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

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 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

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

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.

chosen by the user, determine the initial timing cycle time, the

fault current limit time, and the hot swap retry duty cycle.