PMOSFET switch, in which casethis term is removed.

ICONST is the programmed charge current.

VBAT is the maximum battery voltage.

D is the maximum duty cycle. D = 0.93.

To give the designer preliminary data about the typicallowest input voltage, the circuit in Figure 6 was tested. Theconstant current was adjusted with a trim pot connectedto the PROG pin and the output voltage (VOUT) was forcedwith the battery simulator board (see Appendix B). Theresults are shown in Figure 7.

Figure 6. Minimum Operating Voltage Test

GND

SW

BOOST

GND

OVP

SENSE

GND

GND

GND

VCC2

VCC1

PROG

VC

BAT

GND

GND

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

C1 0.22µF

C3 0.1µF

C4 1µF

C5**

L1 CTX33-2 CR2†

NC

CR1 MBRS140L

CR3 MBRS140L

VIN

C2*

R4 1k

R5 300Ω RPROG

AN68 F06

U1 LT1510

+ BATTERY SIMULATOR * AVX TPSD226M025RO200

** TOKIN 1E106ZY5U-C304F-T † MOTOROLA MMBD914LT1

(15)

Application Note 68

AN68-14

mended that a 2200µF capacitor be connected in place ofthe battery before making the measurement. (Be sure toobserve capacitor polarity and to connect the capacitorwhen the charger is not running.)

Connecting the System Circuitsto the BAT Pin or SENSE Pin

It is possible to connect the portable system circuitrydirectly to the battery (BAT pin), but two facts should betaken into consideration. First, the total current will belimited and so the system will “steal” battery chargecurrent. In this case it is not possible to have a terminationsuch as –∆V that relies on constant charge current.Second, when the charger is active and the system isturned off, the full constant current will charge the battery,so the battery should be able to absorb it.

It is also possible to bypass the sense resistor and connectthe system circuits to the SENSE pin, as shown in Figure9. In this case the sum of the charging current and systemcurrent should not exceed the LT1510 maximum outputcurrent (limited by thermal considerations or switch peakcurrent). However, since the system circuitry is capacitivein nature (input capacitor of a DC-to-DC converter), itshould not be connected directly to the SENSE pin. This isbecause the internal sense resistor between the SENSEand BAT pins will have a large capacitance across it, whichwill cause instability. A 2.2µH inductor, such as theDT1608C-222 by Coilcraft, isolates the input capacitanceof the system circuits well (L2 in Figure 9).

CR4 limits the transient current through the LT1510’sinternal sense resistor when the system is switched on inbattery operation. Q1 is required if the series resistance of0.2Ω between BAT pin and SENSE pin causes the effi-ciency to drop. The Si9433’s on-resistance is 0.075Ω. Thecharge pump (C3/C4/CR5/CR6/R2) biases the gate of Q1.Q2 and R1 turn Q1 off when the system operates from theAC wall adaptor (VIN active). R8 is required if there is nocircuitry connected to VIN.

Switching Between AC and Battery Operation,2-Diode Configuration

Most systems that employ battery chargers also requireglitch-free switching between AC operation and batteryoperation. Figure 10 shows a way to connect the load(switching regulator) to VIN (when it is present) for AC

OUTPUT VOLTAGE (V)5

MIN

IMUM

INPU

T VO

LTAG

E (V

) 25

23

21

19

17

15

13

11

9

7

59 13 15

AN68 F07

7 11 17 19 21

I = 0.25AI = 1.3A

Figure 7. Typical Minimum Operating Voltage of an LT1510Based Charger vs Output Voltage and Output Current

Efficiency

The circuit in Figure 6 was used to test efficiency. The plotsin Figure 8 show the results for output voltage of 8V. Somepower is dissipated in the input diode. See the data sheetfor a circuit with a PMOSFET switch that bypasses theinput diode when VIN is on and saves power.

AN68 F08

INPUT VOLTAGE (V)8

EFFI

CIEN

CY (%

)

12 16 18 2610 14 20 22 24

90

89

88

87

86

85

84

IOUT = 1A

IOUT = 1.3A

VOUT = 8V

Figure 8. Efficiency vs Input Voltage and Output Voltage

Measuring Constant Voltage Without a Battery

At times, it may be necessary to measure the constantvoltage of the charger with high accuracy and with thebattery removed. Since the battery is an integral part of theregulation loop, some low frequency, low amplitude ripplemay appear on the BAT pin and the DC voltage measure-ment at the BAT pin will not be accurate. It is recom-

Application Note 68

AN68-15

Figure 10. LT1510 Charger System, 2-Diode Configuration

SW

BOOSTVC

C1 0.22µF

C4 1µF

Q1 Si9433DY

C5 22µF

BAT 3-CELL NiCd

TO SYSTEM

C3 0.1µF

C2 10µF

L1 33µH CR5

1N914

CR4 1N5819

CR3 1N5819

CR2 1N5819

VIN

VCC

GND

R2 1k

R3 300Ω

R4 4.99k R1

1k

AN68 F10

LT1510

SWITCHING REGULATOR

LT1373 LTC1439

OR SIMILAR

CR1 1N5819

+

SENSE

PROG BAT

Figure 9. Connecting the System to SENSE Pin

BAT1 2-CELL NiCd

GND

SW

BOOST

GND

OVP

SENSE

GND

GND

GND

VCC1

VCC2

PROG

VC

BAT

GND

GND

1

2

3

4

5

6

7

8

16

14

15

13

12

11

10

9

C1 0.22µF

C6 0.1µF

C2 10µF

C3 0.01µF

SW

C4 0.01µF

C7 1µF

L1 33µH

L2* 2.2µH

SYSTEM ON/OFF SWITCH

CR2 1N914

CR4 1N4001

CR1 1N5819

CR3 1N5819

VIN 8V TO 20V

C5 22µF 25V

R3 1.21k

R4 4.99k

R8 10k

R1 100k

R2 100k

R5 1k

R6 300Ω

R7 16.2k

AN68 F09

Q3

Q2 MPS2907

Q1 Si9433

CR6 1N914

CR5 1N914

TO SYSTEM

LT1510

+SYSTEM

SWITCHING REGULATOR

LT1302 LT1304 LT1307

OR SIMILAR

*COILCRAFT DT1608C-222

Application Note 68

AN68-16

system circuits “steal” charge current from the battery.This can be circumvented by boosting the output currentby the same amount that is “stolen.”

Figure 11 shows how the charge current can remainconstant regardless of the load. The system circuits’current is sensed by RS. Q1, U2, R1, R2 and R3 boost theLT1510’s PROG pin current and thus the output currentincreases with the system circuits’ current so that thecurrent charging the battery remains unchanged. TheLT1510 based charger should be designed for an outputcurrent that is the sum of currents into the battery andthe load.

