The UCC28780 is intended to control active clamp flyback (ACF) converters in high efficiency offline applications, and is optimized to be used with universal AC input, from 85 V_AC to 265 V_AC, at 47 Hz to 63 Hz. An external depletion-mode MOSFET connected between the switch node of the converter and the SWS / HVG pins of this controller is required to charge the VDD capacitor during start-up, and to perform ZVS sensing during normal operation. Once the V_VDD reaches the UVLO turn-on threshold at 17.5 V, the VDD rail should be kept within the limits of the Bias Supply Input section of Specifications. To avoid the possibility that the device might stop switching, V_VDD must not be allowed to fall below the UVLO turn-off threshold at 9.8 V.