SMH4811A(2000) 데이터 시트보기 (PDF) - Summit Microelectronics

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SMH4811A
(Rev.:2000)

Distributed Power Hot-Swap Controller

Summit Microelectronics 

SMH4811A

Output Slew-Rate Control

The SMH4811A 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

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 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 resistances of

UV and OV are 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 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.

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.

R1 is calculated from:

R1= vOV

IDMAX

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

2)

The minimum current that flows through the resis-

tive divider, IDMIN, is easily calculated from the ratio of

maximum and minimum supply voltages:

IDMIN

=

IDMAX × VSMIN

VSMAX

Therefore:

IDMIN

=

250µA ×

72V

36V

=

125µ A

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

R3 = VSMIN – VUV

IDMIN

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

therefore:

R3 = 36V – 2.5V = 268kΩ

125µ A

The closest standard 1% resistor value is 267kΩ

4) R2 may be calculated using:

R1+ R2 = VUV

IDMIN

or

R2

=







VUV

IDMIN







–

R1

so

R2

=







2.5V

125µ A







–

10kΩ

=

10kΩ

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 = 10kΩ

250µ A

2044 4.1 8/1/00

SUMMIT MICROELECTRONICS, Inc.

8

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