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TPA3245: C_START value is inconsistent across TI instructional assets

Josh Heiner
Intellectual 860 points
Part Number: TPA3245
Other Parts Discussed in Thread: TAS3251

Hi -

I'm working on a design that employs the TPA3245 in pre-filter PBTL mode. In regards to the C_START capacitor, I see this conflicting instruction from the following sources:

Oct 2016 Datasheet: 470nF
Jan 18 2018 SLVUAT6A: 470nF
"TPA3245 Setup Guide & Configuration Tool" spreadsheet configured for pre-filter PBTL: 10nF

All of these are the most recent versions as far as I can tell. A 10nF cap is substantially different from 470nF - which is correct? I'm inclined to use the larger one as I don't mind a longer startup time, and I see it in two out of the three sources, but thought I'd check to be certain.

Thanks,

Josh

  • 0
    TI__Mastermind 36170 points

    Hi Josh

        Sorry not able to find detailed explanation about this capacitance choice, this device is quite long time ago. But seems both datasheet and EVM recommend different C-START value for different configuration. The value 470nF is only used for SE mode, and 47nF for BTL mode. For PBTL, only 10nF recommended. I'd suggest we use 10nF value for a start, and maybe try others along the test.

  • 0
    Intellectual 860 points

    Thanks for responding Shadow.  I get that the part may have been introduced some years ago, but if it is still active and being sold, I think the datasheet especially should be correct, as that is the accepted standard of "truth" for a given part.  Are you saying it is wrong?  It clearly shows 47nF being used for PBTL mode, not 10nF.  Even more confusingly, if you look at this similar schematic for the BTL case you mention, it shows 10nF in this position, not 47nF.

    Similarly, assuming the dev board schematic actually reflects what is being built and shipped to customers, it has 47nF stuffed in this position.  I actually just noticed a further inconsistency here, as the dev board shows that 47nF x 2 (parallel) is to be used in the SE case.  The evaluation guide text says nothing about removing this 47nF part and soldering down a 10nF part in its place, and given that it clearly supports PBTL mode (see mode pin selection table), I'd think this would be pretty important to mention if 10nF were critical for proper PBTL operation.

    What is the downside of getting this value wrong, or is there even a truly "wrong" value?  If it's larger, wouldn't the part just take somewhat longer to start up?  Or perhaps I'm not understanding the function of the C_START pin fully?  I get that I could try different values, but I'm under a very tight time constraint and would very much prefer to get this right the first time than have to experiment with it.  It doesn't seem like part of the circuit that customers would or should have to "tune" according to their specific application requirements.

    In any case, I think it would be in TI's best interests to correct these various assets (datasheet, eval board guide, and config spreadsheet) so that they all convey the same information in each configuration case.  Otherwise, more confused customers will burden your support staff with these same questions over time.

  • +1 in reply to Josh Heiner
    TI__Mastermind 36170 points

    Hi Josh

       I totally agree your point. But just don't get worried too much, if this cap is that critical to influence the working of the device, the datasheet will definitely explain. For most of the time, not mentioned part is not that important. The circuit are only the example to follow, there's not saying that must using the certain value of the C-START cap.

       Luckily I finally find some explanation in a newer part's datasheet, TAS3251. They belongs to the same product family, so we could say the C_START function is totally the same among these two devices. The capacitor value could vary from a large range, should be no problem to choose the one you like.

  • 0 in reply to Shadow He
    Intellectual 860 points

    Understood Shadow - thanks again.