1

PURPOSE:• This module provides information about the Freescale power integrated circuit

switch training.

OBJECTIVES:• Describe the the basic operation of a mechanical and electrical switch.• Describe switch configurations including, Low-Side Switches, High-Side Switches,

Half-Bridge Circuits, and H-Bridge Circuits• List the different protection features and describe what they do.

CONTENTS:• 31 pages• 4 questions

LEARNING TIME:• 40 minutes

Module Introduction

Welcome to this module on Power Integrated Circuit Switch products. First, we will teach you some of the basics, then acquaint you with some of the operational features of Power IC switches. In later modules we will further acquaint you with the products using product profiles, and datasheets.

The basics will start by first acquainting you with some of the various kinds of switches, their function, and the ideal switch. Important practical switch parameters are pointed out as well as some of the more popular switch configurations used in ICs. The various protection features incorporated in analog ICs will be discussed to acquaint you with their function and operation as well as fault modes and their being reported. Finally, we will discuss the popular input control methods used in analog power IC switches followed by a wrap-up showing some of the more generic applications.

2

Common Switches

MechanicalNPN

BipolarPNP

Bipolar

P-ChannelMOSFET N-Channel

MOSFET

ONOFF

Collector

Base

Emitter

Emitter

Base

Collector

Gate

Drain

Source

Parasitic Diode(Body Diode)

Parasitic Diode(Body Diode)

Gate

Drain

Source

There are many different kinds of switches. In general, they can be thought of as either being mechanical or electronic. It would be fairly safe to say everyone has used a mechanical “wall switch” to turn lights ON in their homes. The most common switches are symbolically shown here.

In the electronic switch category semiconductor devices are made up of NPN and PNP bipolar transistors as well as N and P-Channel Field Effect Transistors (or simply FETs ). All of these device structures are used in various combinations in ICs. When a FET structure is used in an IC, it is, more descriptively referred to as a Metal Oxide Silicon Field Effect Transistor, or more simply, a MOSFET. MOSFET switch structures are used in very large numbers in ICs, and bipolar transistors to a lesser extent. The reason being, bipolar transistors use current as a means of control whereas MOSFET transistors use voltage as a means of control and current conservation in ICs is of paramount importance. The need for MOSFET devices has greatly increased in recent years as a result of the market needing lower and lower current devices brought about as a result of smaller and smaller, yet more and more complicated, battery powered portable products.

NPN bipolar transistors increase their conduction or turn-ON with small positive voltage increases imposed on the Base relative to the Emitter, causing relatively small Base-to-Emitter currents to flow, allowing large amounts of Collector-to-Emitter currents to flow. PNP bipolar transistors turn-ON in a similar manner but with small negative voltage increases imposed on the Base relative to the Emitter. To turn-OFF the Collector-to-Emitter current, the Base-to-Emitter voltage is reduced to zero.

P and N-Channel MOSFET devices use an electric field principle as a means of control and in so doing consume very small amounts of control current. Relatively small decreases in the Gate voltage of a P-Channel MOSFET will cause very large amounts of Source-to-Drain currents to flow. Increases in Gate voltage will turn-OFF the P-Channel Source-to-Drain current. The N-Channel turns-ON with an increase in Gate voltage and turns-OFF with a decrease in Gate voltage. This is just the opposite of that required for the P-Channel and the P and N-Channel devices can be thought of as “mirror opposites” of each other in control.

NPN and PNP transistors might also be thought of as “mirror opposites” of each other in regards to control voltage polarity. N and P-Channel MOSFET transistors, and NPN and PNP transistors can be referred to as respective compliments of each other.

3

Basic Switches (Mechanical)

LOAD

ONOFF

Control

+ VPWR

ONOFF

LOAD

Control

+VPWR

The Mechanical electrical switch element shown here is the most common and most easily understood. Everyone is familiar with the “inner workings” of the mechanical switch element and for that reason the discussion vehicle around which the basics will be reviewed. The basics apply to all switch varieties.

Though it is easily understood that switches connect or disconnect parts of a circuit to control currents, it is another thing to understand in detail what is happening when currents are turned-ON or turned-OFF. Let’s learn a little more about switches.

