Advanced Analog Technology, Inc.
HOW TO SET THE TIME CONSTANT FOR SOFT-START AND SHORT
CIRCUIT PROTECTION
1. Soft-Start
AAT1109 operates in soft-start mode. The PWM comparator monitors the voltage at soft-start setting
and SCP pin, which varies proportionally to each other. This proportion relation controls the ON duty
of the OUT pin and activates the soft-start operation. 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 determined by theVSCP
.
When the soft-started mode is completed, its setting voltage remains high and the circuit enters the
output short circuit protection wait state while VSCP remains low.
Soft-Start Time
s
t
(
sec
)
~ 0.84 × C
µF
( )
SCP
2. Short Circuit Protection
In response to a sudden drop of the switching regulator output, the device locks the error amplifier, OE
pin, at VOS+ , and starts feeding currents into the capacitor at SCP pin to achieve in a voltage value of
approximately 0.84V at this pin. This chain reaction protects the circuit by keeping the SCP pin and
output pin low. Users could reverse this process by restarting the device.
Short Circuit Detection Time
t
(
sec
)
~ 0.84 × C
µF
( )
SCP
SCP
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