SMH4804 Datasheet, PDF (20/41 Pages) Summit Microelectronics, Inc. – -48V Programmable Hot Swap Sequencing Power Controller

Timing Diagram
SMH4804
Operating at High Voltages
The breakdown voltage of the external active and passive
components limits the maximum operating voltage of the
SMH4804 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
controller’s V_{DD pin.}
Over-Voltage and Under-Voltage Resistors
In Figures 21, 22, and 23, 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 V_SS. 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 formula for calculating R1,
R2, and R3 are as follows.
First, a peak current, ID_MAX, must be specified for the
resistive network. The value of the current is arbitrary, but it
cannot be too high (self-heating in R3 becomes 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 is used to illustrate the following calculations.
With V_OV (2.5V) being the over-voltage trip point, R1 is
calculated by the formula:
R1 =
V_OV
ID_MAX
Substituting:
R1 =
2.5V
250µA
= 10 KΩ
Next the minimum current that flows through the resistive
divider, ID_MIN, is calculated from the ratio of minimum and
maximum supply voltage levels_:
ID_MIN =
ID_MAX x VS_MIN
VS_MAX
Substituting:
250µA x 36V
ID_MIN =
2.5 V ^{= 125 µΑ}
Now the value of R3 is calculated from ID_MIN:
R3 =
VS_MIN x V_UV
ID_MIN
V_UV is the under-voltage trip point, also 2.5V. Substituting:
36V x 2.5V
R3 = _{125 µΑ} = 286 kΩ
The closest standard 1% resistor value is 267 kΩ
Then R2 is calculated:
(R1 + R2) =
V_UV
ID_MIN
or
R2 =
V_UV
ID_MIN
- R1
Substituting:
R2 =
2.5V
125 µΑ
- 10 kΩ = 20 kΩ − 10 kΩ = 10 kΩ
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 SMH4804 is powered from the high-voltage supply via
a dropper resistor, R_D. The dropper resistor must provide
the SMH4804 (and its loads) with sufficient operating
current under minimum supply voltage conditions, but must
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2050 3.7 10/30/02
Summit Microelectronics