November 2000

1

MIC4043

MIC4043

Micrel

MIC4043

Low-Voltage Secondary-Side Shunt Regulator

Final Information

General Description

The MIC4043 is a shunt regulator optimized for secondary-

side regulation in low-voltage power supplies. Featuring an

output stage guaranteed to swing within 400mV of ground,

the MIC4043 can be used in power supplies operating down

to 1.8V, even with optoisolators requiring greater than 1.2V of

headroom.

In power supply applications, the MIC4043 normally drives

the LED of an optically isolated feedback circuit. The MIC4043

monitors a resistively-divided output voltage and sinks error

current through the optoisolator’s LED (secondary side); the

optoisolator’s transistor (primary side) provides this signal to

the controller’s feedback input. The MIC4043 is also practical

for other voltage-monitoring applications requiring an open-

collector output.

The MIC4043 replaces conventional ’431-type shunt regula-

tors to allow low-voltage applications where there is inad-

equate headroom for a 2.5V regulator in series with an

optoisolator. Replacing ’431-type devices requires only a

minor change to the way that the resistive-divider values are

calculated.

Typical Application

MIC4043

MIC38HC43BN

GND

COMP

FB

2

1

4

3

8

5

7

6

Return

1

2

7

6

4

3

FB

GND

SNK

IN

OPTICAL

ISOLATION

COMPENSATION

MIC4043

Low-Side Feedback

Control

R1

R2

V

1.245V

R2

R1

1









200kHz DC-DC Flyback Converter

Features

• Ideal for 1.8V switching converters

• Low-voltage operation

400mV maximum saturation over

operating temperature range

• Easy to use

voltage in, current out

• 2% voltage tolerance over operating temperature range

Applications

• Optically isolated low-volage power supplies

• Low-voltage discrete regulator control

Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com