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Introduction
The SMH4811A
can easily provide reliable hot swap control that
allows a circuit board to be safely inserted or
removed from a live backplane. The SMH4811A
includes features which help isolate faults so that
other system assets are protected as well. The
SMH4811A is compat-ible with all members of the
Lucent J-series, Q-series, and other 36 to 75V
board mounted DC-DC converter modules.
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Features
The SMH4811A has the following features:
- Inrush
current and slew rate are controlled by limiting the gate
voltage of an external N-channel MOSFET. This function is
programmable.
- Over-voltage
and under-voltage shutdown are based on programmable
limits.
- An enable
output is provided to drive the DC-DC converter's ON/OFF
pin. The SMH4811A features a programmable Power Good
delay to ensure power stabilization.
- A circuit
breaker protects against routine shorts. Its current
threshold and sampling interval are
program-mable.
- Also, a
programmable quick-trip™ circuit breaker protects
against hard shorts.
- There is a
circuit breaker fault output to notify the host of an
over-current situation.
- Two enable
pins can be used as connector pin detects (to ensure
proper board insertion), card extractor detection, shelf
door detection, air flow switch detection,
etc.
- Master
Enable input can be used for software control over
shutdown, or for a temperature sense input.
Figure 1.
Basic Hot Swap Circuit Using SMH4811A
Basic Hot
Swap Circuit
A simplified circuit is shown in Figure 1. The
over-voltage and under-voltage thresholds are determined by
the values of R1, R2, and R3. Resistor RCB is determined by
the maximum allowable input current specified for the
design. Resistor RD protects the SMH4811A from high voltage.
This is an important consideration for NEBS compliant
design. Consult the SMH4811A
Data Sheet for guidelines in determining the values of these
resistors.
Where a Pin
Detect function is used, PD1# and PD2# should be connected
to short pins at opposite ends of the circuit board
connector, and tied to VSS on the motherboard. Both Pin
Detects function as enables, and must be at VSS to enable
VGATE. The enabling signal supplied to the PD# pins must be
at the same potential as VSS of the SMH4811A in a redundant
supply system. When the card being inserted uses local ORing
diodes (see Figure 2), VSS is fed out one pin of the card
edge and subsequently routed to the recessed pins connected
to PD1# and PD2#. It is imperative that the PD# voltages
never exceed the absolute maximum ratings.
EN/TS doubles
as either a Master Enable input or a Temperature Sense
input. The SMH4811A will turn the N-channel MOSFET on when
the over-voltage and under-voltage conditions are within
their acceptable limits, and the EN/TS, PD1#, and PD2#
signals, if used, are enabled. The DC-DC converter has a
large internal input capacitor for stability. To limit the
inrush current an external resistor and capacitor are
required to prevent the DC/DC converter from going into a
latched current limit mode.
Circuit
Breakers: Fault Detect Options
The SMH4811A features programmable options for both
fault detection and fault response. The circuit breaker
monitors the voltage differential across sense resistor RCB
using the CBSENSE input. The value of the resistor is
calculated so that the voltage drop across RCB will be 50mV
at the desired over-current threshold. Detection of an
over-current situation starts a programmable timer. If the
over-current situation persists when the timer expires,
VGATE is disabled, removing power from the load and the
FAULT# output is activated. The time-out period is
internally programmable from 5µs to 400µs. The
SMH4811A has a second circuit breaker to detect hard shorts.
It uses the same CBSENSE signal as the standard circuit
breaker, but is set to detect a higher voltage drop across
RCB. The trip level is programmable from 60mV to
200mV.
If this higher
threshold level is surpassed the SMH4811A immediately
disables VGATE and activates FAULT# without delay.
Figure 2.
Using the PD# Function with Local ORing Power
Diodes
Driving the
ON/OFF Control
The ON/OFF input of the Lucent device is controlled by
the SMH4811A's PG# output through an external transistor.
Figure 1 shows an enable circuit for an active low ON/OFF
input. Using the external transistor ensures that the ON/OFF
input is floating until voltage is applied to VIN+ and
VIN&endash;. This prevents a sneak inrush current path
through the ON/OFF pin which could damage the DC/DC
converter.
The transistor
also provides additional VDS drop, ensuring that the ON/OFF
input cannot be at lower potential than the VIN&endash;
signal. It is not recommended to drive the ON/OFF pin
directly from the IC. The SMH4811A monitors the Drain Sense
input as power is switched on to the DC/DC converter. Once
the SMH4811A determines that the MOSFET is on, the device
waits for a programmed time before enabling PG#. This
ensures that the power applied to the load is stable and
gives the bulk input capacitor time to charge, assuring a
clean secondary power-up sequence and preventing possible
damage to the DC/DC converter.
Lucent DC/DC
converter products are also available with active high
ON/OFF inputs. The SMH4811A can drive this as well (see
Figure 3).
Figure 3.
Driving an Active High On/Off Signal
Copyright©2003
by Summit Microelectronics, Inc.
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