The UCC28740 is designed to be used with a Universal AC input, from 85 VAC to 265 VAC, at 47 Hz to 63 Hz. Other input line conditions can be used provided the HV pin can be set up to provide 500 µA to charge the VDD capacitor for start-up through the internal startup switch. Once the VDD reaches the 21-V UVLO turnon threshold, the VDD rail should be kept within the limits of the Bias Supply Input section of the Electrical Characteristics table. To avoid the possibility that the device might stop switching, VDD must not be allowed to fall below the UVLO V_VDD(off) range.