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UCC21521: VDDB UVLO issue

user5105395
Prodigy 70 points
Part Number: UCC21521
Other Parts Discussed in Thread: UCC21520

We use ucc21521ADW to drive a half bridge, VOB for high side MOS with a bootstrap diode as typical application, once we disable the driver by EN pin and then restart the IC, VOB also has a 50us delay to output pulse, although both input VIA and VIB always be there. We measured the waveform of VDDB vs VSSB, the voltage was setup by one switching cycle and the value excess the UVLO_ON threshold. It looks like that  needs a response time to generate the output. How do we avoid this response delay and get the two pulses simultaneously when we release EN pin? 

  • 0
    TI__Expert 3355 points

    Hi,

    Thank you for your question. I work on the applications team in the high power drivers group.

    As you have seen, UCC21521 has an approximately 50us UVLO turn-on delay. This delay is necessary for the driver to properly start up internal voltage rails and ensure proper operation. Unfortunately, when using a bootstrap circuit, there is no way around this delay, so it will need to be considered at the system level. We have seen many other customers able to successfully implement a bootstrap pre-charge routine, like described in the image below. The other option is to switch to an isolated bias supply, like a flyback or push-pull, which helps power up the device before switching starts.

     

    I see that this delay has been added to the UCC21520 datasheet, which uses the same silicon. I will work to add this start-up delay and further information to the UCC21521 datasheet as well.

    If this answered your question, could you please press the green button? If not, feel free to ask more questions.

    Thanks and best regards,

    John

  • 0 in reply to John Geiger
    Prodigy 70 points

    Got it, thanks!