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TAS5727EVM output filter questions

Other Parts Discussed in Thread: TAS5727

I'm looking at the reference schematics for the TAS5727 EVM and the output filter values do not seem to match the recommendations in App note SLOA119B "Class D filter design." 

The filter on the board is a 15uH inductor and a 0.68uF capacitor to ground. Since there is no Cbtl (cap between the OUTA and OUTB or OUTC and OUTD), this would appear to be set up for BD modulation. The app note shows Cg for a 15uH inductor as 1.5uF. The eval board has 0.68uF inductors, which would indicate a 22uH inductor should be used. Additionally, the default mode of the part is AD modulation. 

If AD modulation is used, the app note shows a Cbtl of 0.68uF and a Cg of 0.1uF with an inductor value of 15uH. 

I do not have a demo board for this part, but I assume it works with these filter values. Why were these output filter values selected? Can they be used with AD or BD modulation, or does BD modulation need to be used? Is there any advantage or disadvantage of either modulation? The BD modulation scheme has one less component, so there is that. We are currently planning to use AD modulation, so should I select my output filter values based on the reference design or based on the app note? 

Additionally, we are only planning ot use one channel of the amplifier. My understanding is that I need to connect all PVDD pins, but do not need bypass capacitors on the unused channel and the unused channel outputs can be left unconnected. The unused channel then needs to be shut down in software. Is this correct? 

Clarification on this would be appreciated. 

Thank You,

 Ben Missele

  • Hello Ben,

    The EVM of the TAS5727 is a platform used to evaluate the principal characteristics of the device in several conditions. The filter implemented works with these values because the cut-off frequency still matches the requirements for an Audio Application.

    When designing an specific system, It is suggested to follow the Application Note as the speakers used will have a known impedance value and based on that the filter can be selected.

    The filter on the EVM does not have the differential Cap, so it is intended for BD mode. This mode has a slightly better cross-talk and efficiency than AD mode, but based on performance both are very similar. The selection will depend on customer needs. 

    PVDD and bootstrap capacitors must be kept in the design even if  only one channel is used.

    Best Regards,

      -Diego Meléndez López
       Audio Applications Engineer

  • Diego,

     Great. Thanks for the information. 

     The Eval board uses two 10uf capacitors and one 0.1uF capacitor for bypassing each PVDD group. The datasheet recommends one 0.1uF capacitor per pin. Should I follow the Eval board or the datasheet? 

    Thanks,

     Ben

  • Hi Ben,

    The datasheet specifies the usage of 0.1µF capacitors to decouple the high frequency components of the power supply, these should be placed as close as possible of the Amplifier.  The TAS5727 requires also big bulk capacitors to remove low frequency noise. These capacitors should be of 10µF or greater. I suggest you to follow the EVM components and layout. 

    Regards,

      -Diego Meléndez López
       Audio Applications Engineer

  • Okay. Thanks for your help.