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ADM1176EBZ Datasheet(PDF) 13 Page - Analog Devices

Part # ADM1176EBZ
Description  Hot Swap Controller and I2C짰 Power Monitor with Convert Pin
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

ADM1176EBZ Datasheet(HTML) 13 Page - Analog Devices

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ADM1176
Rev. 0 | Page 13 of 24
OVERVIEW OF THE HOT SWAP FUNCTION
When circuit boards are inserted into a live backplane, discharged
supply bypass capacitors draw large transient currents from the
backplane power bus as they charge. Such transient currents can
cause permanent damage to connector pins, as well as dips on
the backplane supply that can reset other boards in the system.
The ADM1176 is designed to turn a circuit board supply voltage on
and off in a controlled manner, allowing the circuit board to be
safely inserted into or removed from a live backplane. The
ADM1176 can reside either on the backplane or on the circuit
board itself.
The ADM1176 controls the inrush current to a fixed maximum
level by modulating the gate of an external N-channel FET placed
between the live supply rail and the load. This hot swap function
protects the card connectors and the FET itself from damage and
limits any problems that can be caused by high current loads on
the live supply rail.
The ADM1176 holds the GATE pin down (and, thus, the FET is
held off) until a number of conditions are met. An undervoltage
lockout circuit ensures that the device is provided with an adequate
input supply voltage. Once the input supply voltage has been
successfully detected, the device goes through an initial timing
cycle to provide a delay before it attempts to hot swap. This delay
ensures that the board is fully seated in the backplane before
the board is powered up.
Once the initial timing cycle is complete, the hot swap function
is switched on under control of the ON pin. When the ON pin
is asserted high, the hot swap operation starts.
The ADM1176 charges up the gate of the FET to turn on the
load. It continues to charge up the GATE pin until the linear
current limit (set to 100 mV/RSENSE) is reached. For some combi-
nations of low load capacitance and high current limit, this limit
may not be reached before the load is fully charged up. If current
limit is reached, the ADM1176 regulates the GATE pin to keep
the current at this limit. For currents above the overcurrent
fault timing threshold, nominally 100 mV/RSENSE, the current
fault is timed by sourcing a current out to the TIMER pin. If the
load becomes fully charged before the fault current limit time is
reached (when the TIMER pin reaches 1.3 V), the current drops
below the overcurrent fault timing threshold. The ADM1176
then charges the GATE pin higher to fully enhance the FET for
lowest RON, and the TIMER pin is pulled down again.
If the fault current limit time is reached before the load drops
below the current limit, a fault has been detected, and the hot
swap operation is aborted by pulling down on the GATE pin to
turn off the FET. The ADM1176-2 latches off at this point and
attempts to hot swap again only when the ON pin is deasserted
and then asserted again.
The ADM1176-1 retries the hot swap operation indefinitely,
keeping the FET in its safe operating area (SOA) by using the
TIMER pin to time a cool-down period in between hot swap
attempts. The current and voltage threshold combinations on
the TIMER pin set the retry duty cycle to 3.8%.
The ADM1176 is designed to operate over a range of supplies
from 3.15 V to 16.5 V.
UNDERVOLTAGE LOCKOUT
An internal undervoltage lockout (UVLO) circuit resets the
ADM1176 if the VCC supply is too low for normal operation.
The UVLO has a low-to-high threshold of 2.8 V, with 80 mV
hysteresis. Above 2.8 V supply voltage, the ADM1176 starts the
initial timing cycle.
ON FUNCTION
The ADM1176-1 has an active-high ON pin. The ON pin is the
input to a comparator that has a low-to-high threshold of 1.3 V,
a 50 mV hysteresis, and a glitch filter of 3 μs. A low input on the
ON pin turns off the hot swap operation by pulling the GATE pin
to ground, turning off the external FET. The TIMER pin is also
reset by turning on a pull-down current on this pin. A low-to-
high transition on the ON pin starts the hot swap operation.
A 10 kΩ pull-up resistor connecting the ON pin to the supply
is recommended.
Alternatively, an external resistor divider at the ON pin can be
used to program an undervoltage lockout value higher than the
internal UVLO circuit, thereby setting a voltage level at the
VCC supply where the hot swap operation is to start. An RC
filter can be added at the ON pin to increase the delay time at
card insertion if the initial timing cycle delay is insufficient.
TIMER FUNCTION
The TIMER pin handles several timing functions with an
external capacitor, CTIMER. There are two comparator thresholds:
VTIMERH (0.2 V) and VTIMERL (1.3 V). The four timing current
sources are a 5 μA pull-up, a 60 μA pull-up, a 2 μA pull-down,
and a 100 μA pull-down. The 100 μA pull-down is a non-ideal
current source approximating a 7 kΩ resistor below 0.4 V.
These current and voltage levels, together with the value of CTIMER
chosen by the user, determine the initial timing cycle time, the
fault current limit time, and the hot swap retry duty cycle.


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