CS51031

http://onsemi.com

8

The fault time is the sum of the slow discharge time the

fast discharge time and the recharge time. It is dominated by

the slow discharge time.

The first parameter is the slow discharge time, it is the time

for the CS capacitor to discharge from 2.4 V to 1.5 V and is

given by:

tSlowDischarge(t) +

CS

(2.4 V

* 1.5 V)

IDischarge

tSlowDischarge(t) + CS

1.5

105

The fast discharge time occurs when a fault is first

detected. The CS capacitor is discharged from 2.5 V to 2.4 V.

tFastDischarge(t) +

CS

(2.5 V

* 2.4 V)

IFastDischarge

tFastDischarge(t) + CS

1515

The recharge time is the time for CS to charge from 1.5 V

to 2.5 V.

tCharge(t) +

CS

(2.5 V

* 1.5 V)

ICharge

tCharge(t) + CS

3787

The fault time is given by:

tFault + CS

(3787

) 1515 ) 1.5

105)

tFault + CS

(1.55

105)

For this circuit

tFault + 0.1

10*6

1.55

105

+ 15.5 ms

A larger value of CS will increase the fault time out time

but will also increase the soft start time.

9) Input Capacitor

The input capacitor reduces the peak currents drawn from

the input supply and reduces the noise and ripple voltage on

output short circuit. A low ESR capacitor of at least 100

µF

is good. A ceramic surface mount capacitor should also be

noise.

10) MOSFET Selection

depends on the operating conditions but for input voltages

below 7.0 V a logic level FET should be used.

than the maximum output current is required.

Source Breakdown Voltage should be chosen based on the

input supply voltage.

The power dissipation due to the conduction losses is

given by:

PD + IOUT2

RDS(ON)

D

where

RDS(ON) is the value at TJ + 100°C

The power dissipation of the PFET due to the switching

losses is given by:

PD + 0.5

VIN

IOUT

(tr)

fSW

11) Diode Selection

The flyback or catch diode should be a Schottky diode

because of it’s fast switching ability and low forward voltage

drop. The current rating must be at least equal to the

maximum output current. The breakdown voltage should be

at least 20 V for this 12 V application.

The diode power dissipation is given by:

PD + IOUT

VD

(1.0

* D min)