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LTM4700
OPERATION
multiple LTM4700s. The PGOOD
the V
voltage bounces around the UV threshold for a
OUT
long period of time it is possible for the PGOOD
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.
RUN 0
START
RUN 1
RUN 0
RUN 1
Figure 4. Event (Voltage) Based Sequencing
SHUTDOWN
The LTM4700 supports two shutdown modes. The first
mode is closed-loop shutdown response, with user de-
fined 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
falling below the VIN_OFF threshold or FAULT
IN
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 regulation states or
through user intervention.
26
has a 100µs filter. If
n
output
n
PGOOD0
LTM4700
PGOOD1
PGOOD0
LTM4700
PGOOD1
4700 F04
TO NEXT CHANNEL
IN THE SEQUENCE
For more information
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 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 then ON.
LIGHT-LOAD CURRENT OPERATION
The LTM4700 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 COMPnb pins. In this mode, the ef-
ficiency at light loads is lower than in discontinuous mode
operation. However, continuous mode exhibits lower output
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,
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Rev. B
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