4

Switch Basics (Function, Operation, and Design Traits)

ON

LOAD

+VPWR

Control

OFF-Resistance(Very Large)

OFF

ON-Resistance(Very Small)

LeakageCurrent

Path

An electrical switch is used to connect or disconnect two points of a system so as to accomplish a goal or make something happen or not happen. Being more specific, the purpose of a switch is to control electrical currents. Switch manufactures diligently strive to make switches electrically more efficient. Every effort is made to minimize the switch’s ON-resistance to current flow, when the switch is CLOSED. Another important trait of an efficient switch is that it exhibit an extremely high resistance to current flow when OPEN (or OFF) and be able to accommodate a large voltage across the switch without breaking down.

When a switch is turned-ON and large currents are made to flow through the switch, the switch will heat up. The heat is generated as a result of the current flowing through the inherent ON-resistance of the switch. The heat can be calculated in theory by applying Ohm’s Law and multiplying the voltage drop across the switch ‘squared’ times the resistance through the switch (Remember the equation P = I2R from basic electrical classes?). In reality, the inherent ON-resistance is hard to determine by direct measurement as a result of it’s temperature dependency. In practice, it is better to measure the voltage drop across the switch and the current flowing through the switch directly and then apply Ohm’s Law by multiplying the voltage across the switch times the current through the switch (Again, do you remember the equation P = IE ?). Both of these power calculations produce the same end result. Heating represents lost system energy and is sought to be minimize by making the switch ON-resistance as low as economically possible.

When a switch is turned-OFF, no current should flow. In practice, a very minute amount of current will flow “around” the switch as a result of leakage current paths having very high resistances. The leakage current again gives rise to heating and represents lost system energy which can again be calculated by applying Ohm’s Law by multiplying the voltage applied across the OPEN switch times the leakage current. Good switches will exhibit many meg-Ohms of OFF-resistance.

5

The Ideal Switch

ON

LOAD

+ VPW R

C on tr ol

O FF

ON -R esis tance

O FF -R esistance(infinity)

(None)

An ideal switch is capable of switching an infinite amount of current, exhibit zero resistance to current flow when CLOSED (or ON) and present infinite resistance when OPEN (or OFF) and be able to accommodate, or stand-off, an infinite voltage. Ideal switches do not occur in real life and for that reason are engineered to meet various parametric requirements.

6

Important Practical Switch Parameters

• Some primary parameters of importance for practical switches are:

1. Switch ON Current Capability • Maximum operating current

2. Switch OFF Voltage Capability • Maximum operating voltage

3. Switch OFF Leakage Current • Maximum switch OFF leakage current with maximum operating voltage applied across the switch.

4. Switch ON Resistance • Maximum resistance across the switch with maximum operating current flowing through the

switch.

5. Switching Speed • Switch transition time.

6. Switch Propagation Delay Time • Switch input stimulus to output response (results) time.

• All switch parameters are influenced by temperature and the parametric temperature is always specified on datasheets.

DC switch parameters of prime importance are maximum operating current, maximum operating voltage, switch-OFF leakage current specified with maximum operating voltage applied, and the switch-ON resistance with maximum operating current flowing.

Dynamic switch parameters of prime importance are Switching Speed and Switch Propagation Delay Time.

Switching Speed is a measure of how fast the switch can transition from ON to OFF or from OFF to ON and is commonly referred to as output rise and fall times. The internal circuitry of IC switches, acting in conjunction with external circuitry greatly influences rise and fall times. Datasheets always specify the values of any external components and conditions having an effect on rise and fall time parametrics. IC switches are routinely characterized for this parametric.

Propagation Delay Time is a time duration measurement from switch input stimulus time to output response (or results) time. Here again, external circuit components have a great influence on Propagation Delay Time measurements and external circuit conditions are always specified on datasheets.

7

Question

What are the correct resistance levels for the ON-Resistance and the OFF-Resistance? Click on your choice.

a) Very small ON-Resistance, very small OFF-Resistance.b) Very small ON-Resistance, very large OFF-Resistance.c) Very large ON-Resistance, very small OFF-Resistance.d) Very large ON-Resistance, very large OFF-Resistance.

Here’s a question on switching basics.

Correct:

The ON-Resistance should be very small while the OFF-Resistance should be very large.

8

Low-Side Switch (LSS)

OFF

LOAD

ONControl

+VPWR

ONOFF

LOAD

Control

+VPWR

OFF

ONControl

LOAD

+ VPWR

NPNBipolar

N-ChannelMOSFET

Mechanical

Parasitic Diode(Body Diode)

E

BC

G

S

D

Low-Side Switch circuits derive their name from the fact that they all have their switching elements connected to ground or to the most negative voltage of the circuit, thus the name LOW SIDE SWITCH. The load is always located between the positive power supply and the switch element and is always connected to the positive power supply. In some systems, having the load always connected to the power supply can present a safety problem.

