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TAS5710 asserts FAULT when exiting shutdown

Paulo Marques
Prodigy 50 points
Other Parts Discussed in Thread: TAS5710

I work for a Portuguese company that is building a device with a couple of small speakers that should need 2 x 3W of output power from the amplifier. The nominal power supply should be around 12V, but we expect some fluctuation on the input voltage, so having a closed-loop amplifier would be a plus.

We were thinking of using the TAS5710 and built a prototype where the audio part of the circuit was identical to the schematic of the evaluation module from TI.

We are able to read and write the I2C registers with no problems, but when we take it out of shutdown, the FAULT pin goes low and the error status register has the "over current error" bit set. There is no short circuit to either ground, pvcc or between any two output terminals. This condition also occurs even when the speakers are not plugged to the circuit.

After reading the datasheet we still have a few doubts that may (or may not) be related to this problem:

 - in the "INTERCHANNEL DELAY REGISTERS" (0x11 - 0x14) it says the default values are for BD mode, but then there is a table with values different from the default values that are also for BD mode. We want to use BD mode, so what values should we use here?

 - the ICD default values are for BD mode, but the amplifier is in AD mode by default according to the "INPUT MULTIPLEXER REGISTER (0x20)". If we want to operate in BD mode, this register needs to be changed to BD mode before coming out of shutdown, correct?

 - in the initialization sequence in the datasheet the timing for powering the PVCC lines is between driving reset=1 and sending the trim oscillator command. However, in the evaluation module, the instructions just say power the 5V first and then the PVCC (manually, I assume). Is this timing really critical? In the prototype we initially were powering up manually the PVCC after the power-up sequence finished. We tried to increase the time between reset and trim oscillator so that we could manually power PVCC during that time, but that didn't make any difference.

 - the device will have the speakers embedded and the connections from the amplifiers to the speakers should be below 10cm. There are some texas amplifier datasheets were it states that in this case the output filters can be discarded. Does the TAS5710 also allow this? The TAS5710 datasheet doesn't state this anywhere, but since this device will only use 3W per channel and the connections are short, I was hoping we could get away with just using a ferrite bead + ceramic capacitor as the output filter.

Thanks in advance,

  • 0
    Prodigy 50 points

    Just one thing I forgot to mention: the datasheet says that the internal PWM oscillator works even when MCLK is absent and that the I2C interface also works without MCLK which led us to believe that the amplifier could work without MCLK, but maybe it can't. I'll try to provide a valid MCLK signal and see if it makes a difference.

  • 0 in reply to Paulo Marques
    TI__Mastermind 33420 points

    Hello Paulo:


    Perhaps the statement is not very clear ... the internal oscillator is always running after the trim.  When MCLK is not present, e.g. MP3 track changes, the oscillator will keep the PWM switching so that there won't be pop noise.  When data is present, MCLK, SCLK and LRCLK are always needed to process that data.

    I2C is running without MCLK.

    Best regards,

    Tuan

  • 0
    TI__Mastermind 33420 points

    Hello Paulo:

    In BD mode, the following capacitors are removed:  C26/27/32/33/34/35.  The ICD settings for BD mode is on the bottom of page 45 in the data sheet. Yes, the settings should be programmed before starting PWM (coming out of shutdown).

    The timing with regard to PVCC is for further mitigating pop noise during power on.  The timing is critical for avoiding any pop/click upon power up.

    With 12V and 3W operation, you can use the filter free configuration as you mentioned above, i.e. ferrite bead + ceramic cap.

    Best regards,

    Tuan

     

     

  • 0 in reply to Tuan
    Prodigy 50 points

    Hi, Tuan

    Thanks a lot for your prompt answer! It really helped to clear some doubts.

    Unfortunately I've applied a proper MCLK signal, but my main problem still remains: when I power up PVCC the amplifier immediately signals FAULT and the "ERROR STATUS REGISTER" has the over current bit set. The signal being sent to the amplifier (I don't know if makes a difference or not) is a stereo 48kHz white noise, full scale wave, with 32 bits per sample (per channel) so that the SCLK and the MCLK are actually the same signal. The "CLOCK CONTROL REGISTER" correctly shows the detection of a 48kHz signal with 64xFs MCLK, and there are no CLK, PLL or frame slip errors on the "ERROR STATUS REGISTER".

    I did one extra test that was to place a "sense" resistor in the PVCC power to see on an oscilloscope if there was any glitch indicating a short burst of current that might trigger the over current status, but the power remains steady at 12V (no glitches) even with a 10 Ohm resistor.

    So I'm starting to think that this particular chip might be damaged and I need to try and replace it. I was just wondering how that might have happened so that I don't do the same thing again to a new chip. There are only a couple of possibilities that come to mind:

    - I've set wrong ICD parameters at some point causing the half bridges to "shoot-through". Is this possible, or should the overcurrent sense circuit prevent that sort of damage to the bridge?

    - the IC was damaged during soldering because of too high temperatures. This is a hand-soldered prototype, that didn't went through a proper reflow oven, so it might have been subjected to temperatures outside the acceptable range. However, the I2C interface is working perfectly and I know that the soldering was done with care, so this is possible but unlikely.

    Just one more data point: the MCLK (and the SCLK) are being generated by a "fractional divider" that doesn't produce a perfectly square wave, but the duty cycle is well inside the 40-60% range allowed in the datasheet. However the period of the clocks is not exactly stable. The period of the clock is between 320 - 340ns to produce a 3.072MHz clock on average. Should this be a problem?

  • 0 in reply to Paulo Marques
    TI__Mastermind 33420 points

    Hello Paulo:

    One question I would like to ask, do you use solder paste to solder down the power pad to the ground on the PCB?  This will help dissipate the heat and will not trip over temperature error (OR'ed) with over current error.

    -Wrong ICD parameter would not damage the device.

    Best regards,

    Tuan

  • 0 in reply to Paulo Marques
    Prodigy 50 points

    Oh, never mind.... I just found out the problem.

    The PVCC_B pin seemed to be soldered ok, but was actually not. I had already tested the pin continuity for all the pins with a multimeter, but the pin was so close to being ok that just the slightest pressure with the multimeter probe would make it contact the pad below and signal continuity. After soldering it correctly I now have a beautiful white noise coming out of the speakers :)

    Thanks for all the help!

  • 0 in reply to Tuan
    Prodigy 50 points

    Yes, the PowerPad is soldered to a central pad on the pcb with vias connecting to the bottom ground plane to dissipate heat. Soldering it by hand was a little tricky, but I'll just quickly explain how we did it in case anyone else is interested in doing the same. The sequence was something like:

    - put a small amount of solder in each pad and a small blob in the central pad on the pcb

    - align the chip over the pads and solder 2 distant pins on one side of the chip, e.g., pins 1 and 12. The chip should now be tilted because it is being pushed up by the central blob of solder

    - heat up the ground plane from below using a powerful soldering iron or preferably a hot air station until the central blob melts and the chip settles in its place. You can apply a small amount of pressure over the chip to be sure it settles properly

    - at this point you can test if the chip is properly attached to the pcb through the central pad by trying to pull it up. You can also test the pins for shorts that might occur if you used too much solder in the central blob

    - with a hot air station or a sharp soldering iron solder the pins all around the chip

  • 0 in reply to Paulo Marques
    TI__Mastermind 33420 points

    Hello Paulo:

    Thank you for the great tips.

    Best regards,

    Tuan