MIC2172/3172

Micrel, Inc.

M9999-042205

10

April 2005

Figure 7 shows how one or more MIC2172s can be locked to

an external reference frequency. The slaves lock to the

negative (falling edge) of the external reference waveform.

Soft Start

A diode-coupled capacitor from COMP to circuit ground

slows the output voltage rise at turn on (figure 8).

MIC2172/3172

COMP

C2

R1

D2

C1

D1

Figure 8. Soft Start

The additional time it takes for the error amplifier to charge the

capacitor corresponds to the time it takes the output to reach

regulation. Diode D1 discharges C1 when V

Current Limit

For designs demanding less output current than the MIC2172/

3172 is capable of delivering, P GND 1 can be left open

reducing the current capability of Q1 by one-half.

MIC2172/3172

COMP

C2

R3

Q1

R2

GND

P1 P2 S

R1

C1

FB

Note: Input and output

returns not common.

Figure 9. Current Limit

Alternatively, the maximum current limit of the MIC2172/3172

can be reduced by adding a voltage clamp to the COMP

output (figure 9). This feature can be useful in applications

requiring either a complete shutdown of Q1’s switching action

or a form of current fold-back limiting. This use of the COMP

output does not disable the oscillator, amplifiers or other

circuitry, therefore the supply current is never less than

approximately 5mA.

Thermal Management

Although the MIC2172/3172 family contains thermal protec-

tion circuitry, for best reliability, avoid prolonged operation

with junction temperatures near the rated maximum.

The junction temperature is determined by first calculating

the power dissipation of the device. For the MIC2172/3172,

the total power dissipation is the sum of the device operating

losses and power switch losses.

The device operating losses are the dc losses associated

with biasing all of the internal functions plus the losses of the

power switch driver circuitry. The dc losses are calculated

from the supply voltage (V

The MIC2172/3172 supply current is almost constant regard-

less of the supply voltage (see “Electrical Characteristics”).

The driver section losses (not including the switch) are a

function of supply voltage, power switch current, and duty

cycle.

P

0.004 + δ

50

























where:

P

V

I

I

(see “ Design Hints: Switch Current

Calculations”)

δ = duty cycle

δ =

V

V

V

V

As a practical example refer to figure 1.

V

I

I

δ = 60% (0.6)

Then:

P

0.004+0.6

50

























P

Power switch dissipation calculations are greatly simplified

by making two assumptions which are usually fairly accurate.

First, the majority of losses in the power switch are due to

on-losses. To find these losses, assign a resistance value to

the collector/emitter terminals of the device using the satura-

tion voltage versus collector current curves (see Typical

Performance Characteristics).

Power switch losses are

calculated by modeling the switch as a resistor with the switch

duty cycle modifying the average power dissipation.

P

From the Typical performance Characteristics:

R