8

Rev. D

04/17/02

HV302 / HV312

The inrush current limit can be calculated as follows:

SENSE

SENSE

CL

-

SENSE

CL

R

50mV

R

V

I

=

=

Thus the inrush current limit for a 2A circuit breaker:

1A

50mΩ

50mV

I

CL

=

=

Compensation components from gate to source of the external

MOSFET may be required to reduce peaking of the inrush current.

Compensation can be accomplished as follows:

1.

Start with a 2nF capacitor from gate to source.

2.

Increase capacitor value up to 10nF if needed.

3.

If needed, add a 1k

Ω resistor in series with the above

capacitor.

Servo Mode Timing

The timing functions are defined by the following equations:

RAMP

RAMP

START

I

C

4

.

2

t

=

RAMP

RAMP

)

th

(

GS

TH

I

C

V

t

=

TH

START

POR

t

t

t

+

=













−

≈

FB

SENSE

LIMIT

RAMP

fs

RAMP

RISE

R

R

I

9

.

0

I

g

C

t

LIMIT

LOAD

IN

LIMIT

I

C

V

t

≈

()

RAMP

RAMP

)

LIMIT

(

GS

INT

A

PWRGD

I

C

2

.

1

V

V

t

−

−

=

−

These equations assume that the load is purely capacitive and the

following definitions apply.

10

from 9V to 11V.

transistor and may be obtained from its datasheet.

required to obtain the limit current. It is dependent on the

pass transistor’s characteristics and may be obtained from

the transfer characteristics on the transistor datasheet.

may be obtained from its datasheet.

Ω.

resistor is 50mV.

These equations may be used to calculate the minimum value of

For maximum contact bounce duration protection choose a value

)

it

(lim

GS

RAMP

POR

RAMP

V

4

.

2

I

t

C

+

×

=

If control of PWRGD active delay is the critical system parameter,

2

.

1

V

V

I

t

C

)

it

(lim

GS

INT

RAMP

A

PWRGD

RAMP

−

−

×

=

−

Supertex, Inc. 1235 Bordeaux Drive, Sunnyvale, CA 94089 TEL: (408) 744-0100 Fax: (408) 222-4895 www.supertex.com