Advanced Analog Technology, Inc.
AAT1109A
How to set the time constant for soft start and short circuit protection
1.Soft start
At power on, the AAT1109A operates in soft start mode. The capacitor Cscp connected to SCP pin starts
charging at a constant current. In the soft start mode, the voltage at SCP pin (VSCP) is input to the PWM
comparator, so that the ON duty of the OUT pin is controlled by the VSCP. On completion of soft start
operation, the voltage at the SCP pin stays low, the input from soft start circuit to the PWM comparator
stay high, and enter the short circuit protection waiting state.
Soft start time
T
s
(sec)
~ 0.84 × C
SCP
(uF)
2.Short circuit protection
If the switching regulator output suddenly drops due to loading effect , the error amplifier output is fixed at
VOM
+
and capacitor C
SCP
starts charging. When the voltage at the SCP pin reaches approximately 1.26V
,the output pin set low and SCP pin stays low.
Once the protection circuit operates, the circuit can be recovered by setting the power supply.
Short circuit detection time
T
scp
(sec)
~ 0.84 × C
SCP
(uF)
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Advanced Analog Technology, Inc
.
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V 0.1
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
