4 UCC25640X has Higher Vcc Startup Voltage to Support Lower Vcc Capacitance

Same as previous section that mentioned the Vcc voltage is charged up to Vccstartself threshold (26 V) and then JFET turn off to let Vcc voltage dropped till VCCReStartJfet (9.65 V). If PFC boosts the Vbus voltage to let BLK pin above the BLK start threshold, the PWM can start switching then Auxwinding voltage charge the Vcc so larger Vcc hysister window is helpful to reduce the Vcc capacitance.

This benefit is more significant at LED lighting PSU application because the LED needs to operate at -40℃ to approximately +90℃ ambient temperature without the air flow for thermal dissipation so the Vcc equivalent capacitance can be dropped by low and high ambient temperature as Figure 4-1 showing.

The capacitor bias voltage can also influence the capacitance as Figure 4-2 showing because the LED lighting PSU also needs to support two times output voltage range and the Vcc voltage is up to two times too so the Vcc equivalent capacitance can be dropped during the high output voltage condition.