September 1999

185

MIC5022

MIC5022

Micrel

Applications Information

The MIC5022 MOSFET driver is designed for half-bridge

switching applications where overcurrent limiting and high

speed are required. The MIC5022 can control MOSFETs that

switch voltages up to 36V.

The MIC5022 functionally includes the MIC5020 and MIC5021

with additional circuitry to coordinate the operation of the high

and low-side drivers. Since most output considerations are

similar,

refer to the MIC5020 and MIC5021 data sheets for

additional applications information.

Supply Voltage

The MIC5022’s supply input (V

supply voltage must be equal to or greater than the voltage

applied to the drain of the external N-channel MOSFET.

A 16V minimum supply is recommended to produce continu-

ous on-state, gate drive voltage for standard MOSFETs (10V

nominal gate enhancement).

When the driver is powered from a 12V to 16V supply, a logic-

level MOSFET is recommended (5V nominal gate enhance-

ment).

PWM operation may produce satisfactory gate enhancement

at lower supply voltages. This occurs when fast switching

repetition makes the boost capacitor a more significant

voltage supply than the internal charge pump.

Overcurrent Limiting

Separate high and low-side 50mV comparators are provided

losses when a power resistor is used for current sensing.

The adjustable retry feature can be used to handle loads with

high initial currents, such as lamps or heating elements, and

can be adjusted from the C

C

ing an overcurrent condition.

C

The default duty cycle (C

high side is slightly greater than the low side). Refer to the

typical characteristics when selecting a capacitor for a re-

duced duty cycle.

C

Increasing the duty cycle increases the power dissipation in

the load and MOSFET under a “fault” condition. Circuits may

become unstable at a duty cycle of about 75% or higher,

depending on conditions.

Caution: The MIC5022 may be

damaged if the voltage applied to C

maximum voltage rating.

Boost Capacitor Selection

For 12V to 20V operation, the boost capacitor should be

0.01

µF; and for 12V to 36V operation, the boost capacitor

should be 2.7nF; both connected between V

MOSFET source. The preferred configuration for 20V to 36V

operation is a 0.1

µF capacitor connected between V

BOOST

and V

Do not connect capacitors between V

source and between V

Larger capacitors than specified may damage the MIC5022.

Circuits Without Current Sensing

Current sensing may be omitted by connecting the high-side

SENSE

ground. Do not connect the high-side sense pins to ground.

Inductive Load Precautions

Circuits controlling inductive loads require precautions when

controlled by the MIC5022. Wire wound resistors, which are

sometimes used to simulate other loads, can also show

significant inductive properties.

Sense Pin Considerations

The sense pins of the MIC5022 are sensitive to negative

voltages. If a voltage spike is too negative (below approxi-

mately –0.5V), current will be drawn from functional sections

of the IC resulting in unpredictable circuit behavior or dam-

age. Resistors and Schottky diodes may be used to protect

the sense pins from the negative spikes.

Refer to the

MIC5021 data sheet for details.

High-Side Sensing

For the high-side driver, sensing the current on the supply

from the inductive spike. Refer to the MIC5021 data sheet for

details.

Low-Temperature Operation

As the temperature of the MIC5022AJB (extended tempera-

ture range version—no longer available) approaches –55

°C,

the driver’s off-state, gate-output offset from ground in-

creases. If the operating environment of the MIC5022AJB

includes low temperatures (–40

°C to –55°C), add an external

2.2M

Ω resistor from gate-to-source or from gate-to-ground.

This assures that the driver’s gate-to-source voltage is far

below the external MOSFET’s gate threshold voltage, forcing

the MOSFET fully off. Refer to the MIC5020 and MIC5021

data sheets for examples.

The gate-to-source configuration is appropriate for resistive

and inductive loads. This also causes the smallest decrease

in gate output voltage.

The gate-to-ground configuration is appropriate for resistive,

inductive, or capacitive loads. This configuration will de-

crease the gate output voltage slightly more than the gate-to-

source configuration.

Full-Bridge Motor Control

An application for two MIC5022s is the full-bridge motor

control circuit.

Two high or two low-side sense inputs may be used for

overcurrent detection. (Low-side sensing is shown in Fig-

ure 2). Sensing at four locations is usually unnecessary.

When switching inductive loads, such as motors, it is desir-

able to place the high-side sense inputs on the supply side of

the MOSFETs. The helps prevent the inductive spikes that

occur upon load shutoff from affecting the sense inputs.