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