SMH4803A
Preliminary
FUNCTIONAL DESCRIPTION
GENERAL OPERATION
System Enables
There are several enabling inputs, which allow a host
system to control the SMH4803A. The Pin Detect pins
(PD1# & PD2#) are two active low enables that are
generally used to indicate that the add-in circuit card is
properly seated. This is typically done by clamping the
inputs to VSS through the implementation of an injector
switch,oralternativelythroughtheuseofastaggeredpins
at the card-cage interface. Two shorter pins arrayed at
opposite ends of the connector force the card to be fully
seated (not canted) before both pin detects are enabled.
Care must be taken not to exceed the maximum voltage
rating of these pins during the insertion process. Refer to
details in the Applications Section for proper circuit imple-
mentation.
The SMH4803A is an integrated power controller for hot
swappableadd-incards. Thedeviceoperatesfromawide
supplyrangeandgeneratesthesignalsnecessarytodrive
isolated output DC/DC converters. As a typical add-in
board is inserted into the powered backplane physical
connections must first be made with the chassis to dis-
charge any electrostatic voltage potentials. The board
then contacts the long pins on the backplane that provide
powerandground. Assoonaspowerisappliedthedevice
starts up, but does not immediately apply power to the
output load. Under-voltage and over-voltage circuits
inside the controller check to see that the input voltage is
within a user-specified range, and pin detection signals
determine whether the card is seated properly.
TheEN/TSinputprovidesanactivehighcomparatorinput
that may be used as a master enable or temperature
sense input. These inputs must be held low for a period of
These requirements must be met for a Pin Detect Delay
period of tPDD, after which time the hot-swap controller
enables VGATE to turn on the external power MOSFET
tPDD before a power-up sequence may be initiated.
switch. The VGATE output is current limited to IVGATE
,
allowing the slew rate to be easily modified using external
passivecomponents. Duringthecontrolledturn-onperiod
Under-/Over-Voltage Sensing
the VDS of the MOSFET is monitored by the drain sense The Under-Voltage (UV) and Over-Voltage (OV) inputs
input. When drain sense drops below 2.5V, and VGATE provideasetofcomparatorsthatactinconjunctionwithan
gets above VDD – VGT, the power good outputs can begin external resistive divider ladder to sense when the host
turning on the DC/DC controllers. Power Good Enable supplyvoltageexceedstheuserdefinedlimits. Iftheinput
inputs may be used to activate or deactivate specific to the UV pin rises above 2.5V, or the input to the OV pin
output loads.
falls below 2.5V for a period of tPDD, the power-up se-
quence may be initiated. The tPDD filter helps prevent
spurious start-up sequences while the card is being in-
serted. If UV falls below 2.5V or OV rises above 2.5V, the
PG and VGATE outputs will be shut down immediately.
Steady state operation is maintained as long as all condi-
tions are normal. Any of the following events may cause
the device to disable the DC/DC controllers by shutting
down the power MOSFET: an under-voltage or over-
voltage condition on the host power supply; an over-
currenteventdetectedontheCBSENSEinput;afailureof
the power MOSFET sensed via the DRAIN SENSE pin;
the pin detect signals becoming invalid; or the master
enable (EN/TS) falling below 2.5V. The SMH4803A may
be configured so that after any of these events occur the
VGATEoutputshutsoffandeitherlatchesintoanoffstate
Under-/Over-Voltage Filtering
The SMH4803A may also be configured so that an out of
tolerance condition on UV/OV will not shut off the output
immediately. Instead,afilterdelaymaybeinsertedsothat
only sustained under-voltage or over-voltage conditions
will shut off the output. When the UV/OV filter option is
enabledanoutoftoleranceconditiononUV/OVforlonger
than the filter delay time, tUOFLTR, activates the FAULT#
output, and the VGATE and PG outputs will be latched in
the off state. To initiate another power-up sequence the
FAULT# output must first be reset. Refer to the appropri-
ate section on resetting the FAULT# output. The Under-
/Over-Voltage Filtering feature is disabled in the default
configuration of the device.
or recycles power after a cooling down period, tCYC
.
Powering VDD
The SMH4803A contains a shunt regulator on the VDD pin
that prevents the voltage from exceeding 12V. It is
necessarytouseadropperresistor(RD)betweenthehost
power supply and the VDD pin in order to limit current into
the device and prevent possible damage. The dropper
resistor allows the device to operate across a wide range
of system supply voltages, and also helps protect the
deviceagainstcommon-modepowersurges. Refertothe
Applications Section for help on calculating the RD resis-
tance value.
Under-Voltage Hysteresis
The Under-Voltage comparator input may be configured
with a programmable level of hysteresis. The compare
SUMMIT MICROELECTRONICS, Inc.
2051 4.4 3/15/01
6
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