N-Channel Low-Side switches are the most common since they can be operated from a single power supply and the Gate voltage needs only to be pulled up (or positive) for turn-ON much like the NPN transistor. Caution needs be exercised so as to not exceed the maximum allowable Gate-to-Source voltage to prevent damage to the device. To turn OFF an N-Channel Switch the Gate is merely pulled down to ground. The diode shown across the N-Channel is inherent and represents the internal Parasitic Diode of the IC switch. P-Channel Low-Side switches are not commonly used because it is somewhat more difficult to obtain a negative Gate voltage for turn-ON, requires more silicon area to accomplish the same ON-Resistance (or RDS(on)) as an N-Channel, and would as a result, be more costly than an N-Channel.

9

High-Side Switch (HSS)

ON

OFFControl

LOAD

+VPWR

LOAD

ON

OFFControl

+VPWR

LOAD

ONOFF

Control

+VPWR

OFF

ONControl

ChargePump

LOAD

+VPWR

Mechanical PNPBipolar

P-ChannelMOSFET

N-ChannelMOSFET

G

D

S

G

S

D

B

E

C

High-Side Switch circuits derive their name from the fact that they have their switching elements connected to the positive power supply of the circuit, thus the name high-side switch. The load is always located between the switch and ground. Unlike the Low Side Switch, power is completely isolated from the load when the switch is OFF. This avoids the safety issues confronted with in some applications using the Low Side Switches.

Both P-Channel and N-Channel switches are used in High Side Switch applications. Circuit wise, the P-Channel operates analogous to the PNP transistor and is most easily controlled since turn-ON merely requires the Gate to be pulled to ground. Turn OFF is accomplished by pulling the Gate up to the power supply.

N-Channel High Side Switches, on the other hand, require a voltage on the Gate that is in excess of the power supply voltage to attain full turn-ON enhancement. Since the Gate requires a relatively small amount of Gate charging current, an internal Charge Pump circuit is frequently incorporated to “pump-up” or increase the Gate voltage. The small cost adder of incorporating the Charge Pump circuit is offset by savings in using the smaller area N-Channel switch. The cost savings can be put to better use by intentionally increasing the device area to further decrease RDS(on).

10

Complementary Output Switches

Gate

Drain

Source

Parasit ic Diode(Body Diode)

Parasitic Diode(Body Diode)

Gate

Drain

Source

P-Channel

N-Channel

+ VPWR

-- VPWR

+

OFF

ON

--

CWMOTOR

CurrentFlow

+ VPWR

-- VPWR

--

+

MOTOR CCW

OFF

ON

Drain Drain

When a P-Channel High-Side and an N-Channel Low-Side power switch are configured as shown, the circuit can be referred to as a common drain Half-Bridge since the drain terminals of both switches are tied together. The term Half-Bridge will be made clear later when H-Bridge circuits are talked about. More commonly, when the two switch elements require opposite polarities of input voltage for turn ON, the combination is said to be a complementary output switch.

Notice the complementary positive and negative power supplies used in the Half-Bridge circuit shown and that both switches will turn-ON when their respective Gate voltages are moved toward ground and be turned OFF when moved toward their respective power supply sources.

Only one switch is allowed to be turned-ON at a time so as to prevent “current shoot-through” from one power supply to the other. For this to occur, a power supply short circuit would occur with catastrophic results.

The DC Motor is referenced to ground and will rotate in a clock-wise direction when the High-Side switch is ON and in a counter clock-wise direction when the Low-Side switch is ON. This occurs as a result of the applied voltage polarity being reversed across the motor. The choice of direction for a given polarity is arbitrary in this discussion. In reality, motor rotation for a given polarity is dependent on the motor winding design.

The advantage of the complementary output switch is that control circuitry is simple when complementary power supplies are available. With a single polarity power supply, control voltages and load grounds become issues to contend with.

11

Totem-Pole Output Switches

Parasitic Diode(Body Diode)

Gate

Drain

Source

N-Channel

-- VPWR

+ VPWR

+

CW

--

CurrentFlow

MOTOROFF

ON

--

CCW

+

MOTOR

-- VPWR

+ VPWR

ON

OFF

The Totem-Pole Output is made up of two like elements used as high and low-side switches. Operation is much the same as with the complementary output configuration except for the control voltage polarities. Again, only one switch is allowed be turned-ON at a time.

