LTC3883 Datasheet, PDF (18/112 Pages) Linear Technology – Single Phase Step-Down DC/DC Controller with Digital Power System Management

LTC3883/LTC3883-1
Operation
PMBus command to turn on, or the V_IN pin voltage rising
above a preprogrammed voltage. Off sequencing is handled
in a similar way. To assure proper sequencing, make sure
all ICs connect the SHARE_CLK pins together and RUN pins
together. If the RUN pins can not be connected together for
some reason, set bit 2 of MFR_CHAN_CONFIG_LTC3883
to a 1. This bit requires the SHARE_CLK pin to be clocking
before the power supply output can start. When the RUN pin
is pulled low, the LTC3883 will hold the pin low for the MFR_
RESTART_DELAY. The minimum MFR_RESTART_DELAY
is TOFF_DELAY + TOFF_FALL + 136ms. This delay assures
proper sequencing of all rails. The LTC3883 calculates
this delay internally and will not process a shorter delay.
However, a longer commanded MFR_RESTART_DELAY
will be used by the part. The maximum allowed value is
65.52 seconds.
Voltage-Based Sequencing
The GPIO pin can be asserted when the UV threshold
is exceeded. It is possible to feed the GPIO pin from
one LTC3883 into the RUN pin of the next LTC3883 in
the sequence. To use the GPIO pin for voltage based
sequencing, set bit 12 of the MFR_GPIO_PROPAGATE_
LTC3883 command = 1. Bit 12 is the VOUT_UVUF which is
the deglitched VOUT_UV comparator. Using the deglitched
VOUT_UV fault limit is recommended because there is
little appreciable time delay between the comparator
crossing the UV threshold and the GPIO pin releasing
This can be implemented across multiple LTC3883s. The
VOUT_UVUF has a 250µs minimum pulse width filter.
If the GPIO_FAULT_RESPONSE command is not set to
Voltage Based Sequencing by Cascading GPIOs into RUN Pins
START
RUN
LTC3883
GPIO = V_{OUT_UVUF}
RUN
LTC3883
GPIO = V_{OUT_UVUF}
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TO NEXT CHANNEL
IN THE SEQUENCE
Figure 2. Event (Voltage) Based Sequencing
18
ignore, the part will latch off and never be able to start.
If the V_OUT voltage bounces around the UV threshold for
a long period of time it is possible for the GPIO 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.
Shutdown
The LTC3883 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. In discontinuous 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.
The other shutdown mode occurs in response to a fault
condition or loss of SHARE_CLK (if bit 2 of MFR_CHAN_
CONFIG_LTC3883 is set to a 1) or V_IN falling below the
VIN_OFF threshold or GPIO pulled low externally (if the
MFR_GPIO_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 either through user intervention
(deasserting RUN or the PMBus OPERATION command)
or in response to a detected fault or an external fault via
the bidirectional GPIO pin, or loss of SHARE_CLK (if bit
2 of MFR_CHAN_CONFIG_LTC3883 is set to a 1) or V_IN
falling below the VIN_OFF threshold.
In hiccup 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 caused 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 GPIO pin,
the decay time of the faulted output determines the retry
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