TPA3111D1 Fault during Power-on

Hello all,

I have read through most of this forum, and I have found some issues which come close to explaining what I am seeing.  As there was no real resolution with those posts, I am creating a new one here :)

I am attempting to use the TPA3111D1 amp in BTL configuration to drive an 8Ohm.  PVCC is 12V.  My audio input is referenced to a virtual ground at 4V.  I have placed large 10uF X7R caps on both input pins, with INN connected through its cap to my virtual ground to take advantage of the chips CMRR.  I am using an LC output filter, with component values chosen for an 8Ohm load.  I also have the option to tap the output before the reconstruction filter to compare functionality with and without the output filter.  SD pin is tied to FAULT.  My input capacitors are both between 3-8mm from the pins (but not equal distance!).  Also, I have 2 identical boards that are showing the same issue, i.e. I don't think it is simply a faulty IC

The problem I am seeing is the following:

Not always, but increasingly more frequent (recently every time), the chip will fault during power on.  Along with the IC shutting down, there is a large current spike and a loud pop-POP (quiet-loud)  from my speaker.   This fault occurs with and without a load attached to the outputs, also with and without an output reconstruction filter.  I suspect it is a DC detect fault, as it requires a hard reset of PVCC to resolve.  Comparing input pin voltages with the working EVM board, there are clear discrepancies found which I would like some guidance with. 

My start up procedure for measuring the input pin voltages is always the same:

1.  Hold down SD switch on the amp

2.  Turn on main power

3.  Wait some time (1-10s) for all voltages to stabilise. 

4. Release SD switch and measure DC pin voltages before the fault occurs (fault occurs in expected time of ~500ms for a DC detect fault)

I see the following discrepancies between the functional EVM board and the problematic custom board.  Note the EVM and my custom board are not identical

Property                   EVM                                Custom

Cin                           1uF                                 10uF

INN ref voltage         GND                               4VDC

I see the following results comparing the two

Functional EVM Board:

SD pin held low, PVCC applied:  INN = 5.18V, INP = 1.28V

SD pin forced to 1, PVCC applied: INN = 6.92, INP = 2.93V

Chip powers-up without fault

Problematic Custom Board:

SD pin held low, PVCC applied:  INN = 0.63V, INP = 0.00V

SD forced to 1, PVCC applied:  INN = ~1.4V (before shutting down), INP=0.00V (no change)

pop-POP and chip faults

----

This INP result is concerning.  it should not be held at 0.0V.   I have replaced the caps on the input but no improvement is found. 

What about the different trace lengths from the INN/INP pins to their respective caps?

Is 10uF too large?  Can there be problems at the inputs due to discharging caps?

Should I design a buffer stage before the class-d input to take care of impedance mismatching?  The INP signal is currently taken from the output of an active filter stage, INN is simply tied to Virtual GND. A buffer would give a similar low impedance output closer matching the virtual ground impedance

So there are my issues, I will attempt to sort them out on my own, yet if someone has seen this sort of behaviour and has figured out a solution, I would be most grateful for some guidance.

Cheers,

Tim van Boxtel