The output voltage is regulated by the Zener diode VR3. Whenthe output voltage exceeds the sum of the Zener and optocouplerLED forward drop, current will flow in the optocoupler LED.This will cause the transistor of the optocoupler to sink current.When this current exceeds the ENABLE pin threshold currentthe next switching cycle is inhibited. When the output voltagefalls below the feedback threshold, a conduction cycle is allowedto occur and, by adjusting the number of enabled cycles, outputregulation is maintained. As the load reduces, the number ofenabled cycles decreases, lowering the effective switchingfrequency and scaling switching losses with load. This providesalmost constant efficiency down to very light loads, ideal formeeting energy efficiency requirements.