The Next Generation Battery Charger IC, the LT1511

The next generation constant-voltage/constant-currentbattery charger IC, the LT1511, performs like the LT1510,but has two additional features: input current limiting andpeak switch current of 4A.

operation or to the battery when VIN is unavailable. WhenVIN is active, CR1 conducts the load current, CR2 isreverse biased and Q1 is off. When VIN is removed, Q1conducts the load current from the battery. The voltagedrop across Q1 is very low. Note that CR2 is in parallel withthe body diode of Q1. The load has typically high inputcapacitance and also demands high current if the batteryvoltage is low. CR2 conducts at the transition to batteryoperation when Q1 is not fully on (the body diode hasexcessive forward voltage drop). R1 is required if there isno circuitry connected to VIN. Low voltage drop across Q1is essential for high efficiency when the system is operat-ing from the battery. It was measured at 33mV with a loadof 0.5A and the battery at 3V.

Switching Between AC and Battery Operation,Current Boost Configuration

Placing the system circuits in parallel with the batteryachieves glitch-free switching between AC wall adaptorand battery operation. In AC operation, however, the

SW

BOOST

SENSE

PROG

VC

BAT

C1 0.22µF

C2 1µF

C5 22µF 25V

BAT 3-CELL NiCd

C4 0.1µF

C3 10µF

L1 33µH

CR3 1N914

CR1 1N5819

CR2 1N5819

VIN

VIN

VCC

GND

R6 1k

R4 200Ω

R3 200Ω

4

3

2

6

7

Q1 2N3904

R5 10k

R2 10k

RS 0.1Ω

AN68 F11

U1 LT1510

SYSTEM CIRCUITS

–

+R1 10k

U2 LT1006

+

Figure 11. LT1510 Charger System, Current Boost Configuration

Application Note 68

AN68-17

A control loop is provided to regulate the current drawnfrom the power adapter. This allows simultaneous opera-tion of the portable system and battery charging withoutoverloading the adapter. When system current increases,the charging current is reduced to keep the adapter currentwithin the specified level.

The internal switch of the LT1511 is capable of delivering3A DC current (4A peak current) for charging the battery.This is a 100% increase over the LT1510.

CHARGING BATTERIES / TERMINATION METHODS

Any portable equipment that requires fast charge needs tohave proper charge termination. Commonly, the LT1510is used in conjunction with a microcontroller that has aninternal analog-to-digital converter (ADC) or in conjunc-tion with a microprocessor and an ADC IC.

Sometimes, however, a microcontroller is not available oris not suitable for fast charge termination. The followingparagraphs describe solutions for both cases.

Charging NiCd Batteries

dT/dt Termination: A safe and reliable way to terminatefast charging of NiCd and NiMH batteries is based ondetecting the rate at which the battery temperatureincreases during constant-current charging. With con-stant-current charging, the battery temperature increasesrapidly as the battery nears full charge status (seeFigure 12).

The circuit in Figure 13 monitors the battery temperaturewith a negative temperature coefficient (NTC) thermistorRT and detects the rate at which the temperature rises overa 20-second period.

The thermistor output is amplified by differentiator U1Aand integrator U1B (which, together, form an AC coupledamplifier). The differentiator is AC coupled and thus elimi-nates the DC voltage of the RT and R1 network; the

integrator is reset every twenty seconds by the timer U2and transistor Q1. The output of the integrator is moni-tored by U1C, a comparator and latch. When U1B’s outputvoltage exceeds a threshold, the output of U1C is latchedhigh and turns Q2 on, pulling the LT1510 VC pin to groundand shutting the charger off.

The bias voltage (VBIAS) for the circuit is generated byvoltage divider (R13/R14) and buffer U1D. The VBIAS levelchosen is close to the thermistor network (RT/R1) outputvoltage in order to minimize the turn-on time needed forcharging C1. This also minimizes the effect of C1’s leak-age. C1 is a ceramic capacitor.

R2 allows C1 to stabilize rapidly upon turn-on. R2, R3, R6and R7 supply bias current to U1A and U1B.

The design equations for the dT/dt termination circuit arepresented in the following box.

CHARGE INPUT (% OF CAPACITY)1

90

80

70

60

50

40

30

20

120

100

80

60

40

20

0

–20

AN68 F12

50 100 150

CELL

TEM

PERA

TURE

(°C)

INTE

RNAL

PRE

SSUR

E (P

SIG)

1.75

1.50

1.25

1.00

0.75

0.50

0.25

0

CELL

VOL

TAGE

(V)

CELL VOLTAGE

PRESSURE

TEMPERATURE

Reproduced with permission by Butterworth-Helnemann, Rechargeable Batteries Applications Handbook, copyright 1992

Figure 12. Voltage, Pressure and TemperatureDuring 1C Charge of NiCd Battery

Application Note 68

AN68-18

–+

– +

CR5

1N58

19

CR1

1N41

48

CR2

1N41

48

Q2

2N39

04

Q1

2N39

04

U1C

U1B

10

6 5

3

9

8

7

12 11 10 9

816V D

D

Q14

RESE

T

CLK

OUT1

OUT2

V SS

2 3

1

4 11

CR3

1N58

19

CR4

1N41

48

AN68

F13

C9

10µF

C11

22µF

25

V

C5

0.1µ

F

L1

30µH

BAT1

**1,

7,8,

9,10

,16††

C7

0.22

µF

C6

0.1µ

F

C10

0.1µ

F

C8

1µF

C3

22µF

C4

0.1µ

F

C2

0.1µ

FC1

† 4.

7µF

R19

300Ω

R10

100k

R9

100k

R7

100k

R6

100k

VRST

VRST

13 12

14

V BIA

S

V BIA

S

V BIA

S

12V

12V

V BIA

S

R20

4.99

k

R11

100k

R8

20k

R5

10M

R4

10M

R T*

R21

1k

R12

10k

R17

10k

R16

10k

100k

R18

10k

R3

100k

R13

10k

R15

100Ω

R14

7.5k

R2

100k

R1

7.5k

SW

BOOS

T

GND

SENS

E

V CC

PROG

V C

U3

LT15

10

GND

BAT

*

KETE

MA

MSC

103K

. THE

RMAL

LY C

ONNE

CTED

TO

B1

**

PAN

ASON

IC 3

-CEL

L Ni

Cd P

130-

SCR

†

CERA

MIC

CAP

ACIT

OR

††

SOL

DER

TO G

ROUN

D PL

ANE

FOR

HEAT

DIS

SIPA

TION

+

U1A

LT10

14

U2

CD40

60

– +

U1D

+

– +

Figu

re 1

3. L

T151

0 Ni

Cd C

harg

er w

ith d

T/dt

Ter

min

atio

n

Application Note 68

AN68-19

Design Equations for dT/dt Termination

Thermistor Design

(1) β =−

T

TT T

InRR

O

O

T

TO(16)

