SMH4814 Datasheet, PDF (21/44 Pages) Summit Microelectronics, Inc. – Dual Feed Active-ORing Programmable Hot Swap Controller

APPLICATIONS INFORMATION (CONTINUED)
SMH4814
Preliminary Information
Operating at High Voltages
The breakdown voltage of the external active and
passive components limits the maximum operating
voltage of the SMH4814 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 Figure 15A, 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 resistor values should be chosen
so that the UV or OV input reaches its corresponding
trip point (Vuv or Vov) when the incoming power feed
reaches its low or high operational limit. 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 (2.864V) being the over-voltage trip point, R1
is calculated by the formula:
V
R1 =
OV
ID
MAX
Substituting:
R1 =
2.864V
250_µA
= 11.46
k_Ω
The closest standard 1% resistor value is 11.8kΩ
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_{M IN} =
ID_MAX x VS_MIN
VS_{MA X}
Substituting:
ID _{M IN} =
250_µA x 36V
72 V
=
125
µΑ
Now the value of R3 is calculated from ID_MIN:
R3
=
VS_MIN x V_{U V}
I D_{M I N}
V_UV is the under-voltage trip point, also 2.864V.
Substituting:
36V x 2.864V
R3 = _{125 µΑ} = 825 k_Ω
The closest standard 1% resistor value is 825kΩ
Then R2 is calculated:
V
(R1 + R2) =
UV
ID
MIN
Or
V
R2 =
UV
I D_{M I N}
- R1
Substituting:
R2 = _{122.856µΑ}^4V- 11.8 k _{Ω =} 20 k_{Ω −} 10 k_{Ω =} 11 k_Ω
An Excel spread sheet is available at:
(http://www.summitmicro.com/) or contact Summit
to simplify the resistor value calculations and tolerance
analysis for R1, R2, and R3.
Summit Microelectronics, Inc
2080 2.0 07/21/05
21