BUF18830: Ringing on out pin

Part Number: BUF18830

Now the customer is using BUF18830.

Below is the SCH.

SCH.docx

When the picture displayed on the panel is changing, a ringing is observed as below.

Below is the waveform on OUTx pin. (The ringing amplitude is a little different from each other)

Below is the power rail of VS (powered by AVDD of TPS65177)

Could you please give some suggestion about that?

Thank you.

  • Hello Robin,

    Could you please identify the different waveforms (blue/pink/yellow=?)

    Best regards,
    Diaara

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  • In reply to Diaara:

    Hi Diaara,

    I'm really sorry that I forget that.
    The yellow line in all figures is the curve I mentioned. (OUTx and Vs)
    All others curve are other signals for the panel, not BUF18830.
    Thank you.

    Robin
  • In reply to Robin Yu:

    Hello Robin,

    It seems that the pink signal is somehow linked to the ringings as your firsts scope plots show. It would be good if we knew which signal it is.
    The ringings on OUTx are most probable due to the ringing on AVDD. You could confirm this by increasing and reducing the output capacitance on AVDD.
    Does the panel not work or have some problem due to these ringings on OUTx?
    Also when you say "Below is the waveform on OUTx pin. (The ringing amplitude is a little different from each other)" do you mean that the ringing is different for each output or it is different for different pictures?

    Best regards
    Diaara

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  • In reply to Diaara:

    Hi Diaara,

    The pink signal is related with the moment when picture is changed. At this time, the load current of AVDD will change from about 400mA to 700mA(Customer told me that). So a voltage spike occurs due to load transient.
    "Below is the waveform on OUTx pin. (The ringing amplitude is a little different from each other)" , I refer to that ringing is different for each output, but not too much.
    Actually we have add the output cap from 10uF*4 (25V rating) to 10uF*4(25V)+ 10uF*5(50V)+47uF(Al eletrolysis).
    Thank you.

    Robin
  • In reply to Robin Yu:

    Hello Robin,

    I am sorry but for some reasons my last answer was not published.

    What is the value of AVDD and the part# of the output caps? I think that this increased output capacitance should not be necessary and the resulting transient on AVDD is poorer that I would expect. Please see the attached scope plots showing the load transient measurement results on AVDD of TPS65177 and OUTx of BUF18830; I used these  devices EVMs for the test and connected AVDD to Vs.

    Could you please measure again with a scope during the transient event AVDD on C260 and C675? I want to make sure that we optimize first this transient response because OUTx should follow then.

    Best regards,

    Diaara

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  • In reply to Diaara:

    Hello Robin,

    Do you have any update regarding this issue?

    Best regards,

    Diaara

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  • In reply to Diaara:

    Hi Diaara,

    Great thanks for your attention for this case.

    The customer reflects that adding cap close to Vs pin doesn't help a lot.

    Now it is still on a holiday of China, no more feedback from them.

    I would go to field to have a look.

    Do you have some more suggestions to figure out the problem?

    Thanks again.

    Robin

  • In reply to Diaara:

    Hi Diaara,

    Below is the waveform at C260 and C675 node.

    Waveforms of BUF18830.docx

    There is some difference between them. After current sensing circuit, some ripple is observed on AVDD rail.

    And also, for OUT1 and OUT2, the waveforms are really different.

    I have tried to add capacitors on several Vs pins (double to 10uF*2), but no obvious improvement is observed.

    And customers themselves have also tried to add capacitors on OUTx pins, a little improvement is observed.

    Any suggestion for this case.

    Thank you.

  • In reply to Robin Yu:

    Hello Robin,

    Thanks for posting the waveforms.

    On the initial schematic you posted I see that on the COMP pin there is an additional capacitor to ground C285  in parallel to C288-R110. Is this one mounted? If yes I would recommend to remove it and redo the transient measurement without it because it is much too big and this could be the reason of the bad transient behavior of AVDD converter. 

    Best regards

    Diaara

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  • In reply to Diaara:

    Hi Diaara,

    I think you refer to C289, actually I had taken it off. Before it was taken off, the transient time was almost 2ms, while now it is about tens of micro seconds.

    All the waveforms I send to you are tested in the condition C289 is taken off.

    Thank you.

    Robin