SMH4812
Preliminary
APPLICATIONS
Operating at High Voltages
The breakdown voltage of the external active and passive
components limits the maximum operating voltage of the
SMH4812 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 se-
ries 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 to high (self-heating in R3 will become a
problem), or too low (the value of R3 becomes very large,
and R3 becomes very expensive). To set the calculations
a nominal value of 250µA will be assumed.
With V
OV
(2.5V) being the over-voltage trip point, R1 is
calculated by the formula:
R1
=
V
OV
ID
MAX
.
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
Dropper Resistor Selection
The SMH4812 is powered from the high-voltage supply
via a dropper resistor, R
D
. The dropper resistor must
provide the SMH4812 (and its loads) with sufficient oper-
ating current under minimum supply voltage conditions,
but must not allow the maximum supply current to be
exceeded under maximum supply voltage conditions.
The dropper resistor value is calculated from:
Substituting:
R1
=
2.5V
=
10k
Ω
.
250
µ
A
R
D
=
VS
MIN
−
V
DD
MAX
I
DD
+
I
LOAD
,
Next the minimum current that flows through the resistive
divider, ID
MIN
, is calculated from the ratio of minimum and
maximum supply voltage levels:
where VS
MIN
is the lowest operating supply voltage,
V
DDMAX
is the upper limit of the SMH4812 supply voltage,
I
DD
is minimum current required for the SMH4812 to
operate, and I
LOAD
is any additional load current from the
2.5V and 5V outputs and between V
DD
and V
SS
.
11
SUMMIT MICROELECTRONICS, Inc.
2055 4.1 03/27/09
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
