SMH4814 Datasheet, PDF (2/44 Pages) Summit Microelectronics, Inc. – Dual Feed Active-ORing Programmable Hot Swap Controller

GENERAL DESCRIPTION
The SMH4814 integrated power controller operates
within a wide supply range, typically –32 to –72 volts,
and generates the signals necessary to drive isolated-
output DC/DC converters.
The device accepts two independent –48V feeds via
input pins FEED_A and FEED_B. The VGATE_A pin
controls the flow of power from FEED_A to the load.
The VGATE_B pin controls the flow of power from
FEED_B to the load.
The SMH4814 continuously monitors the voltage on
FEED_A and FEED_B. The supply arbitration block in
Figure 2 selects which pin drives power to the device
based on the voltage level on each pin and the
acceptable voltage range. Once the FEED_A or FEED_B
pin is selected the SMH4814 asserts the
corresponding VGATE pin. The assertion of this pin
turns on the external low-RDS_ON FETs to supply power
to the load.
Start-up Procedure
The general start-up procedure is as follows:
1. A physical connection must be made with the
chassis to discharge any electrostatic voltage
potentials when a typical add-in board is inserted
into the powered backplane.
2. The board then contacts the long pins on the
backplane that provide power and ground.
3. As soon as power is applied the device starts up,
but it does not immediately apply power to the
output load.
4. Under-voltage and over-voltage circuits inside the
controller verify that the input voltage is within a
user-specified range.
5. The SMH4814 senses the PD1 and PD0 pin
detection signals to indicate the card is seated
properly.
These requirements must be met for a Pin Detect
Delay period of t_PDD. Once this time has elapsed the
hot-swap controller enables VGATE_HS to turn on the
external power MOSFET switch.
SMH4814
Preliminary Information
The VGATE_HS output is current limited to I_VGATE,
allowing the slew rate to be easily modified using
external passive components. During the controlled
turn-on period the V_DS of the MOSFET is monitored by
the DRAIN SENSE input. When DRAIN SENSE drops
below 2.5V, and VGATE_HS rises above V12 – V_GT,
the SMH4814 asserts the PUP_A through PUP_D power
good outputs to enable the DC/DC controllers.
Steady-state operation is maintained as long as all
conditions are normal. Any of the following events
may cause the device to disable the DC/DC controllers
by shutting down the power MOSFETs:
An under-voltage or over-voltage condition on the
host power supply.
A failure of the power MOSFET sensed via the
DRAIN SENSE pin.
The PD1/PD0 pin detect signals becoming invalid.
The master enable (EN/TS) falls below 2.5V.
Any of the FB inputs driven low by events on the
secondary side of the DC/DC controllers.
The occurrence of an overcurrent.
The SMH4814 may be configured so that after any of
these events occurs the VGATE output shuts off, and
either latches into an off state or recycles power after
a cooling down period, t_CYC.
Powering V12
The SMH4814 contains an internal shunt regulator on
the V12 pin that prevents the voltage from exceeding
12V. It is necessary to use a dropping resistor (R_D)
between the host power supply and the V12 pin in
order to limit current into the device and prevent
possible damage. The dropping resistor allows the
device to operate across a wide range of system
supply voltages, typically –32 V to –72V, and also
helps protect the device against common-mode power
surges. Refer to the Applications Section for help on
calculating the R_D resistance value.
Summit Microelectronics, Inc
2080 2.0 07/21/05
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