SMH4803
sequenced by pre-programmed delays. If a sustained
over-current condition occurs during or after the insertion
process, then Vgate is shorted to Vss and the MOSFET
switch is turned off to protect the host supply.
DEVICE OPERATION
Power-Up Sequence
The SMH4803 is an integrated power controller for hot
swappable add-in cards. The device operates from a
singlesupplyrangingfrom20Vto500Vandgeneratesthe
signals necessary to drive isolated output DC/DC con-
verters.
Circuit Breaker Operation
The SMH4803 provides a circuit breaker function to
protect against over current conditions. A sustained over-
current event could damage the host supply and/or the
load circuitry. The board’s load current passes through a
series resistor connected between MOSFET source/
CBSense and Vss on the controller. The breaker will trip
whenever the voltage drop across the series resistor is
greater than 50mV for more than tCBD, and will trip
The SMH4803 hot-swap controller provides under-volt-
age and over-voltage monitoring of the host power sup-
ply, it drives an external power MOSFET switch that
connects the supply to the load. It also protects against
over-currentconditionsthatmightdisruptthehostsupply.
instantaneously if the voltage drop exceeds VQCB
.
When the input and output voltages to the SMH4803
controller are within specification, the SMH4803 provides
three“PowerGood”logicoutputsthatmaybeusedtoturn
ON loads or drive an LED status light. The SMH4803
provides three separate “Power Good” logic outputs that
activate loads in a programmable timed sequence. There
is a master enable/temperature sense input and 2.5V and
5V reference outputs for expanding monitor functions.
There are two “Power Good” enable inputs that may be
used to activate or deactivate output loads, and duty-
cycle or reset over-current protection modes to provide
automatic or manual restart of the controller after over-
current load conditions.
When the breaker trips, the Vgate output is turned off and
CBFault# will be driven LO. The circuit breaker can be
reset by taking CBReset# LO and then back HI when the
circuit breaker is in the reset mode. In the duty-cycle
mode,thecircuitbreakerresetsautomaticallyafterafixed
time period. If the over current condition still exists after
reset, thecircuitwillre-trip. Inbothoperatingmodesofthe
circuit breaker, the MOSFET can be switched off by
holding the CBReset input LO.
The value of the over-current shunt resistor is determined
by the following formula: Rs = 50mV/Ioc where Rs is the
value of the shunt resistor and Ioc is the over current limit
determined by the board’s power requirement or the limit
of the host supply.
Insertion Process
As the add-in board is inserted into the backplane, physi-
cal connections must be made with the chassis to dis-
charge any electrostatic voltage potentials. The board
then contacts the long pins on the backplane that provide
power and ground. As soon as power is applied the
SMH4803startsupbutdoesnotimmediatelyapplypower
to the output load. Under-voltage and over-voltage cir-
cuits inside the controller check to see if the input voltage
is within a user-specified range, and pin detection signals
determine whether the card is seated properly.
Current Sense Resistors
Current sense resistors are available from a number of
sources and come in two basic formats: open air sense
resistors and current sense resistor chips. The open air
resistorsaremetalstripsthatareavailableasboththru-hole
and surface mount. The resistor chips are surface mount
and offer excellent thermal characteristics. Both styles are
available in resistance ranges from 3 milliohm to 1 ohm.
IRC (www.irctt.com) is one source for these resistors. The
open air sense resistors can be found in their OARS series,
and the chip resistors are found in their LRC series.
tPDD after these requirements are met, the hot-swap
controller enables Vgate to turn on the power MOSFET
switch. The Vgate output is current limited to IVGATE
,
Load Control
allowing the slew rate to be easily modified using external
passive components. During the controlled turn-on pe-
riod, the Vds of the MOSFET is monitored by the drain
sense input. When Vds drops below a user-specified
voltage the power output is considered to be ON. The
resistor and diode in series with the drain sense input
determine Vds(ON).
The SMH4803 is designed to control three or more DC/
DCconverters,orotherloads,whichincorporateON/OFF
control. The Power Good outputs activate the loads when
the following conditions have been met: the input voltage
to the SMH4803 monitored by UV and OV is within user-
defined limits and the external MOSFET is switched ON.
The SMH4803 has three Power Good enable outputs,
PG1#, PG2#, and PG3#, that turn on the DC/DC con-
verter loads in sequence. Output PG1# is activated first,
followed by PG2# after a delay of PGD, and finally PG3#
Provided there is no sustained over-current condition
during start-up, the SMH4803 turns on the loads with the
Power Good logic outputs. Three DC/DC converters can
be connected to the outputs and their turn-on is
2041 8.4 6/15/00
SUMMIT MICROELECTRONICS
10
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
