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Fabricante

SMH4811A

Distributed Power Hot-Swap Controller

Summit Microelectronics 

SMH4811A

Preliminary

APPLICATIONS

Operating at High Voltages

The breakdown voltage of the external active and passive

components limits the maximum operating voltage of the

SMH4811A 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 output, and the dropper resistor connected to the

controller’s VDD pin.

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

2.5V relative to VSS. As the input impedance of UV and OV

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.

First a peak current, IDMAX, must be specified for the

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

and UV trip points). The value of IDMAX should be ≥200µA

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

of 250µA for IDMAX will be used to illustrate the following

calculations.

IDMIN

=

IDMAX × VSMIN

VSMAX

.

Substituting:

IDMIN

=

250µA × 36V

72V

=

125µ A

.

Now the value of R3 is calculated from IDMIN:

R3

=

VSMIN − VUV

IDMIN

.

VUV is the under-voltage trip point, also 2.5V. Substituting:

R3

=

36V − 2.5V

125µ A

=

268kΩ

.

The closest standard 1% resistor value is 267kΩ

Then R2 is calculated:

(R1+ R2)

=

VUV

IDMIN

,

or

R2

=

VUV

IDMIN

− R1.

Substituting:

R2

=

2.5V

125µ A

−

10kΩ

=

20kΩ

−

10kΩ

=

10kΩ

.

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.

With VOV (2.5V) being the over-voltage trip point, R1 is

calculated by the formula:

R1 =

VOV

IDMAX

.

Substituting:

Dropper Resistor Selection

The SMH4811A is powered from the high-voltage supply

via a dropper resistor, RD. The dropper resistor must

provide the SMH4811A (and its loads) with sufficient

operating current under minimum supply voltage condi-

tions, but must not allow the maximum supply current to be

exceeded under maximum supply voltage conditions.

R1 =

2.5V

250µ A

=

10kΩ

.

Next the minimum current that flows through the resistive

divider, IDMIN, is calculated from the ratio of minimum and

maximum supply voltage levels:

The dropper resistor value is calculated from:

RD

=

VSMIN − VDDMAX

IDD + ILOAD

,

10

2044 6.1 2/8/01

SUMMIT MICROELECTRONICS, Inc.

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