8/8/2019 A Minature, Low Dropout Battery Charger
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A Minature, Low Dropout Battery Chargerfor Lithium-Ion Batteries
Design Note 239James Herr
09/00/239
Figure 1. Single Cell 500mA Li-Ion Battery Charger
INTRODUCTION
Lithium-ion (Li-Ion) batteries are becoming the powersource of
choice for todays small handheld electronicdevices due to their
light weight and high energy density.However, there are a number of
important issues associ-ated with the charging of these batteries.
As an example,if they are overcharged, they can be potentially
hazardousto the user.
The LTC1731 is a constant-current/constant-voltage lin-ear
charger controller for single cell lithium-ion batteries.Its output
accuracy of1% (max) over the 40C to 85Crange prevents the
possibility of overcharging. The outputfloat potential is
internally set to either 4.1V or 4.2V,without the use of an
expensive external 0.1% resistordivider. Furthermore, the charging
current is user pro-grammable with 7% accuracy.
At the beginning of the charging cycle, if the batteryvoltage is
below 2.457V, the LTC1731 will precharge thebattery with only 10%
of the full-scale current to avoidstressing the depleted battery.
Once the battery voltagereaches 2.457V, normal charging can
commence. Charg-ing is terminated by a user-programmed timer. After
thistimer has completed its cycle, the charging can be re-started
by removing and then reapplying the input voltagesource, or by
shutting down the part momentarily. A built-in end-of-charge (C
/10) comparator indicates that thecharging current has dropped to
10% of the full-scalecurrent. The output of this comparator can
also be used to
stop battery charging before the timer completes its cycle.The
LTC1731 is available in the 8-pin MSOP and SOpackages.
OPERATION AND CIRCUIT DESCRIPTION
Figure 1 shows a detailed schematic of a 500mA singlecell Li-Ion
battery charger using the LTC1731-4.2. Thecharge current is
programmed by the combination of aprogram resistor (RPROG), from
the PROG pin to ground,and a sense resistor (RSENSE), between the
VCC and
SENSE pins. RPROG sets a program current through aninternal,
trimmed 800 resistor, setting up a voltage dropfrom VCC to the
input of the current amplifier (CA). Thecurrent amplifier controls
the gate of an externalP-channel MOSFET (Q1) to force an equal
voltage drop
across RSENSE, which in turn, sets the charge current.When the
potential at the BAT pin approaches the presetfloat voltage, the
voltage amplifier starts sinking current,which decreases the
required voltage drop across RSENSE,thereby reducing the charge
current.
Charging begins when the potential at the VCC pin risesabove
4.1V. At the beginning of the charge cycle, if thebattery voltage
is below 2.457V, the charger goes intotrickle charge mode. The
trickle charge current is 10% ofthe full-scale current. If the
battery voltage stays low forone quarter of the total programmed
charge time, thecharge sequence is terminated.
The charger goes into the fast charge, constant-currentmode
after the voltage on the BAT pin rises above 2.457V.In
constant-current mode, the charge current is set by thecombination
of RSENSE and RPROG. When the battery ap-proaches the final float
voltage, the voltage loop takescontrol and the charge current
begins to decrease. Whenthe current drops to 10% of the full-scale
charge current,
, LTC and LT are registered trademarks of Linear Technology
Corporation.
VCC
VIN5V TO 12V
SENSE
DRV
1k RSENSE
0.21%
RPROG*19.6k1%
Li-IONCELL
LTC1731-4.2
*SHUTDOWN INVOKED BY FLOATING THE PROG PIN
BAT
CHRG
8
7
MBRM120T3
10F
DN239 F01
1F
IBAT = 500mA
CTIMER0.1F
Q1Si9430DY
6
1
5
4
+
2
3TIMER
PROG
GND
