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SMH4803

Distributed Power Hot-Swap Controller

Summit Microelectronics 

SMH4803

after another delay PGD. The delays built into the

SMH4803 allow correct sequencing of power to the loads,

e.g. +3V supply must come up before +5V supply. The

delay times are factory programmed. PG2# and PG3#

can be disabled using the ENPGA and ENPGB inputs.

When these inputs are low they override the enable

function produced when the SMH4803 sees a power

good condition.

The PG1#, PG2#, and PG3# outputs have a 12V with-

stand capability so high voltages must not be connected

to these pins. Inexpensive bipolar transistors will boost

the withstand voltage to that of the host supply, see figure

5 for connections.

Output Slew-Rate Control

The SMH4803 provides a current limited Vgate turn-on.

A fast turn-off is performed by internally shorting Vgate to

Vss. Changing the passive components around the

power MOSFET switch will modify the turn-on slew-rate.

Operating at High Voltages

The breakdown voltage of the external active and passive

components limits the maximum operating voltage of the

SMH4803 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 DrainSense pin, the power MOSFET switch

and capacitor connected between its drain and gate, the

high-voltage transistors connected to the power good

outputs, 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 which can

be several hundred volts. The trip voltage of the UV and

OV inputs is +2.5V relative to Vss. As the input resis-

tances of UV and OV are very high, high 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 uses 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 are shown below.

R1 is calculated from:

Vov

R1 = ID max

VOV is the over-voltage trip point, i.e. 2.5V, therefore:

R1 = 2.5V =10kΩ

250 µA

2) The minimum current that flows through the resistive

divider, IDmin, is easily calculated from the ratio of

maximum and minimum supply voltages:

ID min = ID max x VS min

VS max

Therefore:

ID min = 250 µA x 36V = 125 µA

72V

3) The value of R3 is now calculated using IDmin.

R3 = (VS min – Vuv)

ID min

Where Vuv is the under-voltage trip point, also 2.5V,

therefore:

R3 = (36V – 2.5V) = 268kΩ

125 µA

The closest standard 1% resistor value is 267kΩ

4) R2 may be calculated using:

(R1 + R2) = Vuv

ID min

R2 =

Or

R2 =

Vuv

ID min

2.5V

125µA

–R1

–10kΩ = (20kΩ – 10kΩ) = 10kΩ

1) First select the peak current, IDmax, allowed through

the resistive divider, say 250µA. The value of current

is arbitrary; however, if the current is too high, self-

heating in R3 may become a problem (especially in

high voltage systems), and if the current is too low the

value of R3 becomes very large and may be expensive

at 1% tolerance.

SUMMIT MICROELECTRONICS

11

2041 8.4 6/15/00

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