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Original question:

TPS54620: about EN and PWRGD pin

TPS54620: Sequential Startup of Multiple TPS54520: PWRGD connected to following EN, but PWRGD doesn't go HIGH.

Kevin Jiang
Prodigy 50 points
Part Number: TPS54620

Hi,

I have multiple TPS54620 that I am aiming to power-on sequentially using the technique illustrated in the part manual (reproduced below):

I have connected the PWRGD pin of one of my TPS54620 to the EN pin of the next TPS54620. I have NOT used an external pull-up resistor.

The converter is functioning properly and producing the correct voltage with minimal ripple (NO choppiness). In addition, I checked that the VSENSE pin and measured it to be 0.799V which means it is within 94% to 106% of VREF. So I can't seem to understand why PWRGD isn't going High by itself. The only thing I notice that is unusual is that the inductors are whistling but I'm not sure if that's relevant since the output voltage is correct and stable. Please let me know if you have any insights or further suggestions to try.

Thank you,

Kevin

P.S. I realize there's another forum post about PWRGD and EN pin for sequential start-up, but that forum post has 2 TI employees who provide contradictory answers. One says that no pull-up resistor is necessary, and the other suggests to use a pull-up resistor. Since I've already fabricated my PCB without a pull-up resistor like the manual suggested, I'd like to find a solution that doesn't require me to use a pull-up resistor.

  • 0
    TI__Guru**** 191445 points
    Kevin,

    Let me see if I have this correct.
    You have two TPS54620 circuits. The PWRGD output of the first converter is tied the EN input of the second converter. The first converter starts up normally and regulates to the proper output. The first PWRGD is pulled up to around 2.4 V and the second converter is also regulating. The PWRGD of the second converter is floating and not high even though the second converter is regulating.
    It would help if you can provide your schematic and waveforms that show your issue. Let me know and we can advise.
  • 0 in reply to JohnTucker
    Prodigy 50 points

    Hi John,

    The 1st converter starts up normally and regulates to the proper output. However, the 1st PWRGD remains LOW. Hence, the 2nd converter always remains OFF.

    VSENSE = 0.799V which is within 94% to 106% of VREF (= 0.8V).

    I would provide the schematic, but since we have a lot more than just 2 TPS54620 circuits and they are in a hierarchical patter, it might be too confusing to describe. I can assure you that it is equivalent to the manual though.

    Thanks,

    Kevin

  • 0 in reply to Kevin Jiang
    Prodigy 50 points

    Here is the schematic for the TPS54620. PWRGD goes to the EN of the next TPS54620 (not shown).

  • 0 in reply to Kevin Jiang
    Prodigy 50 points
    I resolved it. I removed R131, but not R132. The 8kOhms resistor was pulling down the PWRGD pin. I had assumed that the 8kOhms resistor was big enough to not be able to do this. Thank you for your help!

    Kevin