LTC2972 datasheet(94/112 Pages) LINER

94 / 112 page

LTC2972

2972fa

For more information www.linear.com/LTC2972

• DAC manual with soft-connect. Non servo mode. The

DAC soft connects to the feedback node. Soft-connect

drives the DAC code to match the voltage at the feed-

back node. After connection, the DAC is moved by

writing DAC codes to the MFR_DAC.

• DAC manual with hard connect. Non servo mode. The

DAC hard connects to the feedback node using the

current value in MFR_DAC. After connection, the DAC

is moved by writing DAC codes to the MFR_DAC.

Margining

The LTC2972 margins and trims the output of a DC/DC

converter by forcing a voltage across an external resistor

connected between the DAC output and the feedback node

or the trim pin. Preset limits for margining are stored in

the VOUT_MARGIN_HIGH/LOW registers. Margining is

actuated by writing the appropriate bits to the OPERA-

TION register.

Margining requires the DAC to be connected. Margin

requests that occur when the DAC is disconnected will

be ignored.

Off Sequencing

An off sequence is initiated using the CONTROL pin or the

OPERATIONcommand.TheTOFF_DELAYvaluedetermines

the amount of time that elapses from the beginning of the

off sequence until each channel’s VOUT_EN pin is pulled

low, thus disabling its DC/DC converter.

VOUT Off Threshold Voltage

The MFR_VOUT_DISCHARGE_THRESHOLD command

the output voltage must decay below before the channel

can enter/re-enter the ON state. The OFF threshold voltage

is specified by multiplying MFR_VOUT_DISCHARGE_

THRESHOLD and VOUT_COMMAND. In the event that an

output voltage has not decayed below its OFF threshold

before attempting to enter the ON state, the channel will

continue to be held off, the appropriate bit is set in the

STATUS_MFR_SPECIFICregister,andtheALERTBpinwill

be asserted low. When the output voltage has decayed

belowitsOFFthreshold,thechannelcanentertheONstate.

Automatic Restart via MFR_RESTART_DELAY

Command and CONTROL Pin

An automatic restart sequence can be initiated by driving

theCONTROLpintotheoffstatefor>10μsandthenreleas-

ing it. The automatic restart disables both VOUT_EN pins

that are mapped to a particular CONTROL pin for a time

period = MFR_RESTART_DELAY and then starts all DC/

DCConvertersaccordingtotheirrespectiveTON_DELAYs.

(see Figure 29). VOUT_EN pins are mapped to one of the

CONTROLpinsbytheMFR_CONFIG_LTC2972command.

This feature allows a host that is about to reset to restart

the power in a controlled manner after it has recovered.

APPLICATIONS INFORMATION

Figure 29. Off Sequence with Automatic Restart

VCONTROL

VOUT_EN0

CONTROL

PIN BOUNCE

TOFF_DELAY0

TON_DELAY0

2972 F29

MFR_RESTART_DELAY

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