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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.

  • In reply to Robin Yu:

    Hello Robin,

    Just for my understanding: You mention that this ringing is observed when a picture is displayed. Would you please give more information about the problem you are observing? Is there some visible effect on the picture?

    For the differences between OUT1 and OUT2, is it possible that the Code for these 2 outputs is set differently? What is the code for these 2 outputs?

    BTW, you might need to add some smaller capacitors to buffer the fast drop. A capacitor with lower capacitance normally has as well smaller series resistance, so for buffering it might be better to add a high quality 1uF capacitor instead of a 10uF capacitor. The dielectric should be X5R or better and please always keep the DC bias of ceramic capacitors in mind.

    You might try as well some filter on AVDD to keep in clean. The output voltage on the buffers is directly dependent on the analog supply voltage, as given in formula 1 in the datasheet on page 11. So a transient on the supply can be observed directly on the output and if the output voltage is very close to Vs, this effect will be worst.

    Best regards,
    Brigitte

  • In reply to Brigitte:

    Hi Brigitte,

    Thanks for replying. See my input.

    Just for my understanding: You mention that this ringing is observed when a picture is displayed. Would you please give more information about the problem you are observing? Is there some visible effect on the picture?

    [Robin] Actually this is a very complex problem. The visible effect on the picture (most are pure color) is the issue they call "mura". But it is not sure that this is caused by ringing of BUF18830. At the start of every frame, current sensing and maybe some other sensing should be done, a fast load transient performance and stable output of AVDD and gamma is appreciated.  

    For the differences between OUT1 and OUT2, is it possible that the Code for these 2 outputs is set differently? What is the code for these 2 outputs?

    [Robin] OUT1 to OUT9 are used for reference of Source driver to generate G1 ~G9. G1 is the highest output which is close to Vs. What is the code? output voltage?

    BTW, you might need to add some smaller capacitors to buffer the fast drop. A capacitor with lower capacitance normally has as well smaller series resistance, so for buffering it might be better to add a high quality 1uF capacitor instead of a 10uF capacitor. The dielectric should be X5R or better and please always keep the DC bias of ceramic capacitors in mind.

    [Robin] You mean add small capacitor at the output of BUF18830? Or at AVDD? At AVDD, they have added a lot of capacitors, including large one and small one. As for BUG18830, they also have do some test, they tell me a little improvement is observed, but I don't know what value has been used.

    You might try as well some filter on AVDD to keep in clean. The output voltage on the buffers is directly dependent on the analog supply voltage, as given in formula 1 in the datasheet on page 11. So a transient on the supply can be observed directly on the output and if the output voltage is very close to Vs, this effect will be worst.

    [Robin] OUT2 has small peak value but more ringing times

    Thank you, again.

    Robin

  • In reply to Robin Yu:

    Hello Robin,

    Did you find some more time to work on this issue? Could you find out if the mura is dependent on the BUF18830 output voltage?

    Code means the portion of AVDD that shall be shown on the output of the gamma buffer. It is normal that the output that is closest to AVDD shows the biggest distortion by a change in AVDD.

    The buffer capacitors would be necessary on AVDD but close to the BUF18830 device to make it more immune to AVDD changes. It is as well important which type of capacitors are used. X5R or better dielectric should be used and be aware of DC bias. Ceramic caps loose a lot of their capacitance if DC voltage is applied. This needs to be checked and sometimes it is even not given in the datasheet of the capacitor and needs to be asked from the manufacturer.

    Mura means a luminance difference of parts of the panel. Is this correct? Is there a possibility to check the picture content at the moment when you observe the transient on AVDD? I could imagine that directly at the start of the frame, the content is black for some lines and then the transient effect should be gone before content is displayed.

    Best regards,
    Brigitte