You may parallel the outputs of Class D amps to achieve 2x the rated output power. We call this configuration PBTL.
To do so, you need to half the resistance of the load and maintain the supply voltage.
i.e. TPA3100D2 is rated for 20Wx2 at 18V, 8ohms, you may parallel the TPA3100D2 to achieve 40Wx1 at 18V, 4ohms.
One common question is where do we tie the outputs together?
For all TI Class-D amps that are currently released (check today's date), you will need to tie the outputs together after the LC filters.
So 4 inductors are still needed for a single PBTL channel.
Please see the app note for an example of parallel outputs:
http://focus.ti.com/lit/an/sloa121a/sloa121a.pdf
The simple explanation is that the outputs are independent and thus not exactly sync.ed, thus there might be large currents flowing between the channels.
In the worst case, the part might experience Short Circuit Protection or even permanent damage.
We are currently working on a new family (TPA3110 and TPA3115) that allows tying the outputs before the LC filters (right at the output pins).
This allows us to use only 2 inductors in the PBTL channel. However, you need to ensure that the inductors are now rated at 2x the current rating of the original BTL channel.
i.e. If you are using 5A rated inductors for 20W BTL channel, you now need 10A rated inductors for the 40W PBTL channel.
It is advisable to check the cost of the higher current rated inductors to ensure it the cost effectiveness of tying the outputs before or after the LC filters (i.e. using 2 or 4 inductors).
