I am using the reference design for the TPA3125D2 in BTL configuration with 20Vdc supply and the circuit performs just fine. I would like to drive 4ohm speakers up to 15 watts and when I tested I could get it to put out 10 watts before it would start to shut down.
I've paralleled the outputs of two of the reference designs simply by connecting the two LEFT OUTs and RIGHT OUTs. At low volumes (less than 2Vpp across outputs) it performs fine. At higher volumes there will be periodic muting of the outputs where the output will turn off for about 25ms. At even higher volumes the DC portion of the output drop to almost 0V for around 200ms.
Is there a better method for connecting the outputs than what I have done.
I'm just guessing here but you mention that you're using this in a BTL configuration. Are you, because it's BTL, not using output coupling capacitors?
If so, how do you avoid DC circulating currents between the outputs due to offset? The offset can be as high as +/-50 mV/output or 100 mV worst case.
When a single TPA3125 is used in a BTL configuration there is the DC resistance of the speaker to limit DC current between the outputs.
But, when the Left output of one TPA3125 is connected to the Left output of a second TPA3125 (in order to increase the drive current) there is no DC current limit other than the output inductor/LPF DCR.
You may be using up a significant portion of available power pushing current into the other output.
If this were a small-signal op amp application I'd suggest the use of ballasting resistors to share current and limit circulating currents. In this application it would reduce efficiency.
Try for the moment adding output coupling capacitors and see what happens. You could also attempt to measure the DC current flow - with no signal - between the two left outputs to confirm the amount of circulating current.
Thanks, after initial testing I think adding the coupling capacitors fixed it. Now I have it configured with each chip in SE mode, as per the reference design, and the corresponding outputs paralleled. The problem has gone away for now but I will do more extensive testing later.
If left in the original BTL configuration, you might be able to reduce the coupling capacitor count from 4 to 2. My thought is that only one TPA3125 needs coupling caps since the DC loop circulates from output to output. It should only take one pair of caps - on only one of the TPA3125s - to break the DC path.
If used in paralleled SE mode, 4 caps would be required since you now have one end of the speaker grounded rather than both ends floating at the same relative DC potential.
Paralleled BTL won't completely eliminate caps but it reduces the number of large-value electrolytics from 4 to 2. It would certainly have a lower BOM and footprint. You might consider it.
Sorry no one from TI chimed in here before...
First off, if you want super drive capability, why don't you use the TPA3106D1?
Are you paralleling the outputs after the output inductors? This should be ok. Can you upload a schematic of what you're trying to do?
Audio Applications Engineering Manager
Dallas, TX USA
Did you get this sorted out?
Thanks Don. I think you may need to check with the OP Grant Moore.
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