(2) RR T

TTO O

O1

2

2=

−( )+

β

β(17)

(3)dV

dTV

T TDIV

DIVTOO O

= − +

β

2

12 (18)

where:β is a constant depending on the thermistor material,T is temperature in °K at which RT is characterized,TO is temperature in °K at which RTO is characterized,dVDIV/dT is the rate of divider output voltage change vstemperature and VDIVTO is the divider DC voltage at TO

Integrator U1A and Differentiator U1B Gain

(4) G R s CR s C

for

= ( )( )( ) ( )( )( )4 11

5 2

R4 = R5, G =C1C2

(19)

Threshold of the Latch U1C Stage

(5) VdTdt

tdV

dTGTH

DIV=

( ) ( )

( ) ( )(20)

where:dT/dt is the selected slopet is the timer period

R8 and R9 Selection

(6)VR

VR

or

R VR

V

TH BIAS

THBIAS

8 9

89

=

≅ ( )

:

(21)

where:

(7) VV R

R RBIASIN

=( )( )

+

14

13 14(22)

For the design shown in Figure 13:RT is a Ketema MSC103k, a 10k thermistor with R25/R125= 29.25.

(1) β =−

( ) =398

298398 298

29 25 4004In . (23)

(2) Rk

k110 4004 2 298

4004 2 2987 41≅

− ( )[ ]+ ( ) = . (24)


The gain of the RT, R1 network is:

(3)

dVdT

V C

DIV =+

−

+

= − °

1210

10 7 54004

2 298

1298

0 132

2.

. /

(25)

For C1 = 4.7µF and C2 = 0.1µF, the gain of the integratorand differentiator can be written as:

(4) GFF

= 4 70 1..

µµ

= 47 (26)

The selected slope that will trigger termination is 0.5°C/min. (a conservative half of the typical 1°C/minute). Theselected timer period is 20 seconds (0.33 minute).

(5) VTH = (0.5)(0.33)(0.132)(47) = 1.023V

VIN is selected as 12V, R13 = RT = 10k, R14 = R1 = 7.5k

(7) V VBIAS =( )+

=12 7 5

10 7 55 14

.

.. (27)


(6) R k8 1 021005 14

19 9≅ =..

. (28)


A secondary termination for the charger is recommended.Depending on system reliability requirements, the sec-ondary termination circuit may use existing componentssuch as RT or U1 for absolute temperature or time-out,

Application Note 68

AN68-20

respectively. Also, to avoid premature termination, thetemperature rise rate that results from bringing the systemindoors from the lowest outdoor temperature should beconsidered.

–∆V Termination: The internal battery temperature risetowards the end of charge, coupled with the negativetemperature coefficient of NiCd and NiMH, causes thebattery voltage to drop. The drop can be detected and usedfor terminating a fast charge with the LT1510.

In the example shown, Figure 14, the charge current wasselected as 0.8A. To determine the voltage droop rate for–∆V termination, a fully charged 3-cell (Panasonic P140-SCR) NiCd battery was connected to an LT1510 chargercircuit programmed for a 0.8A rate. The negative slope involtage, as seen in Figure 14, is calculated to be–0.6mV/s. It can be seen that the total voltage drop isabout 300mV (100mV per cell). After the battery voltagedropped 300mV from the peak of 4.93V (100mV per cell),the charger was disabled.

capacitor to be charged to the input level. U2B and theassociated parts form a latch that requires a momentarynegative voltage at Pin 6 to change state. R15 supplies thenegative feedback and Q2, R16, R17 and C10 reset thelatch on turn-on.

U3’s output voltage droops at a rate proportional to thehold capacitor’s internal leakage and the leakage current atPin 6 (10pA typical).This droop is very low and does notaffect the operation of the circuit.

The minimum negative battery voltage slope required totrigger termination can be calculated from:

− = ( )( )dVdT

Vt G

TRIG

CLK U A2(29)

where:

VV R

R RmVTRIG

REF=

( )( )+

= =12

11 125

1101

49 5. , (30)

tCLK is the clock period, 15 seconds,

G is the gain of the first stage =R8

U2A R R4 511

||= (31)

hence:

− = ( )( ) =dVdT

mVmV s

49 515 11

0 3. . /

(32)

At the heart of the circuit in Figure 15 is U3, a sample-and-hold IC, LF398. The output of U3, Pin 5, samples the inputlevel, Pin 3, at every clock pulse at Pin 8. When the batteryvoltage drops, the input to U3 also drops. If the update stepat the output of U3 is sufficiently negative, U2B latches inthe high state and Q1 turns on and terminates the chargeby pulling VC pin of the LT1510 down and disabling it.

U2A and the associated passive components smooth,amplify and level shift the battery voltage. The timer U4updates the hold capacitor C8 every fifteen seconds. Thetimer signal stays high for 7ms, sufficient for the hold

Charging Sealed-Lead-Acid (SLA) Batteries

Standard Charge: The LT1510 is ideal for standard charg-ing of SLA batteries because of its constant-current andconstant-voltage features. To extend the battery life, thefloat voltage can be temperature matched to the batteryspecifications. The circuit in Figure 16 was designed forthe Panasonic SLC-214P, which is a 2-cell, 2.1AH SLAbattery with a maximum charge current of 0.8 amps.

The thermistor RT, selected for temperature matching, isa Ketema MSC103K. Figure 17 shows minimum andmaximum float voltage vs. temperature, as recommendedby the manufacturer. The output voltage of the charger vs.temperature fits in this range.

TIME (MINUTES)0

BATT

ERY

VOLT

AGE

(V)

10 20 30

AN68 F14

5.0

4.9

4.8

4.7

4.6

4.5

4.4

4.3

4.2

NEGATIVE SLOPE

END OF CHARGE

Figure 14. – ∆V Test

Application Note 68

AN68-21

Figu

re 1

5. L

T151

0 Ni

Cd C

harg

er w

ith –

∆V T

erm

inat

ion

CR2

1N58

19

CR1

1N91

4

AN68

F15

C5

22µF

25

VC6

0.

1µF

L1

30µH

B1**

*1,7

,8,9

, 1

0,16

1 2 3 4 5 6 7 8

16

15

14

13

12

11

10

9

C1

0.22

µF

12V

C4

0.1µ

F

C3

1µF

R1

300Ω

R6

100k

R8

100k

R4

10k

5VRE

F

R5

100k

R2

6.19

k

R21

100k

R22

100k

R20

100k

Q1

2N39

04

CLK

R23

100k

R3

1k

SW

V CC1

V CC2

PROG

V C

BAT

U1

LT15

10

GND

C2

10µF

CR3

1N58

19C7

0.

