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TAS5706A amplifier will not output full power at certain frequencies

BRIAN DOTSON
Prodigy 60 points
Other Parts Discussed in Thread: TAS5706A

I am working on a product that uses three TAS5706A stereo amps in 2.0 BTL mode. Our REV B PCB performed admirably with my worst Audio Precision torture tests, such as running a sweep at 20W x 2 into 8 ohms from 20 Hz to 20 KHz. However, our REV C PCB for this product (slated for release to production), has introduced a weakness. Above 11 W x 2, the amplifier immediately goes into some kind of shutdown mode at frequencies form 16.5 KHz to 19 KHz. It will do full rated power below 16.5 KHz, or if I turn it on at 20 KHz. Other facts:

(1) Idle PWM remains during shutdown, it is the audio modulation on it that the amp cuts off.

(2) The modulated PWM at full power comes back after about 2 seconds, and then the cycle repeats itself.

(3) This happens right off the bat on a relatively cool TAS5706A when you hit it with the full amplitude I2S signal at a vulnerable frequency. It does not seem to be thermally rated.

  • 0
    Prodigy 60 points

     A few more facts that I forgot:

    (4) This seems to be driven by amplitude at the vulnerable frequencies. It happens even with no load if you drive the same levels.

    (5) Monitoring of PVCC rails (nominally about 21.4VDC) shows nothing bad happening with our power supply. Monitoring current coming into all the PVCC pins shows no increase in current when we approach the "bad" frequencies.

  • 0 in reply to BRIAN DOTSON
    TI__Mastermind 22965 points

    Brian,

    It would be good to know what changes were made from the Rev.B to Rev.C boards (schematic & layout etc.). (You can send me the info directly to me via E2E send-message option)

    When the device goes into this mode, what is the status of the VALID pin? Does it stay HIGH or does it toggle LOW?

    Also, what mode is the device being used in (AD or BD)?

    Best Regards,

    Ravi

    Audio Applications Engineer

  • 0 in reply to Ravi Singh
    Prodigy 60 points

    Hello, Ravi.

    I found out today that this was not at all what it seemed. It seems that we were getting a spurious input to our control processor from a stray circuit between the PWM signal out of the TAS5706A's headphone ouptuts, and a line output jack insert sense logic line. The processor was re-initializing the TAS5706A, causing an interruption in the audio. A really strange turn of events that imitated the behavior I have seen with the thermal protection circuits on your PurePath amplifier devices.

    So, case closed! We know how to fix it.

    Thanks for considering this case.  I look forward to working with you again in the future, sir!

    Brian Dotson,

    Senior Design Engineer, Nuvo Technologies