ICE1QS01
Version 1.4 7 27 Apr 2004
Soft start
The internal reference of the IC is switched on when V
CC
exceeds the V
CCon
threshold. The IC
begins to work with soft start mode. Soft start is realized with an internal soft start resistor, an inter-
nal current sink, a current source and the external feedback capacitor connected at pin SRC. The
internal resistor is connected between the internal voltage reference and pin SRC. The current sink
is connected between pin SRC and GND. The value of the current is set with a timer. Immediately
after the IC is switched on the capacitor C
SRC
is charged with a current source up to 2.5V. This cur-
rent source is switched off 12 µsec after beginning of soft start. The current value of the current sink
is set with a timer. Every three msec the current of the current sink is reduced and so V
SRC
can
increase stepwise. The soft start is finished 24 msec after the IC is switched on. At the end of the
soft start the current sink is switched off.
Figure: Soft Start
PCS (primary current simulation)
A voltage proportional to the current of the power transistor is generated at Pin PCS by the RC-com-
bination R2, C2. The voltage at Pin PCS is forced to 1.5V when the power transistor is switched off
and during its switch on time C2 is charged by R2 from the rectified mains. The relation of V
PCS
and
ton tp1 tp2
VCCon
VSRC1
VSRC2
VSRC
VCC
t
t
20k
pin SRC
current
sink
up
down
counter
D/A
timer
tp=3ms
2.5V
ICE1QS01
timer
t=12us
500
5V
timer
t=24ms
