LTM4686BIVPBF datasheet(98/130 Pages) AD

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LTM4686B

Rev. 0

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APPENDIX C: PMBUS COMMAND DETAILS

TOFF_FALL

The TOFF_FALL command sets the time, in milliseconds, from the end of the turn-off delay time until the output volt-

age is commanded to zero. It is the ramp time of the VOUT DAC. When the VOUT DAC is zero, the part will three-state.

The part will maintain the mode of operation programmed. For defined TOFF_FALL times, the user should set the part

to continuous conduction mode. Loading the max value indicates the part will ramp down at the slowest possible rate.

The minimum supported fall time is 0.25ms. A value less than 0.25ms will result in a 0.25ms ramp. The maximum fall

time is 1.3 seconds. The maximum allowed slope is 4V/ms.

In discontinuous conduction mode, the controller will not draw current from the load and the fall time will be set by

the output capacitance and load current.

This command has two data bytes and is formatted in Linear_5s_11s format.

TOFF_MAX_WARN_LIMIT

The TOFF_MAX_WARN_LIMIT command sets the value, in milliseconds, on how long the unit can attempt to turn off

the output until a warning is asserted. The output is considered off when the VOUT voltage is less than 12.5% of the

programmed VOUT_COMMAND value. The calculation begins after TOFF_FALL is complete. TOFF_MAX_WARN is not

enabled in VOUT_DECAY is disabled.

A data value of 0ms means that there is no limit and that the unit can attempt to turn off the output voltage indefinitely.

Other than 0, values from 120ms to 524 seconds are valid.

This command has two data bytes and is formatted in Linear_5s_11s format.

Precondition for Restart

COMMAND NAME

CMD CODE DESCRIPTION

TYPE

PAGED

DATA

FORMAT UNITS

NVM

DEFAULT

VALUE

MFR_RESTART_ DELAY

0xDC

Delay from actual RUN active edge to virtual

RUN active edge.

R/W Word

L11

200

0xF320

MFR_RESTART_DELAY

This command specifies the minimum RUN off time in milliseconds. This device will pull the RUN pin low for this length

of time once a falling edge of RUN has been detected. The minimum recommended value is 136ms.

Note: The restart delay is different than the retry delay. The restart delay pulls run low for the specified time, after which

a standard start-up sequence is initiated. The minimum restart delay should be equal to TOFF_DELAY + TOFF_FALL

+ 136ms. Valid values are from 136ms to 65.52 seconds in 16ms increments. To assure a minimum off time, set

the MFR_RESTART_DELAY 16ms longer than the desired time. The output rail can be off longer than the MFR_

RESTART_DELAY after the RUN pin is pulled high if the output decay bit 1 is enabled in MFR_CHAN_CONFIG and the

output takes a long time to decay below 12.5% of the programmed value.

This command has two data bytes and is formatted in Linear_5s_11s format.

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