SMH4803A
Preliminary
APPLICATIONS
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
controller’s V
DD
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 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 formulae for calculating
R1, R2, and R3 follow.
First a peak current, ID
MAX
, 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 ID
MAX
should be
≥200µA
for the best accuracy at the OV and UV trip points. A value
of 250µA for ID
MAX
will be used to illustrate the following
calculations.
With V
OV
(2.5V) being the over-voltage trip point, R1 is
calculated by the formula:
V
R1
=
OV
.
ID
MAX
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
×
VS
MIN
.
VS
MAX
Substituting:
ID
MIN
=
250
µ
A
×
36V
=
125
µ
A
.
72V
Now the value of R3 is calculated from ID
MIN
:
R3
=
VS
MIN
−
V
UV
.
ID
MIN
V
UV
is the under-voltage trip point, also 2.5V. Substituting:
R3
=
36V
−
2.5V
=
268k
Ω
.
125
µ
A
The closest standard 1% resistor value is 267kΩ
Then R2 is calculated:
(
R1
+
R2
)
=
or
R2
=
V
UV
ID
MIN
,
V
UV
−
R1
.
ID
MIN
Substituting:
R2
=
2.5V
−
10k
Ω =
20k
Ω −
10k
Ω =
10k
Ω
.
125
µ
A
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
via a dropper resistor, R
D
. The dropper resistor must
provide the SMH4803A (and its loads) with sufficient
operating current under minimum supply voltage condi-
Substituting:
R1
=
2.5V
=
10k
Ω
.
250
µ
A
SUMMIT MICROELECTRONICS, Inc.
2051 4.4 3/15/01
11
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
