7.4.5 Shut Down All Rails and Sequence On (Resequence)

In response to a fault, or a RESEQUENCE command, the UCD90120A can be configured to turn off a set of rails and then sequence them back on. To sequence all rails in the system, then all rails must be selected as fault-shutdown slaves of the faulted rail. The rails designated as fault-shutdown slaves will do soft shutdowns regardless of whether the faulted rail is set to stop immediately or stop with delay. Shut-down-all-rails and sequence-on are not performed until retries are exhausted for a given fault.

While waiting for the rails to turn off, an error is reported if any of the rails reaches its TOFF_MAX_WARN_LIMIT. There is a configurable option to continue with the resequencing operation if this occurs. After the faulted rail and fault-shutdown slaves sequence-off, the UCD90120A waits for a programmable delay time between 0 and 1275 ms in increments of 5 ms and then sequences-on the faulted rail and fault-shutdown slaves according to the start-up sequence configuration. This is repeated until the faulted rail and fault-shutdown slaves successfully achieve regulation or for a user-selected 1, 2, 3, or 4 times. If the resequence operation is successful, the resequence counter is reset if all of the rails that were resequenced maintain normal operation for one second.

Once shut-down-all-rails and sequence-on begin, any faults on the fault-shutdown slave rails are ignored. If there are two or more simultaneous faults with different fault-shutdown slaves, the more conservative action is taken. For example, if a set of rails is already on its second resequence and the device is configured to resequence three times, and another set of rails enters the resequence state, that second set of rails is only resequenced once. Another example – if one set of rails is waiting for all of its rails to shut down so that it can resequence, and another set of rails enters the resequence state, the device now waits for all rails from both sets to shut down before resequencing.