• 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

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 opencollector output. The MIC4043 replaces conventional ’431-type shunt regulators to allow low-voltage applications where there is inadequate 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.