SMH4812_09 Datasheet, PDF (12/18 Pages) Summit Microelectronics, Inc. – Distributed Power Hot-Swap Controller

SMH4812
Preliminary
The min/max current limits are easily met using the drop-
per resistor, except in circumstances where the input
voltage may swing over a very wide range (e.g., input
varies between 20V and 100V). In these circumstances it
may be necessary to add an 11V zener diode between
V_DD and V_SS to handle the wide current range. The zener
voltage should be below the nominal regulation voltage of
the SMH4812 so that it becomes the primary regulator.
MOSFET V_DS(ON) Threshold
The drain sense input on the SMH4812 monitors the
voltage at the drain of the external power MOSFET switch
with respect to V_SS. When the MOSFET’s V_DS is below the
user-defined threshold the MOSFET switch is considered
to be ON. The V_DS(ON)_THRESHOLD is adjusted using the
resistor, R_T, in series with the drain sense protection
diode. This protection, or blocking, diode prevents high
voltage breakdown of the drain sense input when the
MOSFET switch is OFF. A low leakage MMBD1401 diode
offers protection up to 100V. For high voltage applications
(up to 500V) the Central Semiconductor CMR1F-10M
diode should be used. The V_DS(ON)_THRESHOLD is calcu-
lated from:
( ) ( ) V_DS ON _THRESHOLD = V_SENSE − I_SENSE × R_T − V_DIODE ,
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:
The dropper resistor value should be chosen such that the
minimum and maximum I_DD and V_DD specifications of the
SMH4812 are maintained across the host supply’s valid
operating voltage range. First, subtract the minimum V_DD
of the SMH4812 from the low end of the voltage, and divide
by the minimum I_DD value. Using this value of resistance
as R_D 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 I_DD current
allowed. If some range of supply voltage is chosen that
would cause the maximum I_DD specification to be violated,
then an external zener diode with a breakdown voltage of
≈12V should be used across V_DD.
As an example of choosing the proper R_D value, assume
the host supply voltage will range from 36 to 72V. The
largest dropper resistor that can be used is: (36V-11V)/
3mA = 8.3kΩ. Next, confirm that this value of R_D also
works at the high end: (72V-13V)/8.3kΩ = 7.08mA, which
is less than 10mA.
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 SMH4812 is configured to turn on –
48V to a DC/DC converter 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 will be
disabled. Ideally, there would be a secondary-side super-
visor similar to the SMS44 that would have its reset time-
out period programmed to be less than 1.6ms. After the
supply turns on the RESET# output of the SMS44 would
be released and FS# pulled high. However, if for any
reason the supply doesn't turn on, the RESET# will not be
released and the SMH4812 will disable the PG output.
( ) 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 resis-
tance.
2055 4.1 03/27/09
SUMMIT MICROELECTRONICS, Inc.
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