Datasheet

Sequencing
UCD9240
SLUS766C – JULY 2008 – REVISED NOVEMBER 2008 ................................................................................................................................................... www.ti.com
There are three methods to squence voltage rails controlled by the UCD9240 that allow for a variety of system
sequencing configurations. Each of these options is configurable in the GUI. These methods include:
1. Use the PMBus to set the soft start/stop parameters for each rail. Multiple start/stop sequences may be
triggered simultaneously. Each voltage rail performs its sequencing in an open-loop manner. If any rail fails
to complete its sequence, all other rails are unaffected.
2. Daisy-chain the Power Good output signal from one controller to the PMBus-CTRL input on another.
3. Use the GPIO_SEQ_CONFIG command to assign dependencies between rails, or to configure unused pins
as sequencing control inputs or sequencing status outputs.
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 Power Good pin can be used to coordinate multiple controllers by running the Power Good 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 initiates 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 UCD9240 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 eight 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 Overcurrent 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.
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Product Folder Link(s): UCD9240