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DI-101

B

11/05

Power Integrations

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not assume any liability arising from the use of any device or circuit described herein. POWER INTEGRATIONS MAKES NO WARRANTY HEREIN AND

SPECIFICALLY DISCLAIMS ALL WARRANTIES INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A

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Design Formulae

Component values can be calculated according to the following

formulae and parameters:

Design parameters were selected as follows:

V

Input under-voltage on-threshold

V

Default under-voltage off-

threshold

V

Diode drop for PoE reverse

protection input diodes

R1 = 10 kΩ

This value is assumed

From the DPA-Switch data sheet we have the following:

I

L-pin current for UV turn on

I

L-pin current for UV turn off

V

L-pin voltage at I

V

Control-pin voltage

We can assume:

V

Transistor base-emitter voltage

V

80% of the max reverse base-

emitter voltage

β = 100

Transistor minimum current gain

Bias Voltage (V

Resistor values R

Key Design Points

• Use a 1% resistor for R

for the turn-on/turn-off thresholds.

• The over-voltage threshold is fixed at I

and can be calculated according to the formula above

(V

• The bias voltage (V

reverse base emitter voltage V

bias voltage minus V

5 V (the transistor rating).

• For PoE applications, make sure to include voltage drops

ofinputdiodes(V

R

• Note: Due to the L-pin synchronization function (with a

1 V threshold), resistor R2 should be sufficiently large to

prevent Q1 turn-on below L-pin voltage 1 V.

R

I

V

V

V

_

_

LS

UV OFF

UV OFF

L

DIN

=

-

-

R

V

V

V

R

V

V

(

)

(

)

BIAS

L

Q BE REV

Q BE REV

L

2

1

1

1

$

1

-

-

+

^

h

For design verification:

V

I

R

V

V

R

V

V

_

_

(

)

UV ON

UV ON

L

Q BE

LS

L

DIN

3

1

$

=

+

-

+

+

c

m

V

I

R

V

V

_

_

UV OFF

UV OFF

LS

L

DIN

$

=

+

+

V

I

R

V

V

_

_

OV OFF

OV OFF

LS

DIN

L

$

=

+

+

R

V

I

R

V

V

V

V

R

_

_

(

)

UV ON

UV ON

LS

L

DIN

L

Q BE

LS

3

1

$

$

=

-

-

-

-

^

h

V

R

R

V

R

V

(

)

Q BE REV

BIAS

L

1

1

2

2

$

=

+

-