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TAS5760M: TAS570M in PBTL Over current fault due to the LC filter capacitors

Intellectual 880 points

Replies: 5

Views: 822

Part Number: TAS5760M

So I spent the last few days working through a fault with my amplifier board, with the TAS5760 going into over current fault as soon as it is enabled.   Verified that the configuration was written successfully (by reading it back out of memory) and examined the soldering as close as my equipment allowed to make sure there were no shorts or bridges, even replaced the amp on the board just in case i had damaged it in development (I had seen it in past projects where the audio chip had its output drivers blown because of some fault or another when working on programming the rest of the board, usually getting to the audio chip last when everything else is working.

anyway.  The circuit schematic is taken from the example application in the PDF for PBTL under software control mode, however in keeping space down, I changed the values of the LC to keep a similar cutoff frequency to the EVM.  instead of 10uH and 0.6uF, i had used 1uH and 4.7uF.   not a large change in my mind  however it was these 4.7uF caps getting charged that was tripping the over current fault (cap would charge to about 4V then fault and slowly bleed down to 0, and reset the fault and recharge about every 3 seconds or so. 

Removing the caps, and the amp powers up happily and controls the speaker now.  kudos to the over current detection.  it reacts very quickly, unfortunately it was shooting me in the foot.  I post this here more as a troubleshooting note for others to read over if they have a similar phantom over current fault that is difficult to figure out what is causing it.  I will change the values of the LC circuit back to the EVM values in future revisions.  Maybe have a note somewhere in the documentation about how sensitive the over current to the LC filter values.

  • Hi Joseph,

    We will take a look and then get back to you shortly. Thanks.

    Andy
  • In reply to Andy Liu SH:

    Hi Joseph,

    Can you tell us what inductors you used for the LC filter and possibly the datasheet for them? I want to make sure there wasn't an issue with the current saturation.


    Regards,

    Robert Clifton
  • Hi Joseph,

    See the screenshot below for recommended LC filter values.  

     Also, you have to make sure the inductors can handle enough current.

    Andy 

  • In reply to Andy Liu SH:

    This is the inductor i used
    PA4332.102NLT

    Datasheet
    products.pulseelex.com/.../P761.pdf
  • In reply to Joseph Woodrell:

    Hi Joseph,

    Thank you for the inductor datasheet.

    I have verified using LC values like the ones you were using can lead to large peak currents during device startup due to the LC filter charging as you were seeing. I also agree that we should add a note in the datasheet explaining why we recommend these values.

    Thank you for pointing this out!

    Best Regards,

    Robert Clifton

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