LTC2921/LTC2922 Series

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General Operation

The LTC2921 and LTC2922 track multiple supplies, moni-

tor multiple inputs, and provide integrated switches for

remote sensing. Once all input voltages lie between moni-

toring and overvoltage threshold levels, in-line FETs are

turned on to simultaneously ramp power to the loads. The

automatic remote sense switches are then activated, and

the power good signal is asserted. After initial power-on

the LTC2921 and LTC2922 continue monitoring the in-

puts. Several types of events will trigger interruption, any

of which will disconnect all supplies, deactivate the power

good signal, and open the remote sense switches.

Monitoring Sequence

A normal power-on sequence comprises the following

steps:

Step 1) Confirm that the circuit breaker has not tripped and

between their programmed monitor and overvoltage

thresholds. Continue checking these conditions.

Step 2) Check that the TIMER pin voltage starts below

150mV. Create a delay by ramping up the TIMER pin until

it trips an internal comparator.

Step 3) Ramp the GATE pin to turn on the external

N-channel FETs, simultaneously ramping the supplies into

their loads. Await confirmation of full GATE enhancement,

this condition.

Step 4) Activate the remote sense switches. Await confir-

mation of full Feedback Switch Gate enhancement.

Step 5) Wait again for another TIMER cycle delay.

Step 6) Release the pull-down on the PG output. Continue

the GATE voltage.

Interrupting Events

Three events can interrupt the sequence and trigger imme-

diate disconnect of all supplies, pull-down of the PG

signal, and deactivation of the remote sense switches. The

three interrupting events are a lockout, a fault, and an

error.

threshold (including hysteresis). Escape from lockout

quence begins at Step 1. A lockout condition supersedes

faults and errors.

A fault occurs when the circuit breaker trips. Escape from

a fault requires pulsing the V1 pin below the reset thresh-

old of 0.5V(nom) for more than 150

µs after the current

falls below the trip point. When V1 returns high, the

sequence begins from Step 1. An undervoltage lockout of

>10

µs also clears the circuit breaker fault latch. A fault

condition supersedes errors.

An error occurs when one or more of the monitor inputs

rises above its overvoltage threshold. A loss of voltage on

the GATE pin, once it has fully ramped up, also causes an

error. An error sends the sequence to Step 1.

Feedback Switches for Remote Sensing

The integrated N-channel switches of the LTC2921/

LTC2922 automatically compensate for the voltage drops

FET switches. This is accomplished by modifying the

normal feedback path of each power supply that is con-

trolled by the LTC2921/LTC2922. When the load-control

switches are off, the remote sense switches are also off,

and the power supply uses its normal feedback path to

sense its output voltage. After the load-control switches

are turned on, the remote sense switches are turned on to

create dominating feedback paths. The feedback loops

include the load-control switches, thus compensating for

their voltage drops.

In order to eliminate glitching on the output of the power

supply, the remote sense switches are turned on at a

controlled rate of about 8V/ms. The gates of these inte-

range of voltages.

OPERATIO