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TAS5760M: TAS5760M - Surge Current Exceeded Expectation

C%20K%20Ong
Expert 1240 points
Part Number: TAS5760M
Other Parts Discussed in Thread: TAS5805M,

We designed a stereo circuit with 4 ohm speaker.

The TAS5805M is configured to output 1.8W per channel powered by a USB 3.0 port. That means maximum current 900mA for whole speaker. (We can allow current draw to 950mA).

However, we notice that for certain song, the surge current at the USB 3.0 port can be as high as 1.8A for the whole speaker.

We have tried using AGL to limit the output signal but only able to suppress to 1.7A.

In SLOA263A, there is AGL Omega Time Constant. What is this time constant? What is it function?

Is there a way we can change the turn ON/OFF slope of the Class-D MOSFET to reduce the surge current.

I attached my PPC3 file and TAS5805M schematic  for your reference. Please refer to channel B parameter. Please advise what other parameters we can change to reduce the surge current.

  • 0
    Expert 1240 points

    The TAS5805M is operated in Hybrid Mode with PVDD = 5V and Switching Freq = 384KHz.

  • 0 in reply to C%20K%20Ong
    TI__Intellectual 2430 points

    Hi CK,

                As you mentioned, you need 1.8W×2 output power with 4ohm load/ 5V PVDD. 

                So for each channel, the RMS current is the square of 1.8/4≈0.67A, which mean whole PVDD current is about 1.34A. It has exceeded your limit 950mA.

                 In addition, when you play low frequency with loud volume, because of high duty cycle of PWM, the current of PVDD is closed to PVDD/load= 5V/ 4ohm ≈1.25A. 

                 So I think the root reason of your high current is that the load is too small. Maybe you can try a 8ohm or larger DC res speaker.

  • 0 in reply to Leon Peng
    Expert 1240 points

    Dear Leon Peng,

    What we are seeing is consistently at one location of a song, the surge current will appear. This surge current might have bring down the PVDD of TAS5805M and caused a 'tick' sound.

    Please refer to attach image for song detail.

    We believe the sudden change of signal amplitude was unable to suppress by DRC and AGL on time, therefore some part of this sudden change of signal amplitude might have caused the PWM to turn ON longer and resulted in surge current.

    Anyway we can reduce this surge current? For example, slow down the switching slop?

  • 0 in reply to C%20K%20Ong
    Expert 1240 points

    Hi Leon Peng,

    I put the wrong part number in the subject. This issue happened to TAS5805M (not TAS5760M).

    Best Regards,

    CK

  • 0 in reply to C%20K%20Ong
    TI__Intellectual 2430 points

    Hi CK,

              Pls try below methods:

                1. Change switching frequency to 768kHz for lower ripple inductor current.

                2. Use Clipper function(Clipper Level is -2.85dB, same as AGL) to limit output power which AGL cannot react in time.

    Best Regards,

    Leon

  • 0 in reply to Leon Peng
    Expert 1240 points

    Dear Leon,

    1. The datasheet mentioned Hybrid mode only support 384KHz.

    2. Will Clipper function increase distortion?

    Best Regards,

    CK

  • 0 in reply to C%20K%20Ong
    TI__Intellectual 2430 points

    Hi CK,

               1. I didn't see there describes that Hybrid mode only support 384kHz, it can support 768kHz, you can check it with PPC3/EVM.

               2. Yes Clipper function will increase distortion, but Clipper function will only work for the signals which AGL/DRC cannot react in time(Usually for several Fs sample signals), I think at most time it won't work so the distortion won't be too more. You can try it to test whether the current will decrease.

    Best Regards,

    Leon

  • 0 in reply to Leon Peng
    Expert 1240 points

    Dear Leon,

    Thank you for the advise.

    We will try on Clipper function.

    Best Regards,

    CK