1µF

C8†

1µF

HOLD

CA

PACI

TOR

150Ω

C9

0.1µ

F

C10

22µF

C11

0.1µ

F

C12

0.01

µF

+

U4

MC1

4536

B

30k

U3

LF39

8

OUTP

UT

NC

2 3

8 4

3

21

OFFS

ET

INPU

T

8LO

GIC

7

CLK

1

46

5

7

6 5

R7

10Ω

R9

30.1

k

R11

100k

R14

10k

LT10

29CZ

R12 1k

R15

100k Q2

2N

3904

R13

10k

R10

30.1

k

R16

100k

R17

100k

+

5VRE

F

– +

LOGI

C RE

FERE

NCE– +

– +

*

SOLD

ER T

O GR

OUND

PLA

NE F

OR H

EAT

DISS

IPAT

ION

**

B1 IS

A N

iCd

3-CE

LL P

ANAS

ONIC

PI4

0-SC

R

†PA

NASO

NIC

ECQV

1HIO

SJL

– +

U2B

LT10

13U2

A

BOOS

T

SENS

E

GND

5VRE

F

V DD

MON

O IN

OSC

INH

DECO

DE D C B A

SET

RESE

T

IN1

OUT1

OUT2

8-BY

PASS

CLK

INH

V SS

Application Note 68

AN68-22

BAT1*

GND

SW

BOOST

GND

OVP

SENSE

GND

GND

GND

VCC2

VCC1

PROG

VC

BAT

GND

GND

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

C1 0.22µF

C3 0.1µF

C4 1µF

L1 30µH

CR3 1N914

CR2 1N5819

CR1 1N5819

VIN

C5 22µF 25V

R2** 10k

R4 1k

R1 162k 0.25%

R3 210k 0.25%

R5 300Ω

R6 6.19k

AN68 F16 * PANASONIC LSC-214P ** KETEMA THERMISTOR MSC103K THERMISTOR IS THERMALLY CONNECTED TO BATTERY BAT1

LT1510

+

C2 10µF

BATTERY TEMPERATURE (°C)0 5 15 25 35 45

BATT

ERY

VOLT

AGE

(V)

4.70

4.65

4.60

4.55

4.50

4.45

4.40

10 20 30 40

AN68 F17

50

MAX

MIN

LT1510 CHARGER VOLTAGE

Figure 17. Output Voltage vs Temperature of SLACharger and MIN/MAX Float Voltage vs Temperature(as Recommended by Panasonic)

Fast Charge: The circuit in Figure 18 is a fast SLA batterycharger. It is based on the standard SLA battery chargercircuit in Figure 16. When the charge current is high, theconstant-voltage level increases from 4.5V to 5V. At aconstant voltage of 5V, the battery reaches a full chargestate faster than at a constant voltage of 4.5V.

R9’s value programs the constant current of the LT1510 to0.8A. R1, R2 and R3 program the constant voltage to 4.5V.U2, an open-collector voltage comparator, is at low statewhen the voltage across the internal LT1510 sense resis-

tor is higher than the voltage across R7 or equivalent to thecharge current being above 0.4A. As long as U1’s outputis low, the charging voltage is boosted to 5V by changingthe OVP voltage divider ratio by switching R4 in parallelwith R3.

Charging Li-Ion Batteries

Li-Ion batteries are charged with a constant-voltage/con-stant-current charger. Constant current is supplied untilthe output voltage reaches 4.1V or 4.2V per cell (depend-ing on the manufacturer) followed by constant-voltagecharging with required accuracy of ±50mV per cell. Thecharging current then tapers down naturally.

IMIN + Timer Termination: To maximize battery cycle life,several lithium-ion battery manufacturers recommendtermination of constant-voltage float mode 30 to 90 min-utes after charge current has dropped below a specifiedthreshold level, IMIN. The float voltage is 4.1V or 4.2V percell and the charge current threshold level is typically50mA to 100mA. Check with the battery manufacturerfor details.

Figure 19 shows a constant-current, constant-voltagecharger with IMIN + 30-minute termination. When theLT1510 is charging, U2 compares the voltage across theLT1510 internal 0.2Ω sense resistor to the voltage across

Figure 16. Temperature Compensated Standard Sealed-Lead-Acid Battery Charger

Application Note 68

AN68-23

BAT1*

VCC2

SW

BOOST

GND

OVP

SENSE

VCC1

PROGC2 0.22µF

C5 22µF 25V

C1 10µF

C6 0.1µF

L1 30µH

CR2 1N914

CR1 1N5819

8V TO 15V

1,7,8,9, 10,16**

R2† 10k

R4 825k

R3 210k 0.25%

2

3

1

7

4

8

R1 162k 0.25%

R10 300Ω

R9 6.19k

R11 1k

R8 1M

AN68 F18

* PANASONIC LSC-214P ** SOLDER TO GROUND PLANE FOR HEAT DISSIPATION † KETEMA MSC103K IS THERMALLY CONNECTED TO BAT1

CR4 1N914

U1 LT1510

C3 1µF

C4 0.1µF

CR3 1N5819

GND

R7 15k

R6 15k

–

+U2

LT1011

C7 0.1µFVC

BAT

+

R7. When the voltage across the sense resistor is lowerthan the voltage across R7 (or, alternatively, when thecharge current drops below 75mA), U2’s output voltagedrops. When U2’s output voltage drops, U2 latches at lowstate via CR5. U2’s output is connected to the RESET pinof U3; when switched, this signal releases the reset (activehigh) of U3. U3, a timer, starts clocking and 30 minuteslater its DECODE output (Pin 13) changes to a high state.The DECODE output is connected to the SET input of U3(active high). This signal latches U3’s DECODE pin high.The high at the DECODE pin also terminates the charge bypulling the LT1510’s VC pin down via Q2.

Q3, R14, C8, R15 and CR6 reset U2 and U3 on turn-on.

A secondary termination can be based on total chargetime.

Terminating with a Microprocessor

The LT1510 gives the designer an easy solution for thepower section of a battery charger and also a smooth

transition from constant-current to constant-voltage op-eration. When a sophisticated charging regimen is re-quired, connecting a dedicated or system microprocessorto the charger is the solution of choice.

