ADM1177-1ARMZ-R7(RevPrD) Просмотр технического описания (PDF) - Analog Devices

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ADM1177-1ARMZ-R7
(Rev.:RevPrD)

Hot Swap Controller and Digital Power Monitor with Soft-Start Pin

Analog Devices 

Preliminary Technical Data

ADM1177

OVERVIEW OF THE HOTSWAP FUNCTION

When circuit boards are inserted into a live backplane,

discharged supply bypass capacitors would draw large transient

currents from the backplane power bus as they charge. Such

transient currents can cause permanent damage to connector

pins, and dips on the backplane supply which could reset other

boards in the system. The ADM1177 is designed to turn a

circuit board’s 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 ADM1177 can reside either on the

backplane or on the circuit board itself.

The ADM1177 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 “hotswap” function protects the card connectors and the

FET itself from damage and also limits any problems which

could be caused by the high current loads on the live supply rail.

The ADM1177 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 being provided with an

adequate input supply voltage. Once this has been successfully

detected, the device goes through an initial timing cycle to

provide a delay before it will attempt to hotswap. 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 hotswap function

is switched on under control of the ON pin. When asserted high

the hotswap operation starts.

The ADM1177 charges up the gate of the FET to turn on the

load. It will continue 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 current limit is reached, the ADM1177 will regulate 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 will drop below the overcurrent fault timing

threshold, the ADM1177 will then charge the GATE pin higher

to fully enhance the FET for lowest RON, and the TIMER pin

will be pulled down again.

using the TIMER pin to time a cool-down period in between

hotswap attempts. The current and voltage threshold

combinations on the TIMER pin set the retry duty cycle to

3.8%.

The ADM1177 is designed to operate over a range of supplies

from 3.15 V to 14 V.

UNDERVOLTAGE LOCKOUT

An internal undervoltage lockout (UVLO) circuit resets the

ADM1177 if the VCC supply is too low for normal operation.

The UVLO has a low-to-high threshold of 2.8 V, with 25 mV

hysteresis. Above 2.8 V supply voltage, the ADM1177 will start

the initial timing cycle.

ON FUNCTION

The ADM1177-1 has an active-high ON pin. The ON pin is the

input to a comparator which has a low-to-high threshold of 1.3

V, an 80 mV hysteresis and a glitch filter of 3 μs. A low input on

the ON pin turns off the hotswap 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 hotswap

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 hotswap 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.

SOFT START (SS PIN)

The SS pin is used to determine the inrush current profile.

A capacitor should be attached to this pin. Whenever the FET

is requested to turn on, the SS pin is held at ground until the

SENSE pin reaches a few mV. A current source is then turned

on, which linearly ramps the capacitor up to 1.3V. The

reference voltage for the GATE linear control amplifier is

derived from the soft start voltage, such that the inrush linear

current limit is defined as ILIMIT = VSS / (20 x RSENSE). A voltage

can also be driven onto the SS pin to clamp the reference at a

different level.

If the fault current limit time is reached before the load drops

below the current limit, a fault has been detected, and the

hotswap operation is aborted by pulling down on the GATE pin

to turn off the FET. The ADM1177-2 latches off at this point

and will only attempt to hotswap again when the ON pin is de-

asserted then asserted again. The ADM1177-1 will retry the

hotswap operation indefinitely, keeping the FET in SOA by

Rev. PrD | Page 7 of 16

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