SMH4812
Preliminary
APPLICATIONS
Operating at High Voltages
IDMAX × VSMIN
IDMIN
Substituting:
IDMIN
=
=
Thebreakdownvoltageoftheexternalactiveandpassive
components limits the maximum operating voltage of the
SMH4812hot-swapcontroller. Componentsthatmustbe
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 VDD pin.
.
VSMAX
250µA × 36V
= 125µA
.
72V
Now the value of R3 is calculated from IDMIN
:
VSMIN − V
UV
R3 =
.
IDMIN
Over-Voltage and Under-Voltage Resistors
V
UV istheunder-voltagetrippoint,also2.5V. Substituting:
Inthefollowingexamples, thethreeresistors, 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
2.5VrelativetoVSS. AstheinputimpedanceofUVandOV
is very high, large value resistors can be used in the
resistive divider. The divider resistors should be high
stability,1%metal-filmresistorstokeeptheunder-voltage
and over-voltage trip points accurate.
36V − 2.5V
125µA
R3 =
= 268kΩ
.
The closest standard 1% resistor value is 267kΩ
Then R2 is calculated:
VUV
R1+R2 =
(
)
,
IDMIN
Telecom Design Example
A hot-swap telecom application may use a 48V power
supplywitha–25%to+50%tolerance(i.e.,the48Vsupply
can vary from 36V to 72V). The formulae for calculating
R1, R2, and R3 follow.
or
V
UV
R2 =
−R1
.
IDMIN
First a peak current, IDMAX, must be specified for the
resistivenetwork. Thevalueofthecurrentisarbitrary, 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,
andR3becomesveryexpensive). Tosetthecalculations
a nominal value of 250µA will be assumed.
Substituting:
2.5V
R2 =
−10kΩ = 20kΩ −10kΩ = 10kΩ
.
125µA
Dropper Resistor Selection
The SMH4812 is powered from the high-voltage supply
via a dropper resistor, RD. 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.
With VOV (2.5V) being the over-voltage trip point, R1 is
calculated by the formula:
VOV
R1 =
.
IDMAX
Substituting:
The dropper resistor value is calculated from:
2.5V
−
VSMIN
V
DDMAX
R1 =
= 10kΩ
.
=
RD
250µA
,
+
IDD ILOAD
Next the minimum current that flows through the resistive
divider, IDMIN, is calculated from the ratio of minimum and
maximum supply voltage levels:
where VSMIN is the lowest operating supply voltage,
VDDMAX is the upper limit of the SMH4812 supply voltage,
IDD is minimum current required for the SMH4812 to
operate, and ILOAD is any additional load current from the
2.5V and 5V outputs and between VDD and VSS
.
SUMMIT MICROELECTRONICS, Inc.
2055 4.0 12/22/00
11
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
