AN2870 datasheet(8/41 Pages) STMICROELECTRONICS

8 / 41 page

Device blocks description

AN2870

8/41

Device blocks description

3.1

Start-up and shut-down

During start-up the chip is supplied through a resistive path from the rectified AC Mains

voltage whereas, during normal operation, a self-supply source is recommended: a charge

pump, an auxiliary winding coupled either with PFC choke or resonant choke, or an auxiliary

converter.

As the voltage at Vcc pin reaches the turn-on threshold (VccON, Figure 3-A), the chip is

enabled and (unless a lamp absence is detected) the Half-Bridge and the PFC sections

start at the same time (independently):

●

The PFC section, as the synchronization signal at pin ZCD is not yet generated by the

external ZCD circuit, is forced to switch by internal starter (fstarter = 6 kHz (typ)) for the

first few switching cycles, until the control loop operates correctly at a frequency higher

than fstarter.

●

The oscillator starts switching at a preheating frequency set by values of COSC, RRUN

and RPRE.

At shut-down (Figure 3-B), when the VCC decreases below the UVLO threshold (either in

case of mains removal or in case of fault):

●

All drivers are off;

●

EOI pin is discharged (the internal switch is on);

●

RF reference is disabled;

●

Tch is discharged.

AN2870 Datasheet(PDF) 8 Page - STMicroelectronics