LTM4680 datasheet(32/132 Pages) AD

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LTM4681

Rev. 0

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OPERATION

MFR_RESTART_DELAY can be used by the part. The maxi-

mum allowed value is 65.52 seconds.

VOLTAGE-BASED SEQUENCING

The sequence can also be voltage-based. As shown

in Figure 4, The PGOODn pin is asserted when the UV

threshold is exceeded for each output. It is possible to

feed the PGOODn pin from one LTM4681 channel into the

RUNn pin of the next LTM4681 channel in the sequence,

especially across multiple LTM4681s. The PGOODn has

a 100µs filter. If the VOUTn voltage bounces around the UV

threshold for a long period of time it is possible for the

PGOODn output to toggle more than once. To minimize

this problem, set the TON_RISE time under 100ms.

If a fault in the string of rails is detected, only the faulted

rail and downstream rails will fault off. The rails in the

string of devices in front of the faulted rail will remain on

unless commanded off.

or FAULT pulled low externally (if the MFR_FAULT_

RESPONSE is set to inhibit). Under these conditions, the

power stage is disabled in order to stop the transfer of

energy to the load as quickly as possible. The shutdown

state can be entered from the soft-start or active regula-

tion states or through user intervention.

There are two ways to respond to faults; which are retry

mode and latched off mode. In retry mode, the controller

responds to a fault by shutting down and entering the inac-

tive state for a programmable delay time (MFR_RETRY_

DELAY). This delay minimizes the duty cycle associated

with autonomous retries if the fault that causes the shut-

down disappears once the output is disabled. The retry

delay time is determined by the longer of the MFR_RETRY_

DELAY command or the time required for the regulated

output to decay below 12.5% of the programmed value.

If multiple outputs are controlled by the same FAULTn

pin, the decay time of the faulted output determines the

retry delay. If the natural decay time of the output is too

long, it is possible to remove the voltage requirement of

the MFR_RETRY_DELAY command by asserting bit 0

of MFR_CHAN_CONFIG. Alternatively, latched off mode

means the controller remains latched-off following a fault

and clearing requires user intervention such as toggling

RUNn or commanding the part OFF then ON.

LIGHT-LOAD CURRENT OPERATION

The LTM4681 has two modes of operation: high efficiency

discontinuous conduction mode or forced continuous

conduction mode. Mode selection is done using the

MFR_PWM _MODE command (discontinuous conduc-

tion is always the start-up mode, forced continuous is the

default running mode).

If a controller is enabled for discontinuous operation, the

inductor current is not allowed to reverse. The reverse

current comparator’s output turns off the bottom MOSFET

just before the inductor current reaches zero, preventing

it from reversing and going negative.

In forced continuous operation, the inductor current

is allowed to reverse at light loads or under large tran-

sient conditions. The peak inductor current is determined

solely by the voltage on the COMPn pins. In this mode,

Figure 4. Event (Voltage) Based Sequencing

LTM4681

1/2

RUN1

RUN0

PGOOD0

PGOOD1

PGOOD2

PGOOD3

START

LTM4681

1/2

4681 F04

RUN2

TO NEXT CHANNEL

IN THE SEQUENCE

RUN3

SHUTDOWN

The LTM4681 supports two shutdown modes. The first

mode is closed-loop shutdown response, with user

defined turn-off delay (TOFF_DELAY) and ramp down rate

(TOFF_FALL). The controller will maintain the mode of

operation for TOFF_FALL. The second mode is discontinu-

ous 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, instead of TOFF_FALL.

The shutdown occurs in response to a fault condition or

loss of SHARE_CLK_nn (if bit 2 of MFR_CHAN_ CONFIG

is set to a 1) or VINnn falling below the VIN_OFF threshold

LTM4680 Datasheet(PDF) 32 Page - Analog Devices

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