At start-up, the IC gets its power directly from the high-voltage bulk, through a high-voltage resistor (R_START). The selection of the start-up resistor is the tradeoff between power loss and start-up time. The current flowing through R_START at the minimum input voltage must be higher than the VDD current under UVLO conditions (100 µA at its maximum value). A resistance of 420-kΩ was chosen for R_START, providing 250 µA of start-up current at low-line conditions. The start-up resistor is physically comprised of two 210-kΩ resistors in series to meet the high voltage requirements and power rating at high-line.

After VDD is charged up above the UVLO-ON threshold, the UCC28C42 starts to consume full operating current. The VDD capacitor is required to provide enough energy to prevent its voltage from dropping below the UVLO-OFF threshold during start-up, before the output is able to reach its regulated level. A large bulk capacitance would hold more energy but would result in slower start-up time. In this design, a 120-µF capacitor is chosen to provide enough energy and maintain a start-up time of approximately 7 seconds. For faster start-up, the bulk capacitor value may be decreased or the R_START resistor modified to a lower value.