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UCC2897A: Output Unstable

Kuan Wei Chen (Kevin Chen)
Genius 9710 points
Part Number: UCC2897A
Other Parts Discussed in Thread: TL431, ATL431

Dear Team,

Here is my previous thread, but I didn't receive a further suggestion from your team. So I create a new thread.

Please take a look of the attached waveform for Vgs and Rsense.

It trips the 0.5V threshold so the Vout didn't regulate at 12V.

They modify the Rsense from 150m to 40m, but it will have audible noise during low load.

Do you have a further suggestion on it?

BTW, is 20uH acceptable here? Because we don't have suitable calculation tool here to check the suitable parameter.

BR

Kevin

  • 0
    TI__Mastermind 34465 points
    Hi Kevin,
    The power supply requires a clean CS signal for control and protection on the CS input pin.
    The signal shown above does not look like it is suitable to do any of this.
    I dont understand your question about 20uH. There is an Excel calculator tool available for the UCC2897A and this should be used in the design.
    Regards
    John
  • 0 in reply to John Griffin1
    TI__Genius 9710 points

    Dear John,

    Ok, I will try to use tip and barrel to measure the waveform again.

    Here are more waveforms maybe you can have some ideas on how to solve the issue here.

    Rsense=40mohm, I think the Rsense is too small to cause the unstable, but I am not sure. (Io=3A)

    Rsense=75mohm, the Vout also unstable and will have audible noise.(Io=3A)

    I thought the Vout unstable is cause by TL431, so I change to ATL431.

    Please kindly suggest some solutions which I can try tomorrow!

    BR

    Kevin

  • 0 in reply to Kuan Wei Chen (Kevin Chen)
    TI__Mastermind 34465 points
    Hi Kevin,
    That is a big change and it is more like what is expected on the CS pin.
    The trip level for CS is 0.5V and you are only getting 0.33V in these waveforms.
    I guess thats why decreasing the current sense resistor was a bad idea.
    It looks like some other condition is causing a shutdown.
    Can you check VDD, LINEUV and LINEOV ?
    These are the most likely inputs that can cause a shutdown and restart.

    Regards
    John
  • 0 in reply to John Griffin1
    TI__Genius 9710 points
    Dear John,

    It turn out to be the customer didn't instal the output cap.
    So I will close the thread here.

    BR
    Kevin