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| NCP1252CDR2G Datasheet, PDF (12/18 Pages) ON Semiconductor – Current Mode PWM Controller for Forward and Flyback Applications | |||
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VCC pin
VCC(on)
BO pin
VBO
SS pin
DRV pin
NCP1252
VCC pin
VCC(on)
Time
SS capacitor is
discharged
Time
Time
BO pin
VBO
SS pin
DRV pin
Soft start
Time
Time
Time
CASE #3
Time
Figure 31. Controller Shuts Down with the Brown Out Pin
CASE #4
Time
When the BO pin is grounded, the controller is shut down
and the SS pin is internally grounded in order to discharge
the soft start capacitor connected to this pin (Case #3). If the
BO pin is released, when its level reaches the VBO level a
new soft start sequence happens.
Soft Start:
As illustrated by the following figure, the rising voltage on
the SS pin voltage divided by 4 controls the peak current
sensed on the CS pin. Thus as soon as the CS pin voltage
becomes higher than the SS pin voltage divided by 4 the
driver latch is reset.
Clock
Rcomp
Rse nse
CS
LEB
Soft Start
Status
Vd d
Fixe d
Iss
Delay
120 ms
UVLO
+
â
SS
Soft start
1/4
Grand Reset
S
Q
Q
R
DRV
Figure 32. Soft Start Principle
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