Shunt regulators of the 431 family (TL431, TL431LI, ATL431 and ATL431LI) behave as a voltage comparator when operated in open loop. Voltage is sensed at the reference pin and output is taken from the cathode pin as shown in Figure 2-1. A shunt voltage reference is commonly used in the feedback loop of the PWM controllers in switching power supplies. Shunt regulators are very popular for its low cost and built-in precise reference. It acts as an error amplifier in the feedback loop with the optocoupler. It senses the output voltage and trigger the optocoupler for the PWM controller feedback. This application note explains the comparator mode operation and discusses the key design considerations for the 431 family in the comparator mode.