MIC2171

Micrel

4-8

1997

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 MIC2171 supply current is almost constant regardless 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= V

I

V

I

I

(bias+driver)

IN Q

IN(min)

SW

IN

×

()

∆

where:

P

V

I

I

∆I

As a practical example refer to Figure 1.

V

I

I

δ = 66.2% (0.662)

Then:

V

= 5 – (2.21

0.37) = 4.18V

IN(min)

×

P

= (

0.007) + (4.18

2.21

.009)

(bias

driver)

+

××

×

5

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

where:

δ = duty cycle

δ =

V+ V – V

V+ V

OUT

F

IN(min)

OUT

F

V

V

V

From the Typical performance Characteristics:

R

Then:

P

2

× 0.37 × 0.662

P

P

P

Applications Information

Soft Start

A diode-coupled capacitor from COMP to circuit ground

slows the output voltage rise at turn on (Figure 3).

MIC2171

IN

COMP

C2

R1

D2

C1

D1

Figure 3. 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

MIC2171

IN

COMP

C2

R3

Q1

R2

GND

R1

C1

SW

FB

Note: Input and output

returns not common.

Figure 4. Current Limit

The maximum current limit of the MIC2171 can be reduced by

adding a voltage clamp to the COMP output (Figure 4). This

feature can be useful in applications requiring either a com-

plete 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 MIC2171 family contains thermal protection

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 MIC2171, the

total power dissipation is the sum of the device operating

losses and power switch losses.