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LM2841 abnormal waveform


I have a question for the waveform of output voltage.  The question has been sent to Juan Paulo by e-mail.  PLS answer it.

  • We will get back to you ASAP.
  • Please send the schematic and layout in pdf file for review. Thank you.

  • Hi, Yang

    I will ask customer to get the schematic and layout to review.
    By the way, It also seems that the regulator stop the regulation by any fault or some event.  Do you have any assumed cause?

    Best Regards, Taki

  • Repeat the reply via email here:

    From the schematic I can see why the part behave like this. When Vout is higher than Vcc, the part cannot start when there is no load. With no load, SW node does not go to ground. It stays at Vout level, so the boot cap cannot be charged. If Vout is lower than Vcc, it is ok. That's why 3.3V Vout start up alright. A solution is to have some minimum load current, as when you lower the FB divider values. With minimum load, the SW node stays at ground level for long enough time, and then the boot cap can be charged. The SW node waveform in your plot I believe is BOOT waveform.
    In the Webench design, the components were selected for the X version (550kHz). If the same BOM is used with Y version (1.5MHz), it may have some issues. The large inductance in the schematic (120uH) makes start up even harder. I've tried a Webench design with the Y version and the inductance recommended is 33uH.

    Hope this helps.

    Best regards,