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TLV320AIC3206: VNEG not supplying a stable negative voltage for ground centered headphones

Stefan Burstrom
Prodigy 140 points
Part Number: TLV320AIC3206

Hi!
I am using TLV320AIC3206 for headphone output in ground centered mode. I am using a modified version of the linux driver that turns on the the headphone charge pump but for some reason, the VNEG voltage is not able to drive enough power when the driver output is outputting a negative signal
This is the trace that I am capturing:

The blue trace is the VNEG voltage and the red line is the HPR signal. As soon as the signal output is negative, this starts to draw more current from VNEG than the VNEG regulator can supply which is causing the signal to clip if I increase the gain. Is this the expected behavior of the charge pump? I have validated the switching frequency of the charge pump and I have also tried different frequencies with minor differences in how much the VNEG supply increases to the GND level. If I mute the either channel, the amount of drop is cut in half, so I am quite sure that the drop happens due to the loading of the headphones. (32 ohm measured btw)
I have also validated the cap sizes for VNEG and FLY_N/FLY_P and they are both 2.2u. 
The signal in my test is a 440 Hz signal and what is weird is that if I use a lower frequency signal the bump on VNEG is just prolonged, ie my theory that VNEG supply is not sufficient might not be the real explanation.
I have modified the linux driver to set up the registers in the codec according to Setup A - High Audio Output Power, High Performance in the reference manual.
What could be the root cause for this behavior?

Best regards,

Stefan

  • 0
    TI__Mastermind 29755 points

    Hi,

    Are both of the holding and fly caps at VNEG and FLY of 2.2uF X7R type? 

    The level looks like below the FS. It's strange, maybe try with 4.7uF cap and see any effect.

    Also check the routing of the cap ground.

    Regards,

  • 0 in reply to pdjuandi
    Prodigy 140 points

    Thanks!
    The caps are X5R (what I had in my inventory when I built the prototype) and I only had 0805 sized X7R caps so I couldn't fit those. I just checked the GND node of the VNEG cap and it turns out that I had this referencing AGND instead of GND which probably makes a bit of difference too. The net-tie that connects AGND and GND is almost next to the VNEG and FLY caps, so the current loop is quite small, but it is still there so that could definitely be cleaned up a bit. When I compare with the reference design, the routing of my design is a lot more tight than the EVK, so I am not sure how much difference this makes, but I will definitely clean up the GND connection.
    My main concern was that since I was using a modified version of the linux driver where I added ground centered mode myself, I wasn't sure if I managed to get all the registers correctly initialized and if this problem somehow could be linked to a register setting.

    I also noticed that the reference design had both analog and digital ground tied together in the same plane. Right now, I have them separated, but it seems as if I should tie them together completely?

    Best regards,

    Stefan

  • 0 in reply to Stefan Burstrom
    TI__Mastermind 29755 points

    It's recommended to use X7R as mentioned in the datasheet, you can try with that first. The ground should be connected somewhere even though you separate the AGND and DGND ground.

    I'd think the register should be straight forward and since you are referring to the example scripts, it should work. You can check by reducing the amplitude and see if the output is good. 

    Regards,