Datasheet

UCD9248
www.ti.com SLVSA33A –JANUARY 2010–REVISED AUGUST 2012
Method 1: Each rail has programmable delay times, TON_DELAY and TOFF_DELAY, before beginning a soft
start ramp or a soft stop ramp, and programmable ramp times, TON_RISE and TOFF_FALL determine how long
the ramp takes. These PMBus commands are defined in the UCD92xx PMBUS Command Reference. The
parameters can also be configured using the Fusion Digital Power™ Designer GUI
(see http://focus.ti.com/docs/toolsw/folders/print/fusion_digital_power_designer.html).
The configurable times can be used to program a time based sequence for each voltage rail. Using this method
each rail ramps independently and completes the ramp regardless of the success of the other rails.
The start/stop sequence is initiated for a single rail by the PMBus_CTRL pin or via the PMBus using the
OPERATION or ON_OFF_CONTROL commands.
The start/stop sequence may be initiated simultaneously for multiple rails within the same controller by
configuring each rail to respond to the PMBus_CTRL pin. Alternatively, after setting the PMBus PAGE variable to
255, subsequent OPERATION or ON_OFF_CONTROL commands applies to all rails at the same time.
To simultaneously initiate start/stop sequences in multiple controllers, a common PMBus_CTRL signal can be
fed into each controller. Alternatively, the PMBus Group Command Protocol may be used to send separate
commands to multiple controllers. All the commands are sent in one continuous transmission and wait for the
final STOP signal in order to start executing their commands simultaneously.
Method 2: The PGood pin can be used to coordinate multiple controllers by running the PGood pin output from
one controller to the PMBus_CTRL input pin of another. This imposes a master/slave relationship between
multiple devices. During startup, the slave controllers initiate their start sequences after the master completes its
start sequence and reaches its regulation voltage. During shut-down, as soon as the master starts its shut-down
sequence, the shut-down signals to its slaves.
Unlike Method 1, a shut-down on one or more rails on the master can initiate shut-downs of the slave devices.
The master shut-downs can initiate intentionally or by a fault condition.
The PMBus specification implies that the Power Good signal is active when ALL the rails in a controller are
above their power-good “on” threshold setting. The UCD9248 allows the Power Good pin to be reprogrammed
using the GPIO_SEQ_CONFIG command so that the pin responds to a desired subset of rails.
This method works to coordinate multiple controllers, but it does not enforce interdependency between rails
within a single controller.
Method 3: Using the GPIO_SEQ_CONFIG command, several sequencing options can be configured using
undedicated pins for input/output. As many as four pins can be configured as inputs, and as many as six as
outputs. The outputs can be open-drain or actively driven with selectable polarity.
Each rail can be configured to respond to a combination of the power-good status of other internal rails and/or
the state of sequencing input pins. The output pins can be configured to reflect the power-good status of a
combination of rails, or to one of several status indicators including power-good, an over-current warning, or the
“open-drain outputs valid” signal.
When using the output signals for sequencing, they may be routed to sequencing control inputs or to the
PMBus_CTRL inputs on other controllers.
Once each rail’s input dependencies are configured, the rail responds to those input pins or internal rails. Like
method 2, shut-downs on one rail or controller can initiate shut-downs of other rails or controllers. Unlike method
2, GPIO_SEQ_CONFIG offers much more flexibility in assigning relationships between multiple rails within a
single controller or between multiple controllers. It is possible for each controller to be both a master and a slave
to another controller.
GPIO_SEQ_CONFIG allows the configuration of fault relationships such that a fault on one rail can result in the
shut down of any selection of rails in addition to the rail at fault. These fault interactions are not constrained to a
single master/slave relationship; for example, a system can be configured such that a fault on any rail shuts
down all rails. If the fault response of the failing rail is to shut down immediately, all dependent rails follow suit
and shuts down immediately regardless of their programmed response code. The fault slaves can be configured
to shutdown when the master first reports a fault or after the master has exhausted its retries.
Copyright © 2010–2012, Texas Instruments Incorporated Submit Documentation Feedback 27