SMH4802 Datasheet, PDF (13/20 Pages) Summit Microelectronics, Inc. – Programmable -48V Hot-Swap Controller with Forced Shut Down

SMH4802
Preliminary Information
APPLICATIONS INFORMATION (Continued)
where VDIODE is the forward voltage drop of the protection
diode. The VDS(ON)THRESHOLD varies over temperature
due to the temperature dependence of VDIODE and ISENSE.
The calculation below gives the VDS(ON)THRESHOLD under
the worst case condition of 85°C ambient. Using a 68kΩ
resistor for RT gives:
V_DS ( ) ON _THRESHOLD = 2.5V – (15µA × 68kΩ) – 0.5V = 1V
The voltage drop across the MOSFET switch and sense
resistor, VDSS, is calculated from:
V_DSS = I_D (R_S × R_ON )
where ID is the MOSFET drain current, RS is the circuit
breaker sense resistor and RON is the MOSFET on
resistance.
The dropper resistor value should be chosen such that the
minimum and maximum IDD and VDD specifications of the
SMH4802 are maintained across the host supply’s valid
operating voltage range. First, subtract the minimum VDD
of the SMH4802 from the low end of the voltage, and divide
by the minimum IDD value. Using this value of resistance
as RD find the operating current that would result from
running at the high end of the supply voltage to verify that
the resulting current is less than the maximum IDD current
allowed. If some range of supply voltage is chosen that
would cause the maximum IDD specification to be vio-
lated, then an external zener diode with a breakdown
voltage of 11V should be used across VDD.
As an example of choosing the proper RD value, assume
the host supply voltage ranges from 36 to 72V. The largest
dropper resistor that can be used is: (36V-11V)/3mA =
8.3kΩ. Next, confirm that this value of RD also works at
the high end: (72V-13V)/8.3kΩ = 7.08mA, which is less
than 8mA.
The FS# input can also be used in conjunction with a
secondary-side supervisory circuit providing a positive
feedback loop during the power up sequence. As an
example, assume the SMH4802 is configured to turn on
–48V to three DC/DC converters and then sequentially
turn on the converters with a 1.6ms delay. Further,
assume all of the enable inputs are true and PG# has just
been sequenced on. If FS# option 4 (100BIN in register
5) has been selected, then FS# must be driven high within
1.6ms after PG# goes low, otherwise the PG# output is
disabled.
supply turns on, the RESET# output of the SMS44 would
be released and FS# pulled high. However, if for any
reason not all of the supplies turn on, RESET# is not
released and the SMH4802 disables the PG# output.
Soft Start Slew Rate Control
The –48V turn on time is controlled by the SMH4802 and
by the values of R8, C1 and C3 in Figure 9. The turn on
time is approximately 10ms with the component values
shown. Increasing the capacitance reduces the output
slew rate and increases the turn on time. The capacitors
prevent the MOSFET from turning on simultaneously with
the application of –48V. Resistor R8 is specified to limit
the current into and the rate of charge of C1. The ratio of
C1 to C3 (20:1) limits the MOSFET’s VGS to approximately
2V once the –48V supply is connected and C1 is fully
charged.
Ideally, there would be a secondary-side supervisor
similar to the SMS44 that would have its reset time-out
period programmed to be less than 1.6ms. After the last
SUMMIT MICROELECTRONICS, Inc.
2062 2.3 6/19/03
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