Voltage-Based Sequencing; Shutdown; Light-Load Current Operation - Analog Devices LTM4683 Manual

OPERATION
However, a longer commanded MFR_RESTART_DELAY
can be used by the part. The maximum 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 LTM4683 channel into the
RUNn pin of the next LTM4683 channel in the sequence,
especially across multiple LTM4683s. The PGOODn has
a 100µs filter. If the V
OUTn
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.
RUN0
START
RUN1
RUN2
RUN3
Figure 4. Event (Voltage) Based Sequencing

SHUTDOWN

The LTM4683 supports two shutdown modes. The first
mode is a closed-loop shutdown response with a 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.
voltage bounces around the UV
PGOOD0
LTM4683
1/2
PGOOD1
PGOOD2
LTM4683
1/2
PGOOD3
4683 F04
TO NEXT CHANNEL
IN THE SEQUENCE
For more information
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 V falling below the VIN_OFF thresh-
INnn
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.
There are two ways to respond to faults, retry mode and
latched-off mode. In retry mode, the controller responds
to a fault by shutting down and entering the inactive 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 shutdown
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 and then ON.

LIGHT-LOAD CURRENT OPERATION

The LTM4683 has two modes of operation: high-effi-
ciency discontinuous-conduction mode or forced continu-
ous conduction mode. Mode selection is done using the
MFR_PWM_MODE command (discontinuous-conduction
is always the start-up mode, and 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.
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LTM4683
Rev. 0
31