NiCd or NiMH Charger: The charger in Figure 20 has twocharge rates that depend on the HI_CHARGE signal andare programmed by R1 and R7 (see the ComponentSelection section). The microprocessor reads the batteryvoltage by clocking U2, a serial data ADC. C7 smoothes theADC input, but averaging a number of ADC readings isrecommended. The voltage divider R4/R5/R6 divides thevoltage at the BAT pin for both the ADC and the OVP pin.The LT1510 is programmed to 5V in constant-voltagemode. The microprocessor can terminate charge basedon –∆V, zero ∆V or dV/dt. After termination, the lowcharge can serve as trickle for NiCd type batteries; thecharger may have to shut down for NiMH cells. Check thebattery manufacturer’s specifications.

Figure 18. Fast, Temperature Compensated SLA Charger

Application Note 68

AN68-24

Figu

re 1

9. L

i-Ion

I MIN

+ T

imer

Cha

rger

BAT1

* 2-

CELL

Li

-Ion

SW

BOOS

T

GND

OVP

SENS

E

V CC1

V CC2

C1

0.22

µF

C5

22µF

25

V

C8

22µF

C6

0.1µ

FC2

10

µF

C9

0.1µ

F

L1

30µH

CR2

1N41

48

11V

TO

15V

1,7,

8,9,

10,

16**

R6

2.2k

R8

560k

R7

2.2k

R9

430k

R2

300ΩR1

4.

12k

R3

1k

R4

11.3

k 0.

25%

R5

4.87

k 0.

25%

AN68

F19

Q1

VN22

22

Q3

2N39

04

Q2

2N39

04

* M

OLI E

NERG

Y IC

R-18

650

** S

OLDE

R TO

GRO

UND

PLAN

E FO

R HE

AT D

ISSI

PATI

ON

CR5

1N41

48

U1

LT15

10

C3, 1

µF

C4, 0

.1µF

V C

CR1

1N58

19

GND

R15

100k

CR6

1N41

48

PROG

V C

BAT

3 2

1

7

4

8

– +

U2

LT10

11

CR3

1N58

19

R14

100k

R10

10k

+

1 2 3 4 5 6 7

15

14

13

12

11

10

9

R13

10k

R11,

10k

V C

R12

4.64

kC7

0.

01µF

U3

MC1

4536

B

816

V DD

SET

RESE

T

IN1

OUT1

OUT2

8-BY

PASS

CLK

INH V S

SMON

O IN

OSC

INH

DECO

DE D C B A

CR4

1N41

48

+

Application Note 68

AN68-25

Where (N)10 is the decimal value of the data entered intothe PWM register of the microprocessor and 28 is themaximum decimal value of an 8-bit register representingD = 1.

BAT1*

VCC2 VCC1

PROG

C2 0.22µF

C1 10µF 25V

C7 0.1µF

C6 0.1µF

L1 30µH

CR3 1N4148

12V TO 24V

1,7,8,9, 10,16**

R5 4.99k

R6 4.99k

R7 7.87k

R4 1k

R2 300Ω

R1 71.5k

R3 1k

AN68 F20

Q1 VN2222

* 3-CELL NiCd OR NiMH ** CONNECT TO GROUND PLANE FOR THERMAL DISSIPATION

U1 LT1510

C3 1µF

C4 0.1µF

CR1 1N5819

CR2 1N5819

GND

R9 100Ω

R8 100k

VC

BAT

OVP

+

SW

GND

BOOST

SENSE

C5 22µF 25V

+

C8 1µF 10V

+

Q2 VN2222

Q3 VN2222

U3 LT1086-5CT

VIN VOUT

GND

VREF VCC VCC

+IN

–IN

CS/ SHDN

CLK

NO_CHARGE

HI_CHARGE

I/O PORTS

U4 MICRO-

PROCESSOR

DOUT

U2 LTC1096

Figure 20. NiCd or NiMH Microprocessor Controlled Charger

Controlling the LT1510 Charger with a Microprocessor

PWM Charge Current Control: Figure 21 shows how tocontrol the charge current with the PWM output of themicrocontroller. The constant-current charge can be cal-culated from:

ID

RCONSTPROG

=( )( )( )

+

2 465 2000

300

.

Ω(33)

Where RPROG is the value of the programming resistor andD is the duty cycle of the PWM signal. The maximum valueof D is 1. D can be calculated from:

DN

=( )10

82(34)

LT1510

PROGIPROG

RPROG

R1 300Ω

FROM PWM OUTPUT OF MCU

Q1 VN2222

AN68 F21

C1 1µF

Figure 21. PWM Charge Current Control

Application Note 68

AN68-26

Parallel and Serial Control: There are many ways tocontrol the constant current and constant voltage of thecharger. Some of them are described here.

In the circuit in Figure 22, U2, U3 and Q1 form a micropro-cessor-controlled current sink. The data on the micropro-cessor parallel bus controls the voltage at the OUT1 pin ofU2. U3 regulates the voltage across R2 to equal that at theOUT1 pin of U2. The current through R2 flows out of the

PROG pin of the LT1510 and thus the charge current iscontrolled by the microprocessor.

In the circuit in Figure 23, U2, U3 and Q1 form a micropro-cessor-controlled current sink. The data on the micropro-cessor serial bus controls the voltage at the OUT1 pin ofU2. U3 regulates the voltage across R2 to equal that at theOUT1 pin of U2. The current through R2 flows out of thePROG pin of the LT1510 and thus the charge current iscontrolled by the microprocessor.

Figure 22. 12-Bit, Parallel Loading Microprocessor Charge Current Control

U1 LT1510

PROG

Q1 2N3904

R2 1.65k

AN68 F22

R1 300Ω

C1 1µF

C2 33pF

–

+

–1.25VREF

MICROPROCESSOR

U3 LT1097

U2 LTC7541A

VREF RFBOUT1

OUT2GND

U1 LT1510

PROG

Q1 2N3904

R2 1.65k

AN68 F23

R1 300Ω

C1 1µF

C2 33pF

–

+

–1.25VREF

MICROPROCESSOR

U3 LT1097

U2 LTC7543

VREFSTB1

SRI

LD1

RFBOUT1

CLOCK

DATA

LOADOUT2

GND

Figure 23. 12-Bit Serial Interface Microprocessor Charge Current Control

Application Note 68

AN68-27

Figure 24 shows a circuit to control the constant-voltageoutput of an LT1510-based battery charger. U2, R1, andR2 invert the polarity of the battery voltage. U3 and U4 actas a voltage divider and also change the polarity of thevoltage back to positive. The divided voltage is fed to theOVP pin of the LT1510.

U3 can be LTC7541A for parallel data interface with themicroprocessor or LTC7543 for serial data interface withthe microprocessor. The programmed voltage VCONST canbe calculated from the following:

VN

CONST = ( )2 465 212

10

. (35)

where (N)10 is the decimal value of the microprocessorbus data and 212 is the maximum data value based on12-bit data.