For reference comparison to the complementary output, both positive and negative power supplies are retained. In this case though, both switches require their Gates to be pulled up relative to their individual Source terminals for turn-ON. So as to not exceed the maximum Gate-to-Source voltage rating, two totally separate Gate voltage sources must be incorporated. This makes the input control circuitry slightly more complex.

The DC Motor is referenced to ground, as before, and will operate the same as in the preceding complementary output example (that is it will rotate clock-wise when the high-side switch is ON and counter clock-wise when the low-side switch is ON).

The N-Channel Totem-Pole Output is frequently used in power ICs, even though requiring slightly more complex control circuitry, than the complementary output approach. The savings in silicon area out weighs any disadvantage.

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H-Bridge

+ VPWR

OFF

ON

ON

OFF

+ --MOTOR

ChargePump

ChargePump

Current Flow

+ VPWR

ON

OFF

OFF

ON

+--

ChargePump

MOTOR

ChargePump

Placing the motor between two N-Channel Totem-Pole outputs as shown above creates an H-Bridge circuit configuration. Note the use of a single power supply. This configuration is frequently used in power IC switches designed to drive bi-directional DC motors. The added area of using P-Channel High-Side switches to obtain a specific RDS(on) frequently gives way to designs incorporating a charge pump used in conjunction with High-Side N-Channel switches. To further the scale of economy, the charge pump is frequently multiplexed or shared between the two High-Side N-Channel switches, since only one High-Side switch is permitted to be ON at any one time.

An H-Bridge circuit configuration can also be created by combining two Half-Bridge circuits using complimentary output switch elements. The load would be placed between the two complimentary Half-Bridge circuit outputs.

13

Question

What connections are necessary to turn ON an N-Channel MOSFET configured as a Low-Side Switch? Click on your choice.

a) Gate to +VPWR, Source to ground and Drain to Load.b) Gate to Ground, Source to +VPWR and Drain to Load.c) Gate to +VPWR, Source to Load and Drain to groundd) Gate to Load, Source to Ground and Drain to +VPWR

Here’s a question on low-side switching.

Correct:

If the Gate has +VPWR and the Source is grounded current will flow from the Source to the Drain and provide power to the Load.

14

Protection (Output Clamp)

GND

Drain-to-Source “ON”Voltage (VDS(on))

Drain-to-Source ClampVoltage (VCL)

VPWR

Drain Current(ID)

Drain Voltage

ClampCurrent

Area(IA)

Time

Clamp Energy(EJ = IA x VCL x t)

Dynamic Output Clamping (N-Channel) Waveform Diagram

The Dynamic Output Clamp circuit protects power MOSFET outputs from avalanche breakdown caused when excessive voltages are present at the Output. This condition is typically encountered when switching inductive loads. When the MOSFET is turned-ON (in the left circuit), current flow through the inductor load causes a magnetic field to build-up around the inductor. When the MOSFET is abruptly turned-OFF (in the right circuit), the magnetic field rapidly collapses causing the inductor to develop a very large voltage across it with a polarity that adds to that of VPWR (or V-Power). If unchecked, the combined V-Power voltage plus the inductively generated voltage, would in many cases, be catastrophic to the MOSFET. Dynamic Clamping is incorporated to prevent this from occurring.

Dynamic Clamping is accomplished by the placement of a blocking diode and zener diode in series with each other and between the MOSFET’s drain-to-gate. The blocking / zener diode combination forms a feedback path from drain-to-gate. The zener diode avalanches whenever an excessive voltage is encountered by the Drain of the MOSFET. The avalanching zener pulls the Gate voltage up causing the MOSFET to turn back ON allowing the inductive energy to be dissipated through the MOSFET to ground. The blocking diode is necessary to allow the gate of the N-Channel to be pulled above drain voltage (and not be clamped to the drain) when full turn-ON enhancement of the MOSFET is required for minimal RDS(on).

Referring to the waveform diagram shown. So long as the drain voltage is being clamped (as represented by the flat portion of the Drain Voltage waveform), the MOSFET Gate is pulled-up and the MOSFET is turned ON for the duration represented by the red area under the current curve. During this current-ON time the excessive inductive energy is dissipated through the MOSFET to ground. The clamp energy can be calculated using the formula shown in the box to the right.

