HV302 参数 Datasheet PDF下载

HV302 / HV312

Design Information-

continued

The inrush current limit can be calculated as follows:

I

CL

=

V

SENSE-CL

50mV

=

R

SENSE

R

SENSE

The timing functions are defined by the following equations:

t

START

=

2.4

C

RAMP

I

RAMP

C

RAMP

I

RAMP

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

I

CL

=

50mV

=

1A

50mΩ

t

TH

=

V

GS( th )

Compensation components from gate to source of the external

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

t

POR

=

t

START

+

t

TH

t

RISE

≈

C

RAMP



I

R

g

fs



RAMP

−

SENSE



0.9I

R

FB

LIMIT



C

LOAD

I

LIMIT









t

LIMIT

≈

V

IN

t

PWRGD

−

A

=

V

INT

−

V

GS(LIMIT )

−

1.2

(

)

C

I

RAMP

RAMP

These equations assume that the load is purely capacitive and the

following definitions apply.

C

RAMP

is the external capacitor connected to the RAMP pin.

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 1kΩ resistor in series with the above

capacitor.

I

RAMP

is the output current from the RAMP pin, nominally

10µA, when the voltage drop on R

SENSE

resistor is zero.

V

INT

is the internally regulated supply voltage and can range

from 9V to 11V.

V

GS(th)

is the gate threshold voltage of the external pass

transistor and may be obtained from its datasheet.

V

GS(limit)

is the external pass transistor gate-source voltage

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.

g

fs

is the transconductance of the external pass transistor and

may be obtained from its datasheet.

R

FB

is the internal feedback resistor and is nominally 5KΩ.

I

LIMIT

is the load current when the voltage drop on R

SENSE

resistor is 50mV.

These equations may be used to calculate the minimum value of

C

RAMP

for the most critical system performance characteristics.

For maximum contact bounce duration protection choose a value

for t

POR

and use the following equation:

C

RAMP

=

t

POR

×

I

RAMP

2.4

+

V

GS(limit )

Servo Mode Timing

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

then choose a value for t

PWRGD-A

and use the following equation:

C

RAMP

=

t

PWRGD

−

A

×

I

RAMP

V

INT

−

V

GS(limit )

−

1.2

8

Rev. D

04/17/02

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