MIC4421/4422

Micrel, Inc.

M9999-081005

8

August 2005

Table 1: MIC4421 Maximum

Operating Frequency

V

S

Max Frequency

18V

220kHz

15V

300kHz

10V

640kHz

5V

2MHz

Conditions:

1. θ

2. T

3. C

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.

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 volt-

ages 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 130°C/W. In a 25°C ambient, then, using

a maximum junction temperature of 150°C, this package

will dissipate 960mW.

Accurate power dissipation numbers can be obtained by

summing the three sources of power dissipation in the

device:

• Load Power Dissipation (P

• Quiescent power dissipation (P

• Transition power dissipation (P

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:

P

2

R

where:

I = the current drawn by the load

R

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)

Figure 5. Switching Time Degradation Due to

Negative Feedback

MIC4421

1

8

6, 7

5

4

+18

0.1µF

0.1µF

TEK CURRENT

PROBE 6302

2,500 pF

POLYCARBONATE

5.0V

0 V

18 V

0 V

300 mV

6 AMPS

PC TRACE RESISTANCE = 0.05Ω

LOGIC

GROUND

POWE R

GROUND

WIMA

MKS-2

1 µF

Input Stage

The input voltage level of the MIC4421 changes the quies-

cent supply current. The N channel MOSFET input stage

transistor drives a 320µA current source load. With a logic

“1” input, the maximum quiescent supply current is 400µA.

Logic “0” input level signals reduce quiescent current to

80µA typical.

The MIC4421/4422 input is designed to provide 300mV of

hysteresis. This provides clean transitions, reduces noise

sensitivity, and minimizes output stage current spiking

when changing states. Input voltage threshold level is ap-

proximately 1.5V, making the device TTL compatible over

the full temperature and operating supply voltage ranges.

Input current is less than ±10µA.

The MIC4421 can be directly driven by the TL494,

SG1526/1527, SG1524, TSC170, MIC38C42, and similar

switch mode power supply integrated circuits. By offloading

the power-driving duties to the MIC4421/4422, the power

supply controller can operate at lower dissipation. This can

improve performance and reliability.

The input can be greater than the V

rent will flow into the input lead. The input currents can be

as high as 30mA p-p (6.4mA

will occur to MIC4421/4422 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 out-

puts 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 MIC4421/4422 on the

other hand, can source or sink several amperes and drive

largecapacitiveloadsathighfrequency.Thepackagepower