CONCLUSION

The LT1510 is a high efficiency charger building block thatrelieves the designer of the burdens of switcher design,heat sinking, and even power-transistor and sense resis-tor selection. In some cases, the LT1510 and a few passiveparts are all that is necessary to build a high efficiencybattery charger. Its high accuracy constant-voltage andconstant-current features make the LT1510 an excellentchoice for Li-Ion, NiCd, NiMH and SLA charging. Itscontrol over all charging parameters makes the LT1510 aneasy device with which to design.

U1 LT1510

BAT

OVP

AN68 F24

R1 200k

R2 200k

B1C1 33pF

–

+–

+

U2 LT1097

U3 LTC7541A/LTC7543*

VREF RFBOUT1

OUT2GND

U4 LT1097

SERIAL OR PARALLEL BUS

MICROPROCESSOR

* LTC7541A FOR PARALLEL INTERFACE OR LTC7543 FOR SERIAL INTERFACE

Figure 24. Microprocessor Voltage Control

Application Note 68

AN68-28

APPENDIX A: TEST RESULTS

Testing a statistically significant number of batteries andcharge/discharge cycles is essential for validating a chargerdesign. The circuits in the body of this document weretested and the results are presented in this appendix.

Output Current Boost Configuration Test

The purpose of this test is to see how stable the chargecurrent is with a dynamic load. The 0.5A charger circuit inFigure 11 was connected to the constant-current load(Figure B2), which was adjusted to 0.5A and connected toa function generator as shown in Figure A1. The functiongenerator switched the load between 0A and 0.5A at 100Hzwith a 50% duty cycle. The battery charge current wasmonitored with an oscilloscope across a 0.1Ω senseresistor connected in series with the battery.

The test results are shown in Figure A2. The battery chargecurrent is affected by the dynamic load only at the transi-

tion times; otherwise it is regulated at 0.5A. The LT1510control loop corrects the charge current at a rate of 0.5Aper 1ms. If the load changes at a slower rate, it will notaffect the charge current.

0mV

AN68 FA2

3-CELL NiCd

SW

BOOST

VCC

0.22µF

C1 22µF

C2 1µF

10µF

0.1µF

TO O’SCOPE

33µH1N9141N5819

VIN

(+)

(+)

(–)

(–)

CONTROL

1k

R4 300Ω RSENSE

0.1Ω

3

2

4

7

6Q1 2N3904

VIN

RS 0.1Ω

AN68 FA1

U1 LT1510

R2 10k

1N5819

GND

–

+U2

LT1006

VC

PROG

SENSE

GND

RPROG 10k

R3 200Ω

BAT

FUNCTION GENERATOR

CONSTANT- CURRENT LOAD

ADJUSTED TO 0.5A (SEE FIGURE B2)

AMPLITUDE: 0V TO 10V FREQUENCY: 100Hz

R1 10k

+

Figure A1. Battery Charging Current System Test Circuit

Figure A2. Battery Charge Current of Current Boost Systemwith Dynamic Load

Application Note 68

AN68-29

dT/dt Termination Test

The purpose of this test is to establish that the chargercircuit in Figure A3 is reliable and terminates consistently.The circuit was connected to a test system that dischargedthe battery to the same level after termination and reacti-vated the charger circuit. Data from nineteen charge/discharge cycles was collected. The test conditions aregiven in Table A1.

Table A1

Battery 3 NiCd Cells of PanasonicP130-SCR in Series (1.3AH)

Constant Charge Current 1 A

Discharge Current 1 A

Minimum dT/dt Slope That Will 0.5°C/minuteTrigger Termination

Required Duration of the Above 20 secondsSlope to Trigger Termination

The test results are presented below. Figure A4 shows atypical battery voltage during one charge/discharge cycle.The data collected was analyzed and presented in TableA2.

TIME (HR)0

BATT

ERY

VOLT

AGE

(V)

2:30 3:00

4.8

4.6

4.4

4.2

4.0

3.8

3.6

3.4

3.2

3.0

AN68 FA4

1:000:30 1:30 2:00 3:30

Figure A4. Typical Battery Voltage at Charge/DischargeCycle of NiCd Charger with dT/dt Termination

–

+

–

+

CR5 1N5819

CR1 1N4148

CR2 1N4148

Q2 2N3904

Q1 2N3904

U1C

U1B10

6

5

3

9

8

7

12

11

10

9

8

16VDD

Q14 RESET

CLK

OUT1

OUT2VSS

2

3

1

4

11

CR3 1N5819

CR4 1N4148

AN68 F13

C9 10µF

C11 22µF 25V

C5 0.1µF

L1 30µH

BAT1**1,7,8,9,10,16††

C7 0.22µF

C6 0.1µF

C10 0.1µF

C8 1µF

C3 22µF

C4 0.1µF

C2 0.1µF

C1† 4.7µF

R19 300Ω

R10 100k

R9 100k

R7 100k

R6 100k

VRST

VRST

13

12

14

VBIAS

VBIAS

VBIAS

12V

12V

VBIAS

R20 4.99k

R11 100k

R8 20k

R5 10M

R4 10MRT*

R21 1k

R12 10k

R17 10k

R16 10k100k

R18 10k

R3 100k

R13 10k

R15 100Ω

R14 7.5k

R2 100k

R1 7.5k

SW

BOOST

GND

SENSE

VCC

PROG

VC

U3 LT1510

GNDBAT

* KETEMA MSC103K. THERMALLY CONNECTED TO B1 ** PANASONIC 3-CELL NiCd P130-SCR † CERAMIC CAPACITOR †† SOLDER TO GROUND PLANE FOR HEAT DISSIPATION

+

U1A LT1014

U2 CD4060

–

+U1D

+

–

+

Figure A3. LT1510 NiCd Charger with dT/dt Termination

Application Note 68

AN68-30

Table A2

Average Charge Time 1:20:22 hours

Standard Deviation of Charge Time 0:01:00 hours (1 minute)

Average Discharge Time 1:19:25 hours

Standard Deviation of Discharge Time 0:00:58 hours (58 seconds)

The consistent discharge time (58 seconds standard de-viation with constant-current load) proves that the chargetermination is repeatable and that the LT1510 chargerperforms well. Also, because there is no “top-off” or tricklecharge, the charge efficiency (discharge ampere-hoursover charge ampere-hours) is close to 100%.

NiCd –∆V Termination Test

The purpose of this test is to establish that the chargercircuit in Figure A5 is reliable and terminates consistently.The circuit was connected to a test system that dischargedthe battery to the same level after termination and reacti-vated the charger circuit. Data from 68 charge/dischargecycles was collected. Test conditions are given in TableA3.

