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LTC2972

2972fa

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Tracking Supplies On and Off

The LTC2972 supports tracking power supplies that are equipped with a tracking pin and configured for tracking.

A tracking power supply uses a secondary feedback terminal (TRACK) to allow its output voltage to be scaled to an

external master voltage. Typically the external voltage is generated by the supply with the highest voltage in the sys-

tem, which is fed to the slave track pins (see Figure 16). Supplies that track a master supply must be enabled before

the master supply comes up and disabled after the master supply comes down. Enabling the slave supplies when

the master is down requires supervisors monitoring the slaves to disable UV detection. Both channels configured

for tracking must track off together in response to a fault on any channel or any other condition that can bring one

or more of the channels down. Prematurely disabling a slave channel via its RUN pin may cause that channel to shut

down out of sequence (see Figure 19)

An important feature of the LTC2972 is the ability to control, monitor and supervise DC/DC converters that are config-

ured to track a master supply on and off.

The LTC2972 supports the following tracking features:

• Track channels on and off without issuing false UV events when the slave channels are tracking up or down.

• Track channels down in response to a fault from a slave or master.

• Track channels down when VIN_SNS drops below VIN_OFF, share clock is held low or RESTORE_USER_ALL is

issued.

• Ability to reconfigure selected channels that are part of a tracking group to sequence up after the group has tracked

up or sequence down before the group has tracked down.

Tracking Implementation

TheLTC2972supportstrackingthroughthecoordinatedprogramingofTon_delay,Ton_rise,Toff_delayandMfr_config_

track_en. The master channel must be configured to turn on after all the slave channels have turned on and to turn off

before all the slave channels turn off. Slaves that are enabled before the master will remain off until the tracking pin

allows them to turn on. Slaves will be turned off via the tracking pin even though their run pin is still asserted. Ton_rise

must be extended on the slaves so that it ends relative to the rise of the TRACK pin and not the rise of the VOUT_EN pin.

Figure 16. LTC2972 Configured to Control, Supervise and Monitor Power Supplies Equipped with Tracking Pin

PMBus COMMAND DESCRIPTION

VOUTP

RUN

VOUTM

VSENSEP0

VSENSEM0

TRACK

FAULTB0

LTC2972

CONTROL0

VOUT_EN0

CONTROL0

DC/DC

LOAD

VOUTP

RUN

VOUTM

VSENSEP1

VSENSEM1

TRACK

VOUT_EN1

DC/DC

LOAD

VFB

VDAC0

VFB

VDAC1

VSENSEP0

VSENSEP1

VSENSEM0

VSENSEM1

2972 F16

R2_1

R1_1

PWRGD

LTC2972 Datasheet(PDF) 42 Page - Linear Technology

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