Micrel, Inc.

MIC4421A/4422A

June 2007

9

M9999-062707

Input Stage

The input voltage level of the MIC4421A changes the

quiescent supply current. The N-Channel MOSFET input

stage transistor drives a 320µA current source load. With

a logic “1” input, the quiescent supply current is typically

500µA. Logic “0” input level signals reduce quiescent

current to 80µA typical.

The MIC4421A/4422A input is designed to provide

600mV of hysteresis. This provides clean transitions,

reduces noise sensitivity, and minimizes output stage

current spiking when changing states. Input voltage

threshold level is approximately 1.5V, making the device

TTL compatible over the full temperature and operating

supply voltage ranges. Input current is less than ±10µA.

The MIC4421A can be directly driven by the TL494,

SG1526/1527, SG1524, TSC170, MIC38C42, and

similar switch mode power supply integrated circuits. By

off loading the power-driving duties to the MIC4421A/

4422A, the power supply controller can operate at lower

dissipation. This can improve performance and reliability.

current will flow into the input lead. The input currents

can be as high as 30mA p-p (6.4mARMS) with the input.

No damage will occur to MIC4421A/4422A however, and

it will not latch.

The input appears as a 7pF capacitance and does not

change even if the input is driven from an AC source.

While the device will operate and no damage will occur

up to 25V below the negative rail, input current will

increase up to 1mA/V due to the clamping action of the

input, ESD diode, and 1kΩ resistor.

Power Dissipation

CMOS circuits usually permit the user to ignore power

dissipation. Logic families such as 4000 and 74C have

outputs which can only supply a few milliamperes of

current, and even shorting outputs to ground will not

force enough current to destroy the device. The

MIC4421A/4422A on the other hand, can source or sink

several amperes and drive large capacitive loads at high

frequency. The package power dissipation limit can

easily be exceeded. Therefore, some attention should be

given to power dissipation when driving low impedance

loads and/or operating at high frequency.

MIC4421A

0.1µF

1

8

6, 7

5

4

0.1µF

2500pF

Polycarbonate

TEK Current

Probe 6302

Logic

Ground

Power

Ground

300mV

6 Amps

PC Trace

+18V

0V

+5.0V

0V

WIMA

MKS-2

1µF

Figure 7. Switching Time Due to Negative Feedback

The supply current vs. frequency and supply current vs.

capacitive load characteristic curves aid in determining

power dissipation calculations. Table 1 lists the

maximum safe operating frequency for several power

supply voltages when driving a 10,000pF load. More

accurate power dissipation figures can be obtained by

summing the three dissipation sources.

Given the power dissipation in the device, and the

thermal resistance of the package, junction operating

temperature for any ambient is easy to calculate. For

example, the thermal resistance of the 8-pin plastic DIP

package, from the data sheet, is 84.6°C/W. In a 25°C

ambient, then, using a maximum junction temperature of

150°C, this package will dissipate 1478mW.

Accurate power dissipation numbers can be obtained by

summing the three sources of power dissipation in the

device:

•

Load Power Dissipation (PL)

•

Quiescent power dissipation (PQ)

•

Transition power dissipation (PT)

Calculation of load power dissipation differs depending

on whether the load is capacitive, resistive or inductive.

Resistive Load Power Dissipation

Dissipation caused by a resistive load can be calculated

as:

where:

I =

the current drawn by the load

the output resistance of the driver when

the output is high, at the power supply

voltage used. (See data sheet)

D =

fraction of time the load is conducting

(duty cycle).