M9999-072205

8

July 2005

MIC4420/4429

Micrel, Inc.

Table 1: MIC4429 Maximum

Operating Frequency

Max Frequency

18V

500kHz

15V

700kHz

10V

1.6MHz

Conditions:

Input Stage

The input voltage level of the 4429 changes the quiescent

supply current. The N channel MOSFET input stage tran-

sistor drives a 450µA current source load. With a logic “1”

input, the maximum quiescent supply current is 450µA.

Logic “0” input level signals reduce quiescent current to

55µA maximum.

The MIC4420/4429 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 approxi-

mately 1.5V, making the device TTL compatible over the

4 .5V to 18V operating supply voltage range. Input current

is less than 10µA over this range.

The MIC4429 can be directly driven by the TL494,

SG1526/1527, SG1524, TSC170, MIC38HC42 and similar

switch mode power supply integrated circuits. By offloading

the power-driving duties to the MIC4420/4429, the power

supply controller can operate at lower dissipation. This can

improve performance and reliability.

current will flow into the input lead. The propagation delay

perature. The input currents can be as high as 30mA p-p

voltage. No damage will occur to MIC4420/4429 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. Care

should be taken so that the input does not go more than 5

volts below the negative rail.

Power Dissipation

CMOS circuits usually permit the user to ignore power dis-

sipation. 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 cur-

rent to destroy the device. The MIC4420/4429 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.

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 2500pF 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 MSOP package, from the

data sheet, is 250°C/W. In a 25°C ambient, then, using a

maximum junction temperature of 150°C, this package will

dissipate 500mW.

Accurate power dissipation numbers can be obtained by

summing the three sources of power dissipation in the

device:

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

is high, at the power supply voltage used. (See data

sheet)

D = fraction of time the load is conducting (duty cycle)

Figure 4. Switching Time Degradation Due to

Negative Feedback

MIC4429

1

8

6, 7

5

4

+18 V

0.1µF

0.1µF

TE K C U R R E N T

PROBE 6302

2,500 pF

POLYCARBONATE

5.0V

0 V

18 V

0 V

WIMA

MKS-2

1 µF

LOGIC

GROUND

POWER

GROUND

6 AMPS

300 mV

PC TRACE RESISTANCE = 0.05Ω