15

Protection (Current Limit)

Source 1(Power)

Source 2(I Sense)

Drain

+ VPWR

ID

GateControl

Interface

Parasitic Diode(Body Diode)

Output

Load

RS

Gate

Vref

CurrentLimitFeedback

AnalogILimit

The Output Current Limit protection circuit limits the switch’s output current to safe operating levels. An increase in the voltage developed across the sensing resistor RS above Vref will cause the output of the Analog amplifier to increase, signaling the Gate Control Interface to decrease the voltage applied to the Gate. This action serves to reduce or limit the output current. The In-rush current associated with an incandescent lamp turning-ON is initially very high.

A frequently used approach to Output Current Limiting is to add a second Source (Source 2 terminal) to the N-Channel MOSFET switch. This MOSFET structure is then referred to as a SENSFET. Source 2 is made up of few MOSFET cells relative to Source 1. The Source 2-to-Source 1 cell ratio establishes the Source 2-to-Source 1 current resulting in small currents being sourced from Source 2. The small Source 2 current flowing through sense resistor RS develops a voltage drop across RS that is proportional to the total output current. Minimal system energy is lost using this current sensing method.

The analog amplifier used in the Current Limit Feedback loop establishes closed loop control of the switch’s output current. The amplifier’s threshold level is established by the reference voltage, Vref. The value of Vref, in turn, establishes the current limit threshold for the cell ratio used.

Increasing Drain currents cause a proportional increase in the voltage developed across the RS resistor. This increase above the reference voltage causes the Analog amplifier output to increase, signaling the Gate Control Interface to decrease the voltage being applied to the Gate. This action serves to limit the output current.

The resistance of an incandescent lamp is initially very low until it’s filament heats up. The initial low resistance gives rise to high in-rush currents up to 10 times the normal steady state current. Output current limiting is very useful to limit turn-ON in-rush currents.

16

Protection (Thermal Shutdown)

Thermal ShutdownLoop

ThermalShutdown

Circuit

GateGateControl

Interface

Source

Drain

+ VPWR

ID

Parasitic Diode(Body Diode)

Output

Load

The power output switch is usually the only element of the IC that is thermally protected, since the power switch is normally the primary source of heat generation. The output switch is normally allowed to automatically turn back ON (called auto retry) when the device cools back to operating temperature. If the cause for overheating has not been removed, the device will again heat up to the point where Thermal Shutdown reoccurs. This process cycle is normal and is usually allowed to continue until either the problem is corrected or the input command is changed so as to turn-OFF the switch. Locating the thermal sensing element in close proximity to the heat generating source affords a quick response to any over temperature condition.

Thermal Shutdown is usually incorporated in a power switch along with current limit protection. This combination will allow the switch to continue to operate in an output current limiting mode so long as the thermal operating limit is not exceeded.

17

Protection (Overcurrent)

Source

Drain

+ VPWR

Excessive ID

GateControl

Interface

Parasitic Diode(Body Diode)

Output

Gate

LOAD

VThres

InputControlLogic

OverrideControl(Shutdown)

Load Short

Overcurrent = Logic HIGH

OverrideControl(Shutdown)

Excessive ID

Load Start

In the Overcurrent Protection circuit shown, a shorted load causes the Drain-to-Source voltage (VDS) developed across the N-Channel switch to rise significantly. A rise in the VDS voltage is monitored by an overcurrent detection circuit made up of a threshold referenced comparator. VThres establishes the current turn-OFF threshold for the IC switch. If the VDS voltage attains VThres, the comparator output goes logic HIGH, signaling the Input Control Logic to turn-OFF the MOSFET switch.

IC switches are usually designed to periodically attempt an “automatic retry” to re-energize the output while others are designed to “latch-OFF” and remain OFF until redirected by a new input command to turn-ON. In either event, the overcurrent condition must be removed for sustained switch-ON operation. Repeated automatic retry is a normal “faulted mode” of operation and is designed to not be damaging to the IC.

18

Protection (Multi-Level)

Source 1(Power)

Source 2(I Sense)

Drain

Vref

RS

+ VPWR

ID

GateControl

Interface

Parasitic Diode(Body Diode)

Output

Gate

ThermalShutdown

Circuit

AnalogILimit

Disable(SFPD) VThres

InputControlLogic

Overcurrent Detect

2

3

1

Load

Many IC switches incorporate schemes which provide multiple levels of output protection. The multiple levels include:

•Output Current Limiting•Thermal Shutdown Protection•Overcurrent Shutdown, and sometimes•A Short Fault Protect Disable (SFPD) pin.

