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Glitching in the TPA3125D2

Intellectual 420 points

Replies: 31

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I am using the TPA3125D2 in exactly the BTL configuration depicted in Fig 30 of the 2008 datasheet. Supplies are perfectly quiet at 24V. I have an additional 20K ohms in series with two audio inputs that are being summed at Rin (return is common at Lin). \SD is tied to 24V; Mute is tied to Gnd. Gain0/1 are both at Gnd for now, but can be switched under firmware control.

I am getting a noise burst at the output every 276 mSec, lasting 52 mSec each. It seems to be white noise, above normal audio frequencies but rail-to-rail and thus audible (deafening, actually) at the output. The board is controlled by a PIC microcontroller with which I have extensive experience, and holding the microcontroller in reset has no impact on the noise, so any interaction with the firmware or other sections of the board seems to be eliminated.

Vclamp is sitting at about 10.6V, and droops by about 200 mV during each noise burst. Not sure if this is a cause, effect, or some other symptom.

Where do I go next?

  • Hi, Steve. Thanks for using the TPA3125D2 in your design.

    Were you able to measure the power supply ripple at the amplifier itself? Can you please share a plot of this? I'm curious if the amplifier is having some issues with its PVCC, because VCLAMP relates to VCC, and you're seeing effects on it.

    Are you able to measure these noise bursts on an oscilloscope and post the results here? Try to get the signal before and after the output filter.

    I assume this is your own schematic and layout. It helps if you can share them. One thing to check in your layout is that the 0.1 µF ceramic capacitor for PVCC coupling is placed as closely to the PVCC (L and R) pins as possible. The same applies for the 0.1 µF and 10 µF capacitors coupled to the AVCC pins. We see very strange things happen when this isn't the case. Check to make sure your 470 µF bulk capacitors are rated for well above 24 V. I suggest 50 V rating in case there is power supply ripple we haven't measured yet. Make sure the bootstrap caps are placed as closely as possible to the switch nodes and bootstrap pins.

    Are GAIN0 and GAIN1 asserted to ground by your microcontroller, or are they actually tied to ground? A part in this line of products called the TPA3106D1 experiences issues when SHUTDOWN goes low, and then GAIN0 and GAIN1 are driven from an external source (such as a microcontroller). Since your \SD is tied to your supply, I have to wonder if something similar is happening, because if \SD goes low with your PVCC, and GAIN0 and GAIN1 are driven by the microcontroller running off a separate power rail (I assume), things could get funny the next time you apply PVCC. See this forum post and this Application Note for more on this error. I am not sure if it applies here, but it sounds similar.

    I hope this helps!
    Matt

    Matt Lauer

    Home Audio Applications

  • In reply to Matt Lauer:

    • Were you able to measure the power supply ripple at the amplifier itself? Can you please share a plot of this? I'm curious if the amplifier is having some issues with its PVCC, because VCLAMP relates to VCC, and you're seeing effects on it.

    Any supply ripple is really small, order of millivolts at most.

    • Are you able to measure these noise bursts on an oscilloscope and post the results here? Try to get the signal before and after the output filter.

    Yes, as described earlier.

    • I assume this is your own schematic and layout. It helps if you can share them.

    It accurately matches the sample schematic given in the datasheet, except for the previously described summation of two audio inputs via 20K resistors.

    • One thing to check in your layout is that the 0.1 µF ceramic capacitor for PVCC coupling is placed as closely to the PVCC (L and R) pins as possible. The same applies for the 0.1 µF and 10 µF capacitors coupled to the AVCC pins. We see very strange things happen when this isn't the case.

    the only way to get them any closer would be to move them up onto the legs of the part. I'm well-experienced with RF, UHF, and microwave layouts where even a via is too long; my layouts tend to get into trouble with the manufacturability boys for being too tight. The caps here are 0603s, right on the pads of the DIP leads.