Table A3

Battery 3 NiCd Cells of PanasonicP140-SCR in Series (1.4AH)

Constant Charge Current 0.8A

Discharge Current 0.8A

–∆V That Will Trigger Termination 4.5mV

End of Discharge Voltage 2.7V

The test results are presented below. Figure A6 showstypical battery voltage during charge/discharge cycle. Thedata collected was analyzed and presented in Table A4.

Table A4


Standard Deviation of Charge Time 0:05:37 hours


Standard Deviation of Discharge Time 0:00:48 hours (48 seconds)

The consistent discharge time (48 seconds standard de-viation with constant-current load) proves that the chargetermination is repeatable and that the LT1510 charger

performs well. Also, because there is no “top-off” or tricklecharge, the charge efficiency (discharge ampere-hoursover charge ampere-hours) is close to 100%.

SLA Fast Charge Test

The purpose of this test is to establish that the SLA fastcharge circuit in Figure A7 is reliable and terminatesconsistently. The circuit was connected to a test systemthat, after about four hours of charging, discharged thebattery to the same level every cycle and then reactivatedthe charger circuit. Data from seventeen charge/dischargecycles was collected. Test conditions are presented inTable A5.

Table A5

Battery 2-Cell SLA PanasonicLSC-214P

Constant Charging Voltage 4.5V

Boosted Constant Charging Voltage (Until 5VCharge Current Drops Below 0.4A)


Average Charge Time 3:53 hours


Discharge Current 1A

End of Discharge Voltage 3.6V

The test results are presented below. Figure A8 showstypical current during the charge and discharge (positiveand negative, respectively) of one cycle. The data collectedwas analyzed and is presented in Table A6.

Table A6


Standard Deviation of Discharge Time 0:02:06 hours

The consistent discharge time (standard deviation is 2minutes) proves that the charger and regimen are reliable.Charge current of 0.8A and charge level of 1.5AH (dis-charge time of 1.5 hours at 1A) are conservative for a2.1AH battery. To reach full charge at the required fourhours, a higher charge current battery can be used, or acurrent threshold lower than 0.4A can be programmed.

Application Note 68

AN68-31

CR2

1N58

19

CR1

1N91

4

AN68

F15

C5

22µF

25

VC6

0.

1µF

L1

30µH

B1**

*1,7

,8,9

, 1

0,16

1 2 3 4 5 6 7 8

16

15

14

13

12

11

10

9

C1

0.22

µF

12V

C4

0.1µ

F

C3

1µF

R1

300Ω

R6

100k

R8

100k

R4

10k

5VRE

F

R5

100k

R2

6.19

k

R21

100k

R22

100k

R20

100k

Q1

2N39

04

CLK

R23

100k

R3

1k

SW

V CC1

V CC2

PROG

V C

BAT

U1

LT15

10

GND

C2

10µF

CR3

1N58

19C7

0.

1µF

C8†

1µF

HOLD

CA

PACI

TOR

150Ω

C9

0.1µ

F

C10

22µF

C11

0.1µ

F

C12

0.01

µF

+

U4

MC1

4536

B

30k

U3

LF39

8

OUTP

UT

NC

2 3

8 4

3

21

OFFS

ET

INPU

T

8LO

GIC

7

CLK

1

46

5

7

6 5

R7

10Ω

R9

30.1

k

R11

100k

R14

10k

LT10

29CZ

R12 1k

R15

100k Q2

2N

3904

R13

10k

R10

30.1

k

R16

100k

R17

100k

+

5VRE

F

– +

LOGI

C RE

FERE

NCE– +

– +

*

SOLD

ER T

O GR

OUND

PLA

NE F

OR H

EAT

DISS

IPAT

ION

**

B1 IS

A N

iCd

3-CE

LL P

ANAS

ONIC

PI4

0-SC

R

†PA

NASO

NIC

ECQV

1HIO

SJL

– +

U2B

LT10

13U2

A

BOOS

T

SENS

E

GND

5VRE

F

V DD

MON

O IN

OSC

INH

DECO

DE D C B A

SET

RESE

T

IN1

OUT1

OUT2

8-BY

PASS

CLK

INH

V SS

Figu

re A

5. L

T151

0 Ni

Cd C

harg

er w

ith –

∆V T

erm

inat

ion

Application Note 68

AN68-32

TIME (HR)0

BATT

ERY

VOLT

AGE

(V)

1 2 3 4

AN68 FA6

5

5.0

4.8

4.6

4.4

4.2

4.0

3.8

3.6

3.4

3.2

3.06

Figure A8. Typical Battery Current at Charge/DischargeCycle of SLA Fast Charger

TIME (HR)0

BATT

ERY

CHAR

GE

CURR

ENT

(A)

BATT

ERY

DISC

HARG

E CU

RREN

T (A

)

1.0

0.5

0

–0.5

–1.0

–1.51 2 3 4

AN68 FA8

5 6

Figure A6. Typical Battery Voltage During Charge/DischargeCycle of NiCd Charger with –∆T Termination

Figure A7. Fast, Temperature Compensated SLA Charger

BAT1*

VCC2

SW

BOOST

GND

OVP

SENSE

VCC1

PROGC2 0.22µF

C5 22µF 25V

C1 10µF

C6 0.1µF

L1 30µH

CR2 1N914

CR1 1N5819

8V TO 15V

1,7,8,9, 10,16**

R2† 10k

R4 825k

R3 210k 0.25%

2

3

1

7

4

8

R1 162k 0.25%

R10 300Ω

R9 6.19k

R11 1k

R8 1M

AN68 F18

* PANASONIC LSC-214P ** SOLDER TO GROUND PLANE FOR HEAT DISSIPATION † KETEMA MSC103K IS THERMALLY CONNECTED TO BAT1

CR4 1N914

U1 LT1510

C3 1µF

C4 0.1µF

CR3 1N5819

GND

R7 15k

R6 15k

–

+U2

LT1011

C7 0.1µFVC

BAT

+

Application Note 68

AN68-33

Li-Ion with Time-Out Termination Test

The purpose of this test is to establish that the chargercircuit in Figure A9 is reliable and terminates consistently.The circuit was connected to a test system that, aftertermination, discharged the battery to the same level andreactivated the charger circuit. Data from thirty-two charge/discharge cycles was collected. Other test conditions aregiven in Table A7.

Table A7

Battery 2 Li-Ion Cells of Moli EnergyICR18650 in Series

Constant Voltage 8.2V


Discharge Current 1A

Charge Current That Will Trigger Timer <100mA

Timer Duration 30 minutes

The test results are presented below. Figure A10 shows atypical charge/discharge battery voltage. The test data isanalyzed and given in Table A8.

Table A8




Standard Deviation of Discharge Time 0:00:40 hours

The consistent discharge time (40 seconds standard de-viation with 1A constant-current load) proves that thecharge termination is reliable and that the LT1510 is agood solution for Li-Ion charging.