Output current limiting protection limits the output current to safe operating levels but allows the IC to operate.Thermal shutdown protection can take priority over other fault conditions. With the output current limited, the IC is allowed to continue operation so long as the Thermal shutdown circuit is not activated. If the limit temperature is reached, the device immediately goes into a thermal shutdown mode of operation.Overcurrent Shutdown protection, sometimes will take priority over other protections and turn the MOSFET switch OFF when an excessive load current condition occurs.A Short Fault Protect Disable (or SFPD) pin is sometimes incorporated to provide a forced command disable of the overcurrent detect circuit. With the SFPD pin logic HIGH, overcurrent shutdown is disabled, and the IC is allowed to continue operating in the current limiting mode so long as thermal shutdown does not occur. With the SFPD pin selected logic LOW, the IC will immediately shut down whenever an overcurrent condition occurs.

19

Protection (Overvoltage)

OVRef

+ VPWR

InputControlLogic

R1

R2

Overvoltage = Logic HIGH

An Overvoltage Shutdown circuit protects the power IC switch by monitoring thepower supply voltage and shutting down the IC prior to the supply voltages ever reaching damaging levels. The Overvoltage detection circuit usually involves a voltage referenced comparator monitoring a voltage that is proportional to the voltage being sensed.

Some IC switch designs call for a shut down of the entire IC for the duration of the overvoltage condition. Other designs shut down the IC not only for the duration of overvoltage but “globally” latch-OFF all switch outputs requiring a new command word to be entered for an attempted shutdown recovery.

Typically in ICs having a logic supply (VDD) separate from the main power (VPWR), only the power stages are shut down, allowing the logic sections to remain active during any V-power overvoltage conditions. This preserves commands in memory and makes automatic recovery possible, eliminating the need for a new command word to be entered.

20

Protection (ESD)

RCharge RZap = 1500 Ohms

CZap = 100 pF

To DeviceTerminals

DeviceUnder Test

Human Body Model

+

+2000 V

Device Terminal

Zener

R

ProtectionRCharge

200 V

RZap = 0 Ohms

CZap = 100 pF

To DeviceTerminals

DeviceUnder Test

Machine Model

+

+

ICs typically incorporate some means of protection against ElectroStatic Discharge voltages or ESD. The Human Body Model and Machine Model are two industry standard test conditions normally encountered. 2000 Volts Human Body Model and 200 Volts Machine Model are minimums for all semiconductor devices and a sector imperative.

The Human Body Model test discharges a 100 pF capacitor, charged to 2000 Volts, through a 1500 Ohm resistor into the device terminals.

The Machine Model test eliminates the resistor used in the Human Body Model test and discharges the 100 pF capacitor, charged to 200 Volts, directly into the device terminals.

All device terminals, where possible, incorporate independent internal protection similar to that shown. Referring to the Protection Circuit on the right, ESD voltages presented to the device terminal break down the Zener diode. A breakdown of the Zener causes the transistor Base voltage to rise, turning-ON the bipolar NPN transistor, dissipating the otherwise destructive energy to ground. This approach allows normal operating voltage excursions of the terminal so long as the Zenerdiode does not break down.

21

Faults and Fault Reporting (Switch OFF Open Load)

The switch OFF Open Load detection circuit uses a comparator to monitor the Drain-to-Source voltage (VDS) developed across the Low-Side switch. The VDS voltage is greater than VOOLref so long as the Output, Load, and VPWR continuity is maintained and the Low-Side switch is OFF. If continuity is broken, an Switch OFF Open Load is indicated. In design, the same comparator is shared for Switch OFF Open Load and ON Overcurrent detection.

A reference comparator is used to monitor the Output Drain-to-Source voltage developed across the Low-Side switch. The monitored Output voltage is compared to an internal Switch OFF Open Load reference voltage (VOOLref). The internal OFF Open Load reference voltage is chosen so as to be between normal VDS(on) and VDS(off) voltage of the Low-Side switch. If the voltage…

VDS < VOOLref, the comparator outputs a logic LOW, indicating an Output OFF Open Load fault to be present.VDS > VOOLref, the comparator outputs a logic HIGH, indicating the Output, Load, and V-power to be connected.