    • Check to make sure your 470 µF bulk capacitors are rated for well above 24 V. I suggest 50 V rating in case there is power supply ripple we haven't measured yet. Make sure the bootstrap caps are placed as closely as possible to the switch nodes and bootstrap pins.

    This is one slipup on my part. The local cap was insufficiently rated, and has been lifted. The bulk capacitance is at the power supply, perhaps an inch of trace away.

    • Are GAIN0 and GAIN1 asserted to ground by your microcontroller, or are they actually tied to ground? A part in this line of products called the TPA3106D1 experiences issues when SHUTDOWN goes low, and then GAIN0 and GAIN1 are driven from an external source (such as a microcontroller). Since your \SD is tied to your supply, I have to wonder if something similar is happening, because if \SD goes low with your PVCC, and GAIN0 and GAIN1 are driven by the microcontroller running off a separate power rail (I assume), things could get funny the next time you apply PVCC. See this forum post and this Application Note for more on this error. I am not sure if it applies here, but it sounds similar.

    Gain0/1 are indeed driven by microcontroller outputs. However, the microcontroller supply is itself derived from the 24V supply that feeds the TPA3125.

    Thanks for taking the time for a detailed response. Any other ideas?

  • In reply to Steve Hendrix:

    My Service Request # 1-821703757 has received no response other than to direct me back here. Any chance of any further support for this issue?

  • In reply to Steve Hendrix:

    Hi, Steve. I'm sorry for not writing back earlier. I am looking further into what might be the root cause of this. I intend to reply back with more information today.

    Matt

    Matt Lauer

    Home Audio Applications

  • In reply to Matt Lauer:

    Hi, Steve. I'm going to need some more information to help you further. Can you please provide the following?

    1. An oscilloscope plot of the voltage at the BYPASS pin of the amplifier
    2. An oscilloscope plot of the bursting output waveform you're seeing
    3. A part number for the capacitor attached to the BYPASS pin (or just a description: X5R, X7R)
    4. Your schematic/layout. Even if it's just the section with the TPA3125D2 (I don't want you to reveal anything you're uncomfortable revealing), it will help tremendously. I understand it's similar to the application circuit in the data sheet, but I have to admit I'm confused about the 20 kΩ input resistors you mentioned. I'm a visual thinker, so your schematic will help me make sense of it.

    I hope I can help you further on getting this information!

    Matt

    Matt Lauer

    Home Audio Applications

  • In reply to Matt Lauer:

    I have prepared images of hte schematic and layout of the relevant section and captured them in a Word doc, but I can't find any way to attach them here. The "Paste from Word" button doesn't work for the pictures, and I don't see anything to attach a file. How?

  • In reply to Steve Hendrix:

    I'm getting pressure from my client to get this resolved, and still can't find any way to paste anything in here. The "Paste from Word" button doesn't work, nor do any of the normal Windows keyboard shortcuts to cut and paste.

  • In reply to Steve Hendrix:

    Hi, Steve. I'm sorry I didn't reply when you first wrote. I'm also sorry your client is pressuring you at this point. I'll do my best to help you.

    Attaching a file is done with the small paperclip icon in the text entry menu. If you hover over it, it will display "Insert File". It is immdiately left of the "Past from Word" button. A menu will pop up giving you the option to insert a file. It may take a few seconds to load. Please let me know if this gives you any trouble.

    Matt

    Matt Lauer

    Home Audio Applications

  • In reply to Steve Hendrix:

    Thanks, Steve. This helps. Are you able to provide the other data I'm asking for below? I have a suspicion this problem has to do with the BYPASS cap, and the waveforms I'm asking for will help me determine if this is correct.

    1. An oscilloscope plot of the voltage at the BYPASS pin of the amplifier
    2. An oscilloscope plot of the bursting output waveform you're seeing
    3. A part number for the capacitor attached to the BYPASS pin (or just a description: X5R, X7R)

    Regards,
    Matt

    Matt Lauer

    Home Audio Applications

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