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TAS6584-Q1: False Positive Open Load

Tom G.
Prodigy 60 points
Part Number: TAS6584-Q1
I'm seeing an Open Load error on the TAS6584 Class D amplifier ship for a speaker (Niles CM4PR) that I've hooked up to it even though I've checked for continuity and the speaker works with other amplifiers
Correct. To clarify, the TAS6584 chip is detecting the speaker connection as an Open Load even though we placed a multimeter to it and got measurements between 4.5 Ohms to 8 Ohms.
When enabling load bypass the audio is able to play successfully. Why is this?
  • 0
    TI__Mastermind 49740 points

    Hi Yutong

       Some speaker type indeed would have this issue, when there's alrge parasitic inductance/capacitance inside of the speaker, not very commonly seen. I haven't able to find out this speaker, do you have the datasheet for it? We could check their impedance curve first.

       And when this phenomenon happens, we could consider using AC load diag, instead of DC diag.

  • 0 in reply to Shadow He
    Prodigy 60 points

    I don't think the manufacturer provides a datasheet for this product. What should I look for in an impedance curve graph when checking to see if a speaker may be problematic or not/

    Also, I've had to resort to disabling automatic load diagnostics by setting LDG_Bypass to 1. Is that safe to do? What are some downsides to doing it this way? 

    And can I instead perform AC load diagnostics every time instead of DC diagnostics?

  • 0 in reply to Tom G.
    TI__Mastermind 49740 points

    Hi Yutong

     An example of impedance curve like below, at DC has very large value because there's internal capacitors. Will make the DC load diag report OL.

    The AC diag is designed to deal with this situation. You could follow datasheet 8.3.5.2, or use PPC3 to quickly have a try.

    It's no problem to bypass LDG. Safety could be fully guaranteed, because device has OC protection, could prevent SL/S2P/S2G condition. Only the open load condition could only be recognized by Load Diag. You could check what would be the requirement from your products.

  • 0 in reply to Shadow He
    Prodigy 60 points

    I see. So I think what I should do instead then is to bypass LDG and rely only AC load diag instead to see if there is an open load and determine if I should play to a particular channel or not. Would this be fine?

  • 0 in reply to Tom G.
    TI__Mastermind 49740 points

    Hi Yutong

      Yes, it sounds no problem.

  • 0 in reply to Shadow He
    Prodigy 60 points

    Hi Shadow, I wanted to reach back out and see if the AC diagnostics would be audible on the speakers when it is performed. Can you inform me on this? Also, is there a way to raise the open load threshold for the tas6584?

  • +1 in reply to Tom G.
    TI__Mastermind 49740 points

    Hi Yutong

      For AC diag, will generate around 10mA current to the output trace, could adjust this current frequency higher than 20KHz, like 21KHz, will be very hard to notice.

      Open load threshold not able to adjust, already set to around 50ohm. It's hard for diag function to recognize too large resistance value, which need much higher sampling precision.

  • 0 in reply to Shadow He
    Prodigy 60 points

    Hi Shadow, quick follow up. how can I use AC diagnostics to determine open load? I notice that the IMG value returns 0x80 or (-0) when I run AC diagnostics an open load but I'm not sure if that's a reliable indicator for an open load.

  • 0 in reply to Tom G.
    TI__Mastermind 49740 points

    Hi Yutong

      No problem to detect open load. Usually check the Real part would be fine. You could set the threshold value in 0xB7, and check 0xCB to find the results.

  • 0 in reply to Shadow He
    Prodigy 60 points

    Got it. It'll try it out. Is it also reliable to check if the IMG value is -0 (ie, 0x80) to determine open load?

  • 0 in reply to Tom G.
    TI__Mastermind 49740 points

    Hi Tom

      The IMG value mostly affected by the LC filters, and LC filter devices all have their own variation. Not suggested to use this one.