Enable FunctionTinySwitch-4 senses the ENABLE/UNDERVOLTAGE pin to determinewhether or not to proceed with the next switching cycle. Thesequence of cycles is used to determine the current limit. Once acycle is started, it always completes the cycle (even when theENABLE/UNDERVOLTAGE pin changes state half way through thecycle). This operation results in a power supply in which the outputvoltage ripple is determined by the output capacitor, amount ofenergy per switch cycle and the delay of the feedback.The ENABLE/UNDERVOLTAGE pin signal is generated on thesecondary by comparing the power supply output voltage with areference voltage. The ENABLE/UNDERVOLTAGE pin signal is highwhen the power supply output voltage is less than the referencevoltage. In a typical implementation, the ENABLE/UNDERVOLTAGEpin is driven by an optocoupler. The collector of the optocouplertransistor is connected to the ENABLE/UNDERVOLTAGE pin and theemitter is connected to the SOURCE pin. The optocoupler LED isconnected in series with a Zener diode across the DC output voltageto be regulated. When the output voltage exceeds the targetregulation voltage level (optocoupler LED voltage drop plus Zenervoltage), the optocoupler LED will start to conduct, pulling theENABLE/UNDERVOLTAGE pin low. The Zener diode can be replacedby a TL431 reference circuit for improved accuracy.