Philips 20PT643F Service Manual page 111

The system enters burst mode standby when the microprocessor activates the
'Stdby_con' line. When this line is pulled high, the base of Q7541 is allowed to go high.
This is triggered by the current from collector Q7542. When Q7541 turns 'on', the opto-
coupler (7515) is activated, sending a large current signal to pin 3 (Ctrl). In response to
this signal, the IC stops switching and enters a 'hiccup' mode. This burst activation
signal should be present for longer than the 'burst blank' period (typically 30 s): the
blanking time prevents false burst triggering due to spikes.
Burst mode standby operation continues until the microcontroller pulls the 'Stdby_con'
signal low again. The base of Q7541 is unable to go high, thus cannot turn 'on'. This will
disable the burst mode. The system then enters the start-up sequence and begins
normal switching behavior.
For a more detailed description of one burst cycle, three time intervals are defined:
t1: Discharge of V
transferred, which result in a ramp-up of the output voltage (V
stabilizer. When enough energy is stored in the capacitor, the IC will be switched 'off' by
a current pulse generated at the secondary side. This pulse is transferred to the primary
side via the opto coupler. The controller will disable the output driver (safe restart mode)
when the current pulse reaches a threshold level of 16 mA into the Ctrl pin. A resistor
R1 (R3519) is placed in series with the opto coupler, to limit the current going into the
Ctrl pin. Meanwhile the V
t2: Discharge of V
discharged to V
UVLO
t3: Charge of V
CC
is reached. The internal current source charges the V
capacitor is recharged). Once the V
driver is activated and a new burst cycle is started.
when gate drive is active. During the first interval, energy is
CC
capacitor is discharged but has to stay above V
CC
when gate drive is inactive. During the second interval, the V
CC
. The output voltage will decrease depending on the load.
when gate drive is inactive. The third interval starts when the UVLO
CC
capacitor (also the soft start
CC
capacitor is charged to the start-up voltage, the
) in front of the
STAB
UVLO
.
is
CC