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TPA3125D2 instability / oscillation

Intellectual 730 points

Replies: 30

Views: 24178

Hi,

 

I am working with the TPA 3125D2 class D amplifier IC.  I am having a difficult time getting the circuit to operate well.   I have worked with a lot of audio amps over the years, but this is the first class D.  The circuit is based on the Evaluation PCB, and mono / BTL configuration.

 

There is a [ approximately ] 40KHz oscillation that I have yet to be able to find the cause of.  It shows up as distortion of course.  After building a prototype, everything looked ok, so we built 100.  Some just have a low constant 40KHz, some are high enough to cause the output components to become warm. 

 

I have tried re wiring the grounding a few different ways so far, nothing seems to have much effect.  Even with the inputs disconnected and terminated.

 

Thank you,

Dave

  • Hi Dave,

    Is it possible that the oscillation is coming from the previous stage feeding into TPA3125D2? Do you have the speaker load connected? What is your speaker load impedance? I can take a look at your schematic if you post it.

     

    Best Regards,

    Paul Chen

    Audio Applications

    Dallas, TX USA 

  • In reply to user5276421:

    Hi Paul,

     

    I have the 2 inputs connected only to a capacitor, then a 4K resistor to GND at this time, to isolate any possibility of the previous stage causing the problem.  An 8 ohm test resistor is connected.  I also have the Audio Precision connected, with 500 ohms  resistors on each lead [balanced] to isolate the cable from the amplifier.

    As I mentioned, all of the other circuitry on the input side is disconnected right now.

    Schematic:

    0728.J-324 Rev 00.pdf

     

     

    Thank you,

    Dave

     

  • In reply to David Tiefenbrunn:

    I forgot to add - The schematic has one error, that is fixed.  The AVCC pins are connected to the +24V supply.

    I have also found that if I disconnect the 10ohm / 1.5nF snubbers, the amp will come up stable, but when I even momentarily touch one of them back to connected, the oscillation starts, and won't stop.

  • In reply to David Tiefenbrunn:

    Hi David,

    Usually it is required to have 1nF high freq decoupling caps. on the AVCC, PVCCL and PVCCR supplies (need to be placed really close to the device).  Without those high freq decoupling caps you might run into issues. Also you might want to use 50V 1uF for C8 and C27.

     

    Best Reg,

    Paul Chen

    Audio Apps

    Dallas, TX USA.

  • In reply to David Tiefenbrunn:

    Hi Dave,

    Frank had asked me to look at this, too.  First question is where is the AVCC, PVCC coming from?  Is it a switching supply?  As Paul mentioned, power supply decoupling is important.   Have you looked at the noise on the power supply? 

    Also, is this on a pcb, or hand-wired?  You mentioned something about re-wiring the grounding, so that leads me to believe this is hand wired, and as such, this will not work.  Page 18 of the datasheet describes grounding, and connecting the GNDs to the thermal pad, which then is soldered to the IC at assembly.  

    If this is a pcb, would you send the layout please?   And 'scope captures of what you are seeing, as well as the power supply, AC component.

    -Leonard

  • In reply to LeonardEllis:

    Dave,

    I second what Leonard says, if you're using one of those white protoboard things, you're never going to get this to work. The high switching speed coupled with high current will kill you every time... That's a big difference between class-AB and -D!

    It might be useful if you could attach a screen shot of the 40 kHz oscillation as well.

    -d2

    Best regards,

    Don Dapkus

    Gate Driver Applications

    Dallas, TX USA

     

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  • In reply to Don Dapkus:

    Ok, I'll get some scope pictures & post the layout. 

     

    It is built on a 4 layer PCB.   We have 100 of them :(   . 

    No switch mode power supply, just DC from a linear benchtop supply.  I wouldn't even try to use a plug in proto board for a switch mode / class D anything.

     

    The oscillation is has a couple of "states".  One is fairly low p-p voltage, looks like a sawtooth.  The other is much bigger, sine wave

    there are 1uF ceramic SMT caps near all the power pins.  Do they still need 100nF or 1nF also?

     

    For clarity,  the suggestion for C8 & C27 is to go from 25 to 50V?  [they are 1uFalready]

     

    Some of the "rewiring" I've tried is things like getting the caps closer and shortening the ground routing, tying them into different points.  I have seen cases where current in the ground routing shows up as a signal to the input.

     

    Thank you,

    Dave

     

     

     

     

  • In reply to David Tiefenbrunn:

    PCB layout images.

     

    Top & bottom:

    5658.J-324 Rev 00 layout 1.pdf

    With GND:

    4722.J-324 Rev 00 layout g.pdf

    With Power:

    4572.J-324 Rev 00 layout p.pdf

     

  • In reply to David Tiefenbrunn:

    The input is terminated, blocking caps & 4K to GND.

     

    The bottom trace is a scope probe directly on the + power rail and scope is 2.0V/div. [it's a non Textronix probe, so the display thinks it's 1:1]

    The middle trace is the output.  About 50mV BTL across the 8ohm test load.  The same waveform is on each output [ L & R ] referenced to GND, it is 8V P-P.

    The top trace is the distortion analyzer output.  Ignore it. 

    The "fuzz" on the sine wave, and the bottom trace is the class D switcher fundamental / ripple.

     

    Thank you,

    Dave

     

     

  • In reply to LeonardEllis:

    LeonardEllis

     

    Page 18 of the datasheet describes grounding, and connecting the GNDs to the thermal pad, which then is soldered to the IC at assembly. 

     

    I noticed that in the data sheet.  But there is no pad on the IC itself.  It is a 20pin DIP.

    ???

     

     

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