Figure 21 shows typical power−up waveforms. Thebrownout timer (tBO(blank)) is enabled once VHV dropsbelow the lower brownout threshold, VBO(stop) and abrown−out fault is detected if VHV doesn’t exceed VBO(stop)before the brownout timer expires. The timer is set longenough to pass line−dropout tests.Figure 21 illustrates a line−dropout event.The circuit operates normally and suddenly, the linereduces to a low level. Due to the dropout, the HV voltagedrops below the VBO(stop) level. The blanking time tBO(blank)is started and a brown−out fault is detected since the HVvoltage has remained below VBO(start) until the timerexpires. As a result, the PFC stage stops operating and thepfcOK pin is grounded. If as sketched in Figure 21, noexternal power source maintains the VCC voltage, VCCswings between VCC(off) and VCC(on).When the linerecovers, the circuit does not immediate resume operationbut first turns on the HV startup to charge VCC up to VCC(on)so that a clean restart is obtained. If VCC is already higherthan VCC(on) when the line recovers, the NCP1618 restartsimmediately.