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TPS23755EVM-894: Superimposed 11kHz at Vout when dithering enabled (amplitude modulated)

Einar Norrman
Prodigy 20 points
Part Number: TPS23755EVM-894
Other Parts Discussed in Thread: TPS23755

Tool/software:

Hello,

I'm currently performing some measurements on a PoE design based on the TPS23755, which I’ve developed—essentially a copy of the reference design. I’ve observed that when increasing the load from around 3 W to over 8 W, I not only see the expected switching frequency at ~250 kHz, but also an amplitude-modulated component at around 11 kHz or so. The output ripple roughly doubles.

I also have access to the TPS23755 EVM, and it exhibits the same behavior—until I disable the switching frequency dithering. Since the dithering frequency is somewhere in the 11 kHz range, that would seem related, but it feels strange that it would result in such a clear amplitude modulation. What’s your take on this?

Please see the attached oscilloscope captures:

Dither. On


Dither. Off

Best Regards,
Einar Norrman

  • 0
    TI__Mastermind 37071 points

    Hi Einar,

    Thanks for reaching out!

    We will check your question and get back by this week.

    Best regards,

    Diang 

  • 0 in reply to Diang Xing
    Prodigy 20 points

    Thanks Diang!

    I can just add that the 11kHz component is also there during low loads actually, but not as clear.

    Br,

    Einar

  • 0 in reply to Einar Norrman
    TI__Mastermind 37071 points

    Hi Einar,

    The 11kHz sounds like the frequency dithering modulation frequency. This frequency will be inside the modulator and result in low frequency oscillation at output. But in most of cases, this low frequency will be significantly reduced by the filter. 

    For your cases, the 11kHz could be caused by probe coupling or insufficient output cap. TPS23755EVM only has 2 x 22uF ceramic caps which can be increased to >100uF mitigate the output ripple. 

    Besides, the Vp-p is still 88mV which is less than 0.7% of 12V output. Will it be a concern for your case? 

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 20 points

    Hello and thanks!

    I just wanted to make sure noting was wrong. For me it should be fine!

    Best Regards,

    Einar

  • +1 in reply to Einar Norrman
    TI__Mastermind 37071 points

    HI Einar,

    Thanks for your check. I think it is fine since the ripple amplitude is not significant. 

    We had seen multiple dithering related noise issues from different customers. Dithering frequency is real in Vin and Vout but normally they are not huge if there are sufficient capacitance.

    Sometimes the noise could be amplified by the coupled flyback inductor and input / output caps and make it audible. Most of these issues can be solved with transformer exchange or input / output capacitance changes. 

    Best regards,

    Diang