During normal un-faulted operation, a few micro-amps of pull-down current sourced from V-power through the load but incapable of pulling the output below VOOLref. If, on the other hand, the output load OPENS, the Drain current ID or the pull-down current will pull the VDS voltage below VOOLref and the comparator will output a logic LOW indicating a switch OFF open load. If the switch is OFF and the load is shorted, the OFF open load detect circuit will report everything as being OK. The load short fault will only be sensed and reported when the switch is attempted to be turned-ON.

22

Faults and Fault Reporting• Many switch ICs report faults for diagnostics and also use the information for protection purposes.

• Below is a summary of Faults and the Fault Reaction Abilities possible in switch ICs.• All devices do not have the same abilities. See the device datasheet for Faults and Fault Reporting specifics.

Overvoltage

Overcurrent

Thermal Limit

OFF Open Load

Detect Limiting ShutdownApplied

“Globally”Output

SpecificFault

ReportingLock-OutAutoRetry Latch-Off

ON Open Load

Undervoltage

Output-to-GndShort

Output-to-VPWRShort

Pin ESD Voltage

Fault

Fault Reaction Ability

Analog Power ICs have many combinations of Fault Detection and Fault Reporting. Some devices detect faults and use the information internally to “trigger” protection systems but do not output the fault. Most ICs do output fault information for diagnostic purposes. This information when available to the user is very valuable for “trouble shooting” purposes.

23

Question

How is Dynamic Clamping accomplished? Click on your choice.

a) Short the Gate to the Drain.b) Place a Charge Pump between to Gate and Drain.c) Place a Blocking diode and Zener Diode in series between the Gate and Drain.d) Place a Blocking diode and Zener Diode in parallel between the Gate and Drain.

Here’s a question on dynamic output clamping.

Correct:

Dynamic Clamping is accomplished by placing a Blocking diode and ZenerDiode in series between the Gate and Drain.

24

Pre-Driver ICs

• The term Pre-Driver relates to ICs that “drive” or control other higher powercontrol devices.

• Microcontrollers serve as the “brains” for systems and do control many devices directly but a microcontroller is not a device that delivers real power. The microcontroller normally controls other higher current devices which in turn provide the system high power controlling functions. In the purest sense, the microcontroller is a Pre-Driver of sorts. Normally though, a Pre-Driver is thought of as being a device which provides the controlling power for much higher powered devices.

• A low current relay may well be controlled by the microcontroller directly, but a microcontroller will require the help of possibly a Pre-Driver, to drive a high current FET transistor, to in turn, control a high current solenoid or motor.

• There are many types of Pre-Drivers used in the control of solenoids, relays, motors, and high current switches (power FETs).– Some Pre-Drivers incorporate features that not only protect the Pre-Driver itself

but also protect the device being driven and it’s load.

The term Pre-Driver relates to ICs that “drive” or control other higher power control devices. Microcontrollers serve as the “brains” for systems and do control many devices directly but a microcontroller is a not a device that delivers much real power. The microcontroller normally controls other higher current devices which in turn provide the system high power controlling functions. In the purest sense, the microcontroller is a Pre-Driver of sorts. Normally though, a Pre-Driver is thought of as being a device which provides the controlling power for much higher powered devices.

A low current relay may be controlled by the microcontroller directly, but a microcontroller will require the help of possibly a Pre-Driver, to drive a high current FET transistor, to in turn, control a high current solenoid or motor.

There are many types of Pre-Drivers used in the control of solenoids, relays, motors, and high current switches (or power FETs). Some Pre-Drivers incorporate features that not only protect the Pre-Driver itself but also protect the device being driven and it’s load.

25

Pre-Driver Example

MCU

SRC

OUT1

OUT2

DRN

GND

MC33285Pre-Driver

IN

VCC

CP

VCC VPWR

Motor

ControlInput

Solenoid

Q2

Q1D

An example of a Pre-Driver is the MC33285. The MCU could, in some cases, control very small motors or solenoids directly but in this example the MCU is serving to control the MC33285 Pre-Driver. The MC33285 is NOT controlling the solenoid or motor directly, but is driving two external N-channel power transistors (shown as Q1 & Q2) which are controlling the solenoid and motor currents. You will notice the two external transistors are serving essentially as High-Side switches. To do this efficiently, the MC33285 incorporates an internal charge pump to provide sufficient “pumped-up” Gate voltages which fully enhance turn-ON of the external FET transistors.

