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UCC256402: Fault Recovery Questions

Part Number: UCC256402

Hi TI experts,

My customer is using the UCC256402. I have an inquiry.

Looking at the waveform below, if power is restored within 1 second after power failure, normal operation will not work.

The data sheet says Fault Recovery in 1 second, is it correct that the output does not come out within 1 second?

Ask if there is a way to save another second.

  • Hi Leo,

    If the controller detects a fault such as input undervoltage, the controller state machine will move to the FAULT state. The controller will wait for 1s. After this 1s wait time, the controller will move to the startup routine, charging up the VCC capacitance.

    The portion in the waveform labeled "not working" shows the controller has charged up VCC all the way to 26V but it does not look like the controller entered soft start. Could you share waveforms of BLK, LL/SS and FB pins during the fault/recovery? Something is telling the controller to not begin soft start in this portion of the waveform.

    Best Regards,

    Ben Lough

  • Hi, Ben Lough

    Thanks for the answers to your questions.

    We will measure and share additionally requested BLK, LL/SS, and FB waveforms.

    Please provide additional confirmation answers.

    Thank you.

  • Hi Leo,

    Thanks for the waveforms. It looks like the controller is detecting a fault condition on the recovery attempt at the beginning of the "not working" time period. The LL/SS voltage begins to rise (and the output voltage is starting to rise) but then LL/SS is pulled low and the controller is waiting another 1s before attempting to restart again. I would suggest taking a look at the ISNS and BW pins. Perhaps more margin on the OCP or OVP threshold is needed to achieve reliable and consistent restart.

    Best Regards,

    Ben Lough