27
Rev. B
OPERATION
threshold is exceeded for each output. It is possible to
feed the PGOOD pin from one LTM4680 into the RUN pin
of the next LTM4680 in the sequence, especially across
multiple LTM4680s. The PGOOD
n
has a 100µs filter. If
the V
OUT
voltage bounces around the UV threshold for a
long period of time it is possible for the PGOOD
n
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 con-
troller responds to a fault by shutting down and enter-
ing 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 dis-
abled. 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
n
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 volt-
age 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
n
or commanding the part OFF then
ON.
LIGHT-LOAD CURRENT OPERATION
The LTM4680 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 transient
conditions. The peak inductor current is determined solely
by the voltage on the COMP
n
pins. In this mode, the effi-
ciency at light loads is lower than in discontinuous mode
operation. However, continuous mode exhibits lower out-
put ripple and less interference with audio circuitry, 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,
Figure 4. Event (Voltage) Based Sequencing
LTM4680
RUN 1
RUN 0
PGOOD0
PGOOD1
PGOOD0
PGOOD1
START
LTM4680
4680 F04
RUN 0
TO NEXT CHANNEL
IN THE SEQUENCE
RUN 1
SHUTDOWN
The LTM4680 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 (if bit 2 of MFR_CHAN_ CONFIG
is set to a 1) or V
IN
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.
