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TPA2015D1 Gain increasing on start-up

Other Parts Discussed in Thread: TPA2015D1

 Hello

My customer is asking why output of TPA2015D1 is gradually increased even with fixed gain on from start-up.

I thought input might be cause of issue, but I found I can reproduce the issue on our EVM with AP audio analyzer.

Input: 1kHz 1 Vrms(UNBAL), Gain: 20dB(High), AGC: Open, Vin: 4.2V(Power supply), END pin low=>high at about 3 seconds during sweep

From my understanding, AGC pin open means disabling AGC function, but it looks gain is increasing just like AGC release on start-up.

I tried 10 times as below sweep graph, and it takes about 6~8 seconds to get proper gain.  Is it normal operation?

Could you help me to explain this to customer?

  • Hi Kenneth

    Have you tried to disable the AGC function tied AGC pin to VBAT? because It is another way to disable AGC function.

    Best Regards
    Jose Luis Figueroa
    Audio Applications Engineer | A-Team
  • Hello Jose,

    I tried AGC pin tied to Vbat, but it looks same. 10 times test result is attached as below.

  • Hello Kenneth

    I have searched and it is normal operation of the device. When AGC pin is left floating or connected to VBAT, that just disables the inflection point, It means that the limiter level is kept constant over all supply voltage. You could refer the Battery Tracking SpeakerGuard subsection in order to find more information.

    Best Regards
    Jose Luis Figueroa
    Audio Applications Engineer | A-Team

  • Hello Jose,

    Thank you for your answer.
    As I understand about the battery tracking speaker guard from datasheet, AGC should be applied when input voltage is reaching on the inflection point. It would reduce gain(attack) when input voltage drops to the inflection point and release gain slowly as setting on GAIN pin when input voltage goes higher than the inflection point.
    In this case, the inflection point is disabled, so gain should not be decreased in any input voltage. However, it looks like gain is decreased and increased slowly as like AGC release on start-up without any input voltage change. My customer thinks it is unexpected behavior and needs our explanation why this happens. Please advise.
    Thank you.
  • Hi Kenneth

    This behavior is normal. When AGC pin is left floating or connected to VBAT, that doesn't mean that the AGC function is disable, that just disable the point inflection and the limiter level will be kept constant over the power supply. For instance, if the output signal exceeds the power supply level (power supply level=limiter level), the SpeakerGuard will decrease the gain of the amplifier.
    Please let me know if you have more questions.

    Best Regards
    Jose Luis Figueroa
    Audio Applications Engineer | A-Team
  • Hello Jose,

    Thank you for your kind answer.

    During start up, power supply level is constant, so limitter level should be constant. Input signal and external gain setting is constant, so output signal level should be constant as I understand. I can't explain clearly why output is limited by limiter on start up to customer yet.

    Could you please help me understand with more detail?

    Thank you

  • Hi Kenneth

    It is possible that when you start the device up the output signal is very high, then, limiter level is activated. That could be happening because the Gain and the input signal are very high. Have you checked the signals levels when that is happening?

    Best Regards
    Jose Luis Figueroa
    Audio Applications Engineer | A-Team
  • Hello Jose,

    Gain is 20dB and input signal is 1Vrms unbalanced.

    I guess TPA2015D1 output is recognized high enough to activate limiter on start-up even with this constant level input.

    There would be some unknown mechanism or internal process on start-up I think.

    Thank you for your answer.  

  • Kenneth,

    I have performed some tests to our EVM and this kind of start-up is part of the AGC that is working correctly.
    You should take into account that the AGC can't be disabled, you can just change the inflection point.

    Best Regards
    Jose Luis Figueroa
    Audio Applications Engineer | A-Team
  • Hello Jose,

    From your great support and kind explanation, I was able to explain the long sound level change to the customer.

    It seems there's no claim for the issue any more, and thank you for your help.

  • Hello Jose,

    My customer is still using TPA2015D1 in many smartphone projects.
    I receive some questions about AGC and limiter, and I could answer based on your explanation.
    There're some questions I cannot easily answer yet, so I'd like to have your explanation.

    1. You wrote 'power supply level=limiter level'. As I understand, this power supply level is Vboost and Vp of output exceeds this limiter level, AGC will decrease gain not to clip output too much. Am I correct?

    2. If Vboost=limiter level as above, is limiter level constant? Can I assume the constant limiter level is black line in Figure 7 and Figure 9 in the datasheet?

    3. What the limiter threshold voltage(VBAT>Inflection point) in electrical characteristics means? Is this related to the sentence in the datasheet 'The maximum limiter level is fixed'? It has typical value 6.15V, but I don't understand why this is that high while maximum boost voltage is 6V.

    Thank you in advance.
  • Hi Kenneth,

    1) That's correct. The limiter level allows the output signal to slightly clip roughly 6% THD+N.
    2) Yes, you can. This black line is the behavior when you disable the inflection point.
    3) This part of the datasheet is referring the Table 1 where you can find the different inflection point. Regarding the voltage levels, the maximum power supply is 6V, but the boost peak voltage can achieve a value until 6.15V.

    Best Regards
    José Luis Figueroa
    Audio Applications Engineer
  • Hello Jose,

    Thank you very much.

    Now, It's very clear to understand.