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9.2.2.4

Overcurrent

Protect

Circuit

The

overcurrent

protect

circuit

monitors

the

+S V

dc

output

to

sense

an

undervoltage

condition,

and

the

dc

output

circuits

sensing

resistor

(R507)

potential

to

sense

overcurrent

condition

resulting

from

a

short

in

either

the

+5 V

or

+12

V

output

circuits.

The

overcurrent

protect

circuit

(Figure

9-8)

consists

of

the

following

circuits.

• Summing

network

(R514,

R523,

and

R524)

•

Voltage

comparator

components

(ES06

and

RS09)

The

sensing

resistor

potential,

+5.1

V

reference

voltage

input

from

the

pulse

width

modulator,

and

+S V

input

are

summed

to

provide

biasing

to

the

plus

input

of

the

comparator.

This

bias

is

reflected

in

the

output

from

the

comparator,

which

is

sent

to

the

pulse

width

modulator.

If

overcurrent

or

overvoltage

conditions

exist,

the

output

of

the

comparator

will

go

low,

caused

by

a

negative

value

present

at

the

comparator's

positive

input.

This

will

cause

the

pulse

width

modulator

to

output

a

minimum

pulse

width,

effectively

shutting

off

the

power

supply

to

prevent

further

damage.

FROM

+12V

START UP

R5D9

TO

r-7

_3"""3

....0 _

..

PULSE

WIDTH

MODULATOR

R514

15K

1%

I

5V

R524

16K

1%

R523

4871%

FROM

CURRENT

SENSING

RESISTOR

IR507,

+51

V

FRO~~!\

PULSE

WiDTH

MODULATOR -

__

-....,

Figure

9-8

Overcurrent

Protect

Circuit

Diagram

9-11

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