Micrel, Inc.

MIC2171

May 2007

6

M9999-051107

Application 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

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 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 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.

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

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.

where:

As a practical example refer to Figure 1.

δ = 66.2% (0.662)

then:

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 saturation voltage versus collector

current curves (see Typical Performance Character-

istics). Power switch losses are calculated by modeling

the switch as a resistor with the switch duty cycle

modifying the average power dissipation.

where:

δ = duty cycle

F

OUT

IN(min)

F

OUT

V

V

V

V

V

+

−

+

=

δ

From the Typical performance Characteristics:

then: