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TPA3126D2: Output drops by 6dB

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Replies: 34

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Part Number: TPA3126D2

My design using the tpa3126d2 is showing an issue.  The output looks good but after playing a few minutes it drops by about 6dB.  Input voltages are the same.  Doesn't appear to be thermal related.  The audio input signal is at the same level.  The input is single ended.  Checked the GVDD voltage and see no change when the level drops.  Any ideas?

  • Hi,

    Could you please share your schematic to us? You attached picture can not be shown.

    One question, did the output power reduce slowly or quickly when reach one power point?

    Regards,

    Derek

  • In reply to Yanming Gong:

  • In reply to user5838394:

    Hi,

    Did you check power before common choke (after LC filter)?

    TPA3126 output will reduce slowly on high power since of thermal, but the power reduce will be very slowly and very low.

    I am afraid that the peripheral circuit will have influence.

    And you can also check on EVM board.

    Regards,

    Derek 

  • In reply to Yanming Gong:

    There doesn't appear to be any drop on the common mode choke.  Additionally, this is a configuration for this part I've used on a couple other designs and have not ran into this issue.  The drop in output is a definite step change, and stays constant after no longer how long I stream signal.  I've seen thermal cycling before, and the fault line triggers and the amp goes into total shut down, during that case.  The amp is running off of a 24V PVCC rail.  Before the fold back, I'm getting about 28Vp-p into a 4 ohm load.  After the fold back I'm getting about 14Vp-p.

  • In reply to user5838394:

    Hi 

    What's the test point? Did you check on EVM board yet? 

    Regards,

    Derek

  • In reply to Yanming Gong:

    I don't know what "test point" you refer to.  If you mean where am I taking my voltage measurements at, I'm comparing input voltages at the input cap to the amp, and the output voltages across the 4 ohm load.  I did check, as you requested, the input voltage to the common mode choke in the output emi filter, but it was the same as the voltage across the load during this "fold back" condition.  I'm not sure what the evaluation board would have to do with this at all since its different circuitry.

    As I mentioned before, this is my 4th implementation of this same output configuration.  I have not seen this issue before.  So yesterday I was able to have some thermal images done comparing this design to one of the previous implementations.

     This shows a the max temperature of a similar implementation of the chip without the heatsink .

    This shows the new design in the same condition (without heatsink) just prior to the gain reduction .

    And this shows the new design after the gain reduction

    In comparing the two layouts the only difference is that the first design has vias under the chip.  The new design has them removed because we were/are fighting emi issues and I was told that stitching the ground planes together can cause worse emi than not.  Obviously this is telling me that the thermal conduction of those missing vias is needed, and that our heat sink design is not performing as required.  According the the spec sheet on this part, I should be able to run 50W continuous though this amp chip, and yet I have not been able to do that without going into full thermal cycling which I now believe is due to the heat sink design not properly keeping the chip temperature down, which is what I always believed but have not actually required to run it at 50W yet.  What I don't understand is why this doesn't go into a fault condition due to thermal, but instead seems to fold back about 3dB based on actual voltage calculations, then stabilize.   I had pictured the thermal protection to be (and witnessed this function as) more of a step condition.  Violate the temperature and the chip shuts down, will restart after a time delay due to how we've connected the SDZ line to the FAULT line.  I've ordered some freeze spray and will attempt to cool the chip to see if the gain corrects itself, but I'd like to understand exactly what I should expect from a over temperature condition.

  • In reply to user5838394:

    Hi,

    Thanks for your clear description!

    I can't believe that thermal vias under chip can affect EMI performance. I think the vias can not affect EMI. The thermal vias under chip is very important for thermal, if you can add enough vias under chip, it will have great help for thermal.

    Device internal Rdson impedance will increase with thermal, so on high power, the power dissipation will increase quickly if bad thermal design.

    I think the fault is OTSD fault when high power.

    Regards,

    Derek 

  • In reply to Yanming Gong:

    So that was my first thought also.  I obtained some freeze spray and after the part reduced its output level, I applied the coolant.  The part did not recover.  So my questions are:

    1. If this is thermal, why does the part not pull its FAULTZ line low?
    2. Are there thermal conditions that would require a power reset to recover from?

    On additional test I ran was that while operating in what we believe is this thermal condition, I manually pulled the FAULTZ line low (which is tied to the SDZ line), while applying the freeze spray.  When I released the line, the amp performed with the proper gain (until it heated up again of course).  This would imply that there is a thermal condition that does not actually activate the fault line.

    Thoughts?

  • In reply to user5838394:

    Hi 

    The OTSD fault is latched, it can not be auto-recover.

    The only thing I can thought is OT issue. Could you check on EVM board with same condition?

    Regards,

    Derek

  • In reply to Yanming Gong:

    First how can I duplicate this apparent thermal issue on the dev board?  Second, if its an OTSD fault, why is the FAULTZ line not being pulled low?  Lastly, the FAULTZ line is tied to the SDZ line so that, based on my understanding and testing, it will reset the fault latch.   So the main issue I see is why is the FAULTZ line not being pulled low if there's a thermal fault?  Also I didn't get email notification that  you replied, so sorry for the late response.