Micrel, Inc.

MIC4421A/4422A

June 2007

7

M9999-062707

Functional Diagram

IN

OUT

MIC4421A

INVERTING

MIC4422A

NONINVERTING

0.1mA

0.3mA

Q1

VS

GND

Q2

Q3

Q4

Figure 4. MIC4421A/22A Block Diagram

Functional Description

Refer to the functional diagram.

The MIC4422A is a non-inverting driver. A logic high on

the IN produces gate drive output. The MIC4421A is an

inverting driver. A logic low on the IN produces gate

drive output. The output is used to turn on an external N-

channel MOSFET.

Supply

capacitors are recommended to decouple noise.

Input

IN (control) is a TTL-compatible input. IN must be forced

high or low by an external signal. A floating input will

cause unpredictable operation.

A high input turns on Q1, which sinks the output of the

0.1mA and the 0.3mA current source, forcing the input of

the first inverter low.

Hysteresis

The control threshold voltage, when IN is rising, is

slightly higher than the control threshold voltage when

CTL is falling.

When IN is low, Q2 is on, which applies the additional

0.3mA current source to Q1. Forcing IN high turns on Q1

which must sink 0.4mA from the two current sources.

The higher current through Q1 causes a larger drain-to-

source voltage drop across Q1. A slightly higher control

voltage is required to pull the input of the first inverter

down to its threshold.

Q2 turns off after the first inverter output goes high. This

reduces the current through Q1 to 0.1mA. The lower

current reduces the drain-to-source voltage drop across

Q1. A slightly lower control voltage will pull the input of

the first inverter up to its threshold.

Drivers

The second (optional) inverter permits the driver to be

manufactured in inverting and non-inverting versions.

The last inverter functions as a driver for the output

MOSFETs Q3 and Q4.

Output

voltage) is either approximately the supply voltage or

approximately ground, depending on the logic state

applied to IN.

will be floating (unpredictable).