Advanced Analog Technology, Inc.

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Page 5 of 17

V 1.0

AAT1110

How to Set the Time Constant for Soft-Start and Short Circuit Protection

1. Soft-Start

At power on, the AAT1110 operates in soft-start mode. The capacitor

SCP

C

connected to SCP pin starts

V

) is input to the PWM

V

. At completion of the soft-start

operation, the voltage at the SCP pin stays low; the input from soft-start circuit to the PWM comparator

stays high, and enters the short circuit protection waiting state.

Soft-Start Time

)

F

(

C

84

.

0

~

(sec)

t

SCP

s

µ

×

2. Short Circuit Protection

If the switching regulator output suddenly drops due to loading effect, the error amplifier output will be

fixed at

+

OM

V

and capacitor

SCP

C

will start charging. When the voltage at the SCP pin reaches

approximately 1.26V, the output pin will set low and SCP pin will remain low.

Once the protection circuit starts operating, the circuit can be recovered by setting the power supply.

Short Circuit Detection Time

)

F

(

C

84

.

0

~

(sec)

t

SCP

scp

µ

×