LTM4664 datasheet(39/138 Pages) AD

39 / 138 page

LTM4664A

Rev. 0

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UV threshold is exceeded for each output. It is possible

to feed the PGOOD_Cn pin from one LTM4664A PSM

channel into the RUN_Cn pin of the next LTM4664A

PSM channel in the sequence, especially across multi-

ple LTM4664As. The PGOOD_Cn has a 60µs filter. If the

VOUT voltage bounces around the UV threshold for a long

period of time it is possible for the PGOOD_Cn 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.

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 FAULT_Cn

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

RUN_Cn or commanding the part OFF then ON.

LIGHT-LOAD CURRENT OPERATION

The LTM4664A PSM Regulators have two modes of oper-

ation: high efficiency discontinuous conduction mode or

forced continuous conduction mode. Mode selection is

done using the MFR_PWM _MODE command (discon-

tinuous conduction 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 transient

conditions. The peak inductor current is determined solely

by the voltage on the COMP_Cn pins. In this mode, the

efficiency at light loads is lower than in discontinuous

mode operation. However, continuous mode exhibits

lower output ripple and less interference with audio cir-

cuitry, but may result in reverse inductor current, which

can cause the input supply to boost. The VIN_OV_FAULT_

LIMIT can detect this and turn off the offending channel.

However, this fault is based on an ADC read and can take

Figure 8. Event (Voltage) Based Sequencing

LTM4664A

RUN_C1

RUN_C0

PGOOD_C0

PGOOD_C1

PGOOD_C0

PGOOD_C1

START

LTM4664A

4664A F08

RUN_C0

TO NEXT CHANNEL

IN THE SEQUENCE

RUN_C1

SHUTDOWN

The LTM4664A PSM Regulators 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 dis-

continuous 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 (if bit 2 of MFR_CHAN_ CONFIG

is set to a 1) or VIN falling below the VIN_OFF thresh-

old 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.

DUAL 25A/30A PSM OPERATION

LTM4664 Datasheet(PDF) 39 Page - Analog Devices

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