The MC33285 provides some protection features. Gate-to-Source over voltage protection is afforded external FET (Q1) by the incorporation of an internal Zener diode to clamp the over voltage. The SRC contact of the MC33285 monitors the external FET Source voltage. When Q2 turns the solenoid current OFF, any source over voltage is dynamically clamped by the internal Zener’s turning Q1 back ON for the duration of the over voltage. You will notice the same protection is NOT afforded external FET (Q2). Gate-to-Source over voltage dynamic clamp protection is provided externally by Zener diode D.

In addition, Q1 is provided over current protection by the MC33285 monitoring Q1’s Drain-to-Source voltage drop via the DRN and SRC inputs. When an over current condition is sensed, a timer inside the MC33285 is started. If the over current condition goes away before the internal timer “times-out”, the output stays ON. If the over current condition persists and the internal timer “times-out”, OUT1 switches Q1 OFF. Q1 can (in this case) only be turned back ON by the MCU re-commanding OUT1 to turn Q1 ON.

Pre-Drivers provide the additional power necessary to drive high current devices and also provide some protection features.

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Parallel and Serial Peripheral Interfaces (or SPI) are two input communications protocols used extensively to control analog Power IC switches.

Parallel input control requires an input to control each output and for that reason is primarily used in devices having few outputs. Parallel input control is used in high speed IC devices requiring minimal switching and propagation delay times as required in Pulse Width Modulation (or PWM) control of motors speeds and for solenoid positioning. As a result, of each output normally has a dedicated input.

SPI control can readily accommodate ICs with multiple outputs and is normally used in microcontroller based systems. SPI control requires four digital signals, for full implementation, in addition to internal data registers. A command instruction Word is serially clocked into an internal Data-In register on a bit-by-bit basis while a return status Word is simultaneously serially clocked out of an internal Data-Out register. The size of the internal data registers determines the number of bits in the Command Word.

Parallel and SPI combinations are sometimes incorporated to provide high speed and PWM control of critical outputs and to establish a redundant means of control for use in applications requiring input fault tolerant or override control capability.

Fault Reporting information is contained in the return status Word clocked out of the SO pin of the IC Switch.

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Static load control is the function of fully or partially powering-up the elements of a primarily non-moving load. Typical Static load applications include:

Security SystemsHVACWater HeatersCharging systemsLighting displaysIrrigation controlsWhite goods appliancesAutomotive Body electronicsAnd more.

Click on the Block Diagram reference button to see a generic system diagram for the Static Load Control circuit. Each block will identify several components which will meet different circuit design requirements.

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(reference for previous page)

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Motor control is the function of regulating the speed, rotation, and torque of a constant duty motor. Typical motor control applications include:

BlowersFansVacuumsPumpsCompressorsGolf CartsMixersWinchesElevatorsand many more.

Click on the Block Diagram reference button to see a generic system diagram for the Motor Control circuit. Each block will identify several components which will meet different circuit design requirements.

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(reference for previous page)

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Motion control is the function of fully or partially powering-up the elements of a primarily non-moving load. Typical Static load applications include:

CNC MachiningRoboticsHVAC Mix LouversAutopilotsServos and radio controlDish positionersPrinter paper handlingand many more.

Click on the Block Diagram reference button to see a generic system diagram for the Motion Control circuit. Each block will identify several components which will meet different circuit design requirements.

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(reference for previous page)

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Question

Which of the following is NOT an application area for Power IC switching? Click on your choice.

a) Static Control.b) Motor Control.c) Motion Control.d) Pneumatic Control.

Here’s a question about application areas.

Correct:

Pneumatic Control is Not an application area.

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Module Completion

• Common Switches (Mechanical, NPN, PNP, N-Channel and P-Channel)• Basic Switches

• Mechanical• Function, Operation, and Design Traits• The Ideal Switch

• Important Practical Switch Parameters• Switch Configurations

• Low-Side Switches• High-Side Switches• Half-Bridge Circuits• H-Bridge Circuits

• Protection Features• Faults and Fault Reporting• Pre-Drivers• Input Control Communications• Application Areas

This concludes our discussion of Power Integrated Circuit Switch products. We have covered the basics of switching by introducing the various kinds of switches, their function, and the ideal switch. We also saw some of the more popular switch configurations used with ICs. The various protection features incorporated in analog ICs were discussed to acquaint you with their function and operation as well as fault modes. Finally, we discussed the applications that use power IC switch products.