SUMMIT
MICROELECTRONICS, Inc.
Application Note 18
Preliminary
SMH4811A
CONFIGURABLE HOT SWAP CONTROLLER
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 mod-
ules.
FEATURES
The SMH4811A has the following features:
l
Inrush current and slew rate are controlled by limiting
the gate voltage of an external N-channel MOSFET.
This function is programmable.
l
Over-voltage and under-voltage shutdown are based
on programmable limits.
l
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.
l
A circuit breaker protects against routine shorts. Its
current threshold and sampling interval are program-
mable.
l
Also, a programmable quick-trip™ circuit breaker
protects against hard shorts.
l
There is a circuit breaker fault output to notify the
host of an over-current situation.
l
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.
l
Master Enable input can be used for software control
over shutdown, or for a temperature sense input.
0V
R
D
R3
Pin Detect
PD1#
ENPG
EN/TS
–V
IN
+V
OUT
0.01µF
VDD
CBFAULT#
Lucent
DC/DC
Converter
0.01µF
0.01µF
UV
R2
OV
5.0VREF
PG#
Pin Detect
R1
100nF
*10Ω
R
CB
10nF
1N4148
1kΩ
100kΩ
100nF
25V
PD2#
VSS
CBSENSE VGATE
DRAIN
SENSE
100kΩ
47µF
+V
IN
On/Off
–V
OUT
0.01µF
47µF
SMH4811A
0.01µF
MMBTA-
06LT1
–48V
3018 Fig01
* The 10Ω resistor must be located as close as possible to the MOSFET.
Figure 1. Basic Hot Swap Circuit Using SMH4811A
©SUMMIT MICROELECTRONICS, Inc., 2001 • 300 Orchard City Dr., Suite 131 • Campbell, CA 95008 • Phone 408-378-6461 • FAX 408-378-6586 • www.summitmicro.com
Characteristics subject to change without notice
3018 1.0 1/24/01
1
Application Note 18
Preliminary
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 R
CB
is determined by
the maximum allowable input current specified for the
design. Resistor R
D
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 V
SS
on the mother-
board. Both Pin Detects function as enables, and must be
at V
SS
to enable VGATE. The enabling signal supplied to
the PD# pins must be at the same potential as V
SS
of the
SMH4811A in a redundant supply system. When the card
being inserted uses local ORing diodes (see Figure 2),
V
SS
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 con-
verter 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 R
CB
using the CBSENSE input. The value of the resistor is
calculated so that the voltage drop across R
CB
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 inter-
nally 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
R
CB
. The trip level is programmable from 60mV to 200mV.
If this higher threshold level is surpassed the SMH4811A
immediately disables VGATE and activates FAULT# with-
out delay.
0V
PD1#
VDD
SMH4811A
PD2#
VSS
CBSENSE VGATE
–48V Feed A
–48V Feed B
3018 Fig02
Figure 2. Using the PD# Function with Local ORing Power Diodes
2
3018 1.0 1/24/01
SUMMIT MICROELECTRONICS, Inc.
Application Note 18
Preliminary
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 V
IN
+ and
V
IN
–. This prevents a sneak inrush current path through
the ON/OFF pin which could damage the DC/DC con-
verter. The transistor also provides additional V
DS
drop,
ensuring that the ON/OFF input cannot be at lower poten-
tial than the V
IN
– signal. It is not recommended to drive the
ON/OFF pin directly from the IC. The SMH4811A moni-
tors 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).
5.0VREF
47kΩ
On/Off
1N4148
PG#
–48V Out
3018 Fig03
Figure 3. Driving an Active High On/Off Signal
NOTICE
SUMMIT Microelectronics, Inc. reserves the right to make changes to the products contained in this publication in order
to improve design, performance or reliability. SUMMIT Microelectronics, Inc. assumes no responsibility for the use of
any circuits described herein, conveys no license under any patent or other right, and makes no representation that
the circuits are free of patent infringement. Charts and schedules contained herein reflect representative operating
parameters, and may vary depending upon a user’s specific application. While the information in this publication has
been carefully checked, SUMMIT Microelectronics, Inc. shall not be liable for any damages arising as a result of any
error or omission.
SUMMIT Microelectronics, Inc. does not recommend the use of any of its products in life support or aviation applications
where the failure or malfunction of the product can reasonably be expected to cause any failure of either system or to
significantly affect their safety or effectiveness. Products are not authorized for use in such applications unless
SUMMIT Microelectronics, Inc. receives written assurances, to its satisfaction, that: (a) the risk of injury or damage has
been minimized; (b) the user assumes all such risks; and (c) potential liability of SUMMIT Microelectronics, Inc. is
adequately protected under the circumstances.
© Copyright 2001 SUMMIT Microelectronics, Inc.
This document supersedes all previous versions.
SUMMIT MICROELECTRONICS, Inc.
3018 1.0 1/24/01
3
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