LTC3882_15 Datasheet, PDF (18/104 Pages) Linear Technology – Dual Output PolyPhase Step-Down DC/DC Voltage Mode Controller with Digital Power System Management

LTC3882
OPERATION
Soft-Start
The RUN pins are released for external control after the part
initializes and VINSNS is greater than the VIN_ON threshold.
If multiple LTC3882 ICs are used in an application, shared
RUN pins are held low until all units initialize and VINSNS
exceeds the VIN_ON threshold for all devices. A common
SHARE_CLK signal can also ensure all connected devices
use the same time reference for initial start-up even if RUN
pins cannot be shared due to other design requirements.
SHARE_CLK is not released by each IC until the conditions
for power sequencing have been fully satisfied.
After a channel RUN pin rises above 2V and any specified turn
on delay (TON_DELAY) has expired, the LTC3882 performs
an initial monotonic soft-start ramp on that channel. This is
carried out with a digitally controlled ramp of the regulated
output voltage from 0V to the commanded voltage set point
over the programmed TON_RISE period, allowing inrush
current control. During the soft-start ramp, the LTC3882
does not initiate PWM operation until the commanded output
exceeds the actual rail voltage. This allows the regulator
to start up into a pre-biased load even when using gate
drivers or power blocks that do not support discontinuous
operation. The soft-start feature is disabled by setting the
value of TON_RISE to any time less than 0.25ms.
Time-Based Output Sequencing
The LTC3882 supports time-based on and off output se-
quencing using a shared time reference (SHARE_CLK).
Following a valid qualified command to turn on, each output
is enabled after waiting its programmed TON_DELAY. This
can be used to sequence outputs in a prescribed order
that can be preprogrammed as needed without hardware
modification. Channel off-sequencing is accomplished in
a similar way with the TOFF_DELAY command.
Output Ramping Control
The LTC3882 supports synchronized output on and off
ramping control using a shared time reference (SHARE_
CLK). Power rail on and off relationships similar to those of
conventional analog tracking functions can be achieved by
using programmed delays and TON_RISE and TOFF_FALL
times. However, with LTC3882 digital control, on and off
ramping methods need not be the same, and ramping
configurations can be reprogrammed as needed without
hardware modification.
Programmable fault responses and fault sharing can
ensure that any desired time-based output sequencing
and ramping control is properly accomplished each time
the system powers up or down. Refer to the Applications
Information section for various LTC3882 hardware and
PMBus command configurations needed to fully support
synchronization for time-based sequencing and output
ramping when using multiple ICs.
Voltage-Based Output Sequencing
It is also possible to sequence outputs on using cascaded
voltage events. To do this, the GPIO pin monitoring one
PWM channel can be used to control the RUN pin of a down-
stream channel. The controlling GPIO pin can be configured
to hold low if V_OUT is below the VOUT_UV_FAULT_LIMIT
or if POWER_GOOD conditions are not being met. This
keeps the downstream channel off until acceptable output
conditions exist on the controlling channel. The LTC3882
does not readily support voltage-based off-sequencing.
Refer to the Applications Information section for more
details on voltage-based sequencing.
Output Disable
Both PWM channels are disabled any time VINSNS is below
the VIN_OFF threshold. The power stages are immediately
shut off to stop the transfer of energy to the load(s) as
quickly as possible.
A PWM channel may also be disabled in response to cer-
tain internal fault conditions, an external fault propagated
through a GPIO pin, or loss of SHARE_CLK. In these
cases the power stage is immediately shut off to stop the
transfer of energy to the load as quickly as possible. Refer
to the following Fault Detection and Handling section for
additional details related to fault recovery.
Each PWM channel can be disabled with a PMBus OPERA-
TION command at any time if enabled by ON_OFF_CONFIG.
This will force a controlled turn-off response with defined
delay (TOFF_DELAY) and ramp down rate (TOFF_FALL).
The controller will maintain the programmed mode of
operation for TOFF_FALL. In DCM, the controller will not
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For more information www.linear.com/LTC3882