Philips 7629, Service Manual

Page: 81 / 374

output power-, but also input voltage dependent. The higher the input voltage, the
smaller t
PRIM
, so the higher the frequency will be.
Figure:
Point P1 is the minimum frequency f
MIN
that occurs at the specified minimum input
voltage and maximum output power required by the application. Of course the minimum
frequency has to be chosen above the audible limit (>20 kHz).
Start-Up Sequence
When the rectified AC voltage V
IN
(via the center tap connected to pin 8) reaches the
Mains dependent operation level (Mlevel: between 60 and 100 V), the internal ‘Mlevel
switch’ will be opened and the start-up current source is enabled to charge
capacitorC2521 at the V
CC
pin as shown below.
The ‘soft start’ switch is closed when the V
CC
reaches a level of 7 V and the ‘soft start’
capacitor C
SS
(C2522, between pin 5 and the sense resistor R3526), is charged to 0.5
V.