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2051 4.4 3/15/01

SMH4803A

Preliminary

SUMMIT MICROELECTRONICS, Inc.

Operating at High Voltages

The breakdown voltage of the external active and passive

components limits the maximum operating voltage of the

SMH4803A hot-swap controller. Components that must

be able to withstand the full supply voltage are: the input

and output decoupling capacitors, the protection diode in

series with the DRAIN SENSE pin, the power MOSFET

switch and the capacitor connected between its drain and

gate, the high-voltage transistors connected to the power

good outputs, and the dropper resistor connected to the

Over-Voltage and Under-Voltage Resistors

In the following examples, the three resistors, R1, R2, and

R3, connected to the OV and UV inputs, must be capable

of withstanding the maximum supply voltage of several

hundred volts. The trip voltage of the UV and OV inputs is

is very high, large value resistors can be used in the

resistive divider. The divider resistors should be high

stability, 1% metal-film resistors to keep the under-voltage

and over-voltage trip points accurate.

Telecom Design Example

A hot-swap telecom application may use a 48V power

supply with a –25% to +50% tolerance (

i.e., the 48V supply

can vary from 36V to 72V). The formulae for calculating

R1, R2, and R3 follow.

resistive network. The value of the current is arbitrary, but

it can't be too high (self-heating in R3 will become a

problem), or too low (the value of R3 becomes very large,

and leakage currents can reduce the accuracy of the OV

≥200µA

for the best accuracy at the OV and UV trip points. A value

calculations.

calculated by the formula:

OV

MAX

V

R1

ID

=

.

Substituting:

2.5V

R1

10k

250 A

µ

==

Next the minimum current that flows through the resistive

maximum supply voltage levels:

MAX

MIN

MIN

MAX

ID

VS

ID

VS

×

=

.

Substituting:

MIN

250 A 36V

ID

125 A

72V

µ

µ

×

==

.

MIN

UV

MIN

VS

V

R3

ID

−

=

.

36V

2.5V

R3

268k

125 A

µ

−

==

The closest standard 1% resistor value is 267k

Ω

Then R2 is calculated:

()

UV

MIN

V

R1 R2

ID

+=

,

or

UV

MIN

V

R2

R1

ID

=−

.

Substituting:

2.5V

R2

10k

20k

10k

10k

125 A

µ

=−

Ω =

Ω −

Ω =

An Excel spread sheet is available on Summit's website

(

www.summitmicro.com) to simplify the resistor value

calculations and tolerance analysis for R1, R2, and R3.

Dropper Resistor Selection

The SMH4803A is powered from the high-voltage supply

provide the SMH4803A (and its loads) with sufficient

operating current under minimum supply voltage condi-

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