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Output Level Different 7dB when change Damp from TAS5342LA to TAS5612LA

Wee Keat Chin
Intellectual 435 points
Other Parts Discussed in Thread: TAS5612LA, TAS5342LA

Customer study using same DSP (BD mode) input to TAS5612LA, then switch to TAS5342LA with the same DSP PWM output, the level is lower by 7dB.

Both TAS5612LA and TAS5342LA are used in BTL mode.

DSP (BD mode) + TAS5612LA (BD mode)

Same DSP (BD mode) + TAS5342LA( AD mode - Hardware setting[m2,m1,m0- 000] ) : Output level lower by 7dB (at 1kHz).

 Both frequency response are same across 20~20kHz except level different by 7dB.

 

Output filter

TAS5612LA(BD mode) : L=10uH, Cg=0.47uF

TAS5342LA(AD mode) : L=10uH, Cg=0.47uF , CBTL = Nil

I had checked both circuitry, except CBTL not connected in TAS5342LA, others are OK.

Question:

1) Will the mismatch of output filter cause the output level different?

2) What is the other possible causes that will result in the output level different? 

3) What will happen to output if DSP (BD mode) is connected to Damp+Filter (AD mode)?

Thanks.

Best regards,

Chin

  • 0
    TI__Genius 9175 points

    Hi Chin,

    The filter capacitance required for AD mode is roughly half of what is required for BD mode. Therefore excessive capacitance due to a filter designed for BD mode may change the filter performance. I would first recommended that you adjust the filter for each modulation scheme AD vs BD first in order to properly evaluate the amplifiers.

    Please review this app note if you have any questions: http://www.ti.com/lit/an/sloa119b/sloa119b.pdf

    The gain difference you are experiencing is caused by a difference in the modulation Index, or PWM duty cycle scaling from PWM input to PWM output between the two devices. This is expected as devices will have a different modulation index based on the feedback and maximum output power of the device.

    Best Regards,

    Matt

  • 0 in reply to Matthew Beardsworth
    Intellectual 435 points

    Dear Matthew,

    Thanks for your reply and recommendation.

    i) Based on your explanation, shall I say that the output filter (either AD or BD) will not affect the output level significantly if the value is designed properly?

    ii) Is there any data we can refer for the PWM duty cycle scaling from product datasheet?

    iii) From the measurement, can I conclude that the duty cycle for TAS5612LA is higher than TAS5342LA since TAS5612LA has higher level of output?

    Thanks.

    Best regards,

    Chin

     

  • 0 in reply to Wee Keat Chin
    TI__Genius 9175 points
    Hello Chin,

    i) Yes this is correct AD vs. BD modulation should not affect the output level with the proper filter.
    ii) I am looking into this to see if I can find a spec on the modulation index for both amplifiers.
    iii) Yes the TAS5612LA should have a high duty cycle. This can be measured with an oscilloscope at the switching output of the amplifiers.

    Best Regards,
    Matt
  • 0 in reply to Matthew Beardsworth
    Intellectual 435 points

    Hello Mattew,

    Thanks for your informative explanation. I still have few unclear points that need your advice:

    i)  Both TAS5612LA and TAS5342LA are power stage devices, right? How does customer know the gain difference since it is not indicated in datasheet? 

       Example in my customer's case, same DSP output used but get different output level. So customer needs to compensate the gain by changing the setting of        DSP.

    ii) What is the PWM output frequency for both TAS5612LA and TAS5342LA? Is it following the input frequency( from DSP output)?

    iii) For the measurement to check for higher duty cycle, what type of signal should be used? 1kHz signal or -infinity ?

     

    Please send me any documents that related to the modulation index (even with similar amplifiers) and output level so that I can show to my customer.

    Thanks.

    Best regards,

    Chin