At start-up, the IC gets its power directly from the high-voltage bulk, through a high-voltage resistor RSTART. The selection of the start-up resistor is the trade-off between power loss and start-up time. The current flowing through RSTART at the minimum input voltage must be higher than the VCC current under UVLO conditions (1 mA at its maximum value). A resistance of 100-kΩ was chosen for RSTART, providing 1 mA of start-up current at low-line conditions. The start-up resistor is physically comprised of two 50-kΩ resistors in series to meet the high voltage requirements and power rating at high-line.
After VCC is charged up above the UVLO-on threshold, the UC2842 starts to consume full operating current. The VCC 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 2 seconds.