At start-up, the IC gets its power directly from the high voltage bulk, through a high-voltage resistor RH. The selection of start-up resistor is the tradeoff between power loss and start-up time. The current flowing through RH at minimum input voltage must be higher than the VCC current under UVLO condition (0.2 mA at its maximum value). A 300-kΩ resistor is chosen as the result of the tradeoff.
After VCC is charged up above the UVLO turnon threshold, UCC2813-0 starts to operate and consumes full operating current. At the beginning, because the output voltage is low, VCC cannot get energy from the auxiliary winding. The VCC capacitor is required to hold enough energy to prevent its voltage drop below UVLO during the start-up time, until the output reaches high enough. A larger capacitor holds more energy but slows down the start-up time. In this design, a 120-µF capacitor is chosen to provide enough energy for the start-up purpose.