The LM3411 is a low-power fixed-voltage (3.3 V or 5 V) precision shunt regulator designed specifically for driving an optoisolator to provide feedback isolation in a switching regulator. The LM3411 circuitry includes an internally compensated operational amplifier, a bandgap reference, NPN output transistor, and voltage setting resistors. A trimmed precision bandgap reference with temperature drift curvature correction provides a ensured 1% precision over the operating temperature range (A grade version). The inverting input of the amplifier is externally accessible for loop frequency compensation when used as part of a larger servo system. The output is an open-emitter NPN transistor capable of driving up to 15 mA of load current. Because of its small die size, the LM3411 has been made available in the subminiature 5-pin SOT-23 surface-mount package. This package is ideal for use in space-critical applications. Although its main application is to provide a precision output voltage (no trimming required) and maintain very good regulation in isolated DC-DC converters, it can also be used with other types of voltage regulators or power semiconductors to provide a precision output voltage without precision resistors or trimming.