Application Note 68

AN68-34

Figu

re A

9. L

i-Ion

Cha

rger

with

Tim

e-Ou

t Ter

min

atio

n

BAT1

* 2-

CELL

Li

-Ion

SW

BOOS

T

GND

OVP

SENS

E

V CC1

V CC2

C1

0.22

µF

C5

22µF

25

V

C8

22µF

C6

0.1µ

FC2

10

µF

C9

0.1µ

F

L1

30µH

CR2

1N41

48

11V

TO

15V

1,7,

8,9,

10,

16**

R6

2.2k

R8

560k

R7

2.2k

R9

430k

R2

300ΩR1

4.

12k

R3

1k

R4

11.3

k 0.

25%

R5

4.87

k 0.

25%

AN68

F19

Q1

VN22

22

Q3

2N39

04

Q2

2N39

04

* M

OLI E

NERG

Y IC

R-18

650

** S

OLDE

R TO

GRO

UND

PLAN

E FO

R HE

AT D

ISSI

PATI

ON

CR5

1N41

48

U1

LT15

10

C3, 1

µF

C4, 0

.1µF

V C

CR1

1N58

19

GND

R15

100k

CR6

1N41

48

PROG

V C

BAT

3 2

1

7

4

8

– +

U2

LT10

11

CR3

1N58

19

R14

100k

R10

10k

+

1 2 3 4 5 6 7

15

14

13

12

11

10

9

R13

10k

R11,

10k

V C

R12

4.64

kC7

0.

01µF

U3

MC1

4536

B

816

V DD

SET

RESE

T

IN1

OUT1

OUT2

8-BY

PASS

CLK

INH V S

SMON

O IN

OSC

INH

DECO

DE D C B A

CR4

1N41

48

+

Application Note 68

AN68-35

TIME (HR)0 0.5

BATT

ERY

VOLT

AGE

(V)

2.0 2.5

8.5

8.0

7.5

7.0

6.5

6.0

5.5

5.0

AN68 FA10

1.0 1.5 4.0 4.53.0 3.5

Figure A10. Typical Li-Ion Battery Voltage at Charge/Dischargewith Time-Out Termination

APPENDIX B: AUXILIARY CIRCUITS FOR TESTINGBATTERIES AND CHARGERS

While working on battery charger design and testing, two“home-brewed” circuits were frequently used: a batterysimulator and a controlled constant-current load. Theformer was used for checking the operation of the chargerand the latter for charge/discharge tests. A variety oftimers, charge/discharge controllers and the like wereconstructed as the need arose.

The Battery Simulator

There are two advantages to using the battery simulator(Figure B1) over a battery in board tests. In case of anaccidental short, the power supply has current limiting,whereas a shorted battery can conduct more than 20

Figure B1. Battery Simulator

amperes and vaporize everything, including traces. Also,the simulator voltage is static and controllable, whichmakes it easy for testing (efficiency, for instance).

The user has to adjust the power supply to the desiredvoltage and connect the positive and negative “batteryinput” terminals of the simulator in place of the battery.

In discharge mode, the battery simulator uses the currentlimited lab power supply PS1 as the source and thesimulator circuit is inactive. In charge mode, current isforced through the battery input terminals. The low volt-age that develops across R8 is amplified by U1 and causesQ1 to shunt the charge current while maintaining the

+

–

–

+BATTERY

INPUT

+

R10 0.1Ω 2W WW

R1 1Ω 2W WW

R9 200Ω 2W WW

PS1 ADJUSTABLE LAB POWER SUPPLY WITH CURRENT LIMIT

R7 10Ω

8

1

3 3SW1

SW2 GND

SENSE

VIN

ILIM8

4 5

1

2

6 7

NC NC

2

4C5 4700µF ELECT

C2 0.1µF

C1 0.1µF

+ C4 47µF ELECT

+ C3 47µF ELECT

CR1 1N5818

R3 100k

NOTE: UNLESS OTHERWISE SPECIFIED: CAPACITORS ARE 25V, CERAMIC

R2 1k U1

LT1078Q1

BUZ11

L1 120µH

R5 1k

R6 1k

R8 0.1Ω 2W WW

R4 10k

U2 LT1073-12

AN58 B1

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

Application Note 68

AN68-36an68f LT/GP 1296 5K • PRINTED IN USA

LINEAR TECHNOLOGY CORPORATION 1996

power supply PS1 voltage. U2, L1, CR1, C3 and C4produce housekeeping 12V that is required to operate U1and drive Q1. The power supply range is 1.5V to 15V. Q1requires a heat sink. R1 is used for measuring the chargecurrent. R10 and C5 simulate the AC characteristics of thebattery.

Controlled Constant-Current Load

Although a constant-power load is a more realistic load,the constant-current load (Figure B2) works better forbattery discharge testing because the discharge timegives immediate data on the battery charge level. All thatneeds to be done is to multiply the current by the dischargetime.

For operation in DIRECT mode, the control input has to beabove 7V. If the control voltage available is less than 7V,

switch to BOOST. The minimum control voltage is then3.3V. The user has to adjust the potentiometer to thedesired current. This can be done by connecting a powersupply or battery at above 2V in series with a current meterto the (+) and (–) terminals and adjusting R4.

When the load is connected to a battery, Q1 and Q2 operatein a negative feedback mode and maintain the VBE of Q1 at0.5V. The voltage across R6 must be between 0.5V and1.5V, depending on the wiper position of R4, a trim pot.Since the value of R6 is 1Ω, this translates to a constantcurrent of 0.5A to 1.5A. Q1 requires a heat sink.

A voltage doubler (U1, C1, C2, C3, C4, CR1, CR2, R1 andR2) boosts the voltage for the gate of Q2.

R3 100k

R4 500Ω R6

1Ω 2W WW

R5 249Ω

Q2 MTP50N06E

Q1 MPS2222

0.5A TO 1.5A LOAD

(–)

(+)2

CONTROL

6

AN68 FB2

1

3

4 5C2

0.1µF

C1 1000pF

CR1 1N914

CR2 1N914

R1 100k

R2 10k

U1 LMC555

C3 1µF

SW1 BBOOST

DIRECTSW1 A

C4 0.1µF

Figure B2. Controlled Constant Current Load

Linear Technology Corporation1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900FAX: (408) 434-0507 TELEX: 499-3977 www.linear-tech.com

Note: Linear Technology would like to thank Arie Ravid forcontributing the majority of the material in this application note.

Documents

AN68 - LT1510 Design Manual · 2018-03-19 · order to design a reliable battery charger. A fast battery charger must quickly recharge a battery to full charge. Fast charging batteries