JAJSH44C April 2011 – March 2019 UCD90120A
The UCD90120A can control the turn-on and turn-off sequencing of up to 12 voltage rails by using a GPIO to set a power-supply enable pin high or low. In PMBus-based designs, the system PMBus master can initiate a sequence-on event by asserting the PMBUS_CNTRL pin or by sending the OPERATION command over the I2C serial bus. In pin-based designs, the PMBUS_CNTRL pin can also be used to sequence-on and sequence-off.
The auto-enable setting ignores the OPERATION command and the PMBUS_CNTRL pin. Sequence-on is started at power up after any dependencies and time delays are met for each rail. A rail is considered to be on or within regulation when the measured voltage for that rail crosses the power-good on (POWER_GOOD_ON (1)) limit. The rail is still in regulation until the voltage drops below power-good off (POWER_GOOD_OFF). In the case that there isn't voltage monitoring set for a given rail, that rail is considered ON if it is commanded on (either by OPERATION command, PMBUS CNTRL pin, or auto-enable) and (TON_DELAY + TON_MAX_FAULT_LIMIT) time passes. Also, a rail is considered OFF if that rail is commanded OFF and (TOFF_DELAY + TOFF_MAX_WARN_LIMIT) time passes.