7

Rev. D

04/17/02

HV302 / HV312

Programming Current Limit and Circuit Breaker

Feedback Capacitor Mode Operation

In this operating mode the circuit breaker trip current and the

inrush current limit can be independently programmed. In fact the

Ω.

The circuit breaker will trip in less than 5

µs when the voltage on

the sense resistor may be calculated from the following equation:

CB

CB

CB

-

SENSE

SENSE

I

100mV

I

V

R

=

=

For an 8A circuit breaker:

12.5mΩ

8A

100mV

R

SENSE

=

=

The power rating of the sense resistor must be greater than or

The following diagrams depict the equivalent circuitry to clarify the

feedback capacitor operation for programming the inrush current

limit.

The inrush current limit may be programmed as follows:

Calculate

A

5

.

2

k

5

m

5

.

12

A

1

k

5

R

I

SENSE

CL

SINK

µ

=

Ω

Ω

×

=

Ω

×

=

I

Ω,

µA.

Calculate C2 (feedback capacitor) discharge current

A

5

.

7

A

5

.

2

A

10

I

A

10

I

SINK

2

C

µ

=

µ

−

µ

=

−

µ

=

A

6

.

1

M

5

.

2

V

4

R

V

I

DISABLE

t

AUTO

µ

=

Ω

=

=

Where Vt is the maximum threshold voltage of the MOSFET.

AUTO

SINK

2

C

I

I

A

10

I

−

−

µ

=

A

9

.

5

A

6

.

1

A

5

.

2

A

10

I

2

C

µ

=

µ

−

µ

−

µ

=

In this example we assume that Auto-Retry is enabled and

µA.

Note that

dt

dv

2

C

2

C

×

=

I

and

dt

dv

C

LOAD

CL

×

=

I

dt

dv

dt

dv

across C2 as they share a common node

and their other terminals are at fixed voltages during inrush current

limiting. Therefore,

LOAD

CL

2

C

C

I

2

C

I

=

or

CL

LOAD

2

C

I

C

I

2

×

=

C

.

As previously calculated and by conservation of charge on RAMP

µF the required value for C2 can be

calculated.

Therefore

nF

75

.

0

A

1

F

100

A

5

.

7

I

C

I

2

CL

LOAD

2

C

=

µ

×

µ

=

×

=

C

Note that during initial power application the RAMP pin is voltage

Servo Control Mode Operation

The circuit breaker will trip in less than 5

µs when the voltage on

the sense resistor may be calculated from the following equation:

CB

CB

CB

-

SENSE

SENSE

I

100mV

I

V

R

=

=

For a 2A circuit breaker:

50mΩ

2A

100mV

R

SENSE

=

=

The power rating of the sense resistor must be greater than or

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