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TPA3255EVM: Works Great But Lots of HF Noise

John La Grou
Intellectual 320 points
Part Number: TPA3255EVM
Other Parts Discussed in Thread: TPA3255

Just started testing the TPA3255EVM in diff-in / diff-out mode (stereo), and the results are good, except we're getting a large amount of oscillation in the 800kHz range. We've not touched the clock freq, nor anything else. Using the board as it was received (except changing the 2 diff-in jumpers). Any clues?

  • 0
    TI__Mastermind 30030 points
    Hi John, to clarify, you are seeing the 800khz on the output? Can you give me some more information on where you are seeing the oscillation and your test setup? This is not normal behavior.
    Best regards,

    -Steve Wilson
  • 0 in reply to Steve-Wilson
    Intellectual 320 points
    Steve, oscillation measured on the output, not the input. The output is is terminated with a purely resistive 40 ohms (we have a unique audio application). Power supply is 50V. Testing with an AP, at 22kHz bandwidth the THD is .004%, but at 500kHz b/w, THD rises to 4%.
  • 0 in reply to John La Grou
    TI__Guru 61380 points
    John,

    A few things:

    1. Please measure the PWM switching frequency before the LC filter and let us know what you see.
    2. We expect that the oscillation may be the second fundemental of the PWM switching frequency. Usually this isn't an issue since we care mostly about the audio band up to 20kHz. What is your application?
    3. Please send us a screenshot of the oscillation post LC filter given a single tone input to the TPA3255 from the AP, something like a 1kHz.

    Regards,

    -Adam
  • 0 in reply to Adam Sidelsky
    TI__Intellectual 1470 points
    Subscribing to this thread
  • 0 in reply to Adam Sidelsky
    Intellectual 320 points

    Hi Adam,

    1.) Here is the switching frequency before the filter, PWM-A

    2.) Here is the 1 kHz output waveform, with obvious HF noise.

    3.) And here is a screen shot of the noise riding on the output. Sorry, it's not 800kHz range, but rather the 460kHz PWM frequency. About 7Vpp of switching noise on the output. We're using diff-in, diff-out. Is this normal / acceptable behavior?

  • 0 in reply to John La Grou
    TI__Guru 61380 points
    John,

    Output voltage ripple is not something we spec as it's highly dependent on the customer's output LC filter, load, and similar.

    The highest loads we focus on are 8 ohm.

    If the output voltage ripple you are seeing is an issue, you may try a higher order LC filter to control more of the higher frequency content.

    Regards,

    -Adam