ADM1178 Просмотр технического описания (PDF) - Analog Devices

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ADM1178

Hot Swap Controller and Digital Power Monitor with ALERTB Output

Analog Devices 

ADM1178

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 ADM1178 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 ADM1178 can reside either on the backplane or on the

circuit board itself.

The ADM1178 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 the high current

loads on the live supply rail.

The ADM1178 holds the GATE pin down (and therefore holds

off the FET) until certain conditions are met. An undervoltage

lockout circuit ensures that the device is provided with an adequate

input supply voltage. After the input supply voltage is successfully

detected, the device goes through an initial timing cycle to

provide a delay before it attempts a hot swap. This delay ensures

that the board is fully seated in the backplane before the board

is powered up.

After 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 ADM1178 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

combinations of low load capacitance and high current limit,

this limit may not be reached before the load is fully charged

up. If the current limit is reached, the ADM1178 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 to the TIMER pin.

If the load becomes fully charged before the fault current limit

time elapses (when the TIMER pin reaches 1.3 V), the current

drops below the overcurrent fault timing threshold. The ADM1178

then continues to further charge up the GATE pin 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 the GATE pin to

turn off the FET.

Data Sheet

The ADM1178-2 is immediately latched off and attempts a hot

swap only when the ON pin is deasserted and then asserted again.

The ADM1178-1, however, retries the hot swap operation

indefinitely, keeping the FET in its safe operating area (SOA) by

using the TIMER pin to time a cooldown period between hot swap

attempts.

The current and voltage threshold combinations on the TIMER pin

set the retry duty cycle to 3.8%. The ADM1178 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

ADM1178 if the voltage on the VCC pin is too low for normal

operation. The UVLO has a low-to-high threshold of 2.8 V, with

80 mV hysteresis. When there is a supply voltage greater than 2.8 V,

the ADM1178 starts the initial timing cycle.

ON FUNCTION

The ADM1178 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,

an 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 that is higher

than the internal UVLO circuit, thereby setting the hot swap

operation to start at a specific voltage level on the VCC pin. 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 (1.3 V) and VTIMERL (0.2 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 nonideal

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.

Rev. D | Page 12 of 24

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