http://datasheets.maximintegrated.com/en/ds/MAX9744.pdf
I'm trying to figure out how much PCB I'd need to dissipate heat from this amplifier when running it at 12V into a 4 ohm speaker.
The datasheet says on page 2:
Continuous Power Dissipation (TA = +70°C)
44-Pin Thin QFN (derate 27mW/°C above +70°C, single-layer board) ...................................................2162mW
So the chip can dissipate 2.1W continuously? But only with thermal vias and a PCB of adequate size, I assume?
Page 5 of this document says a good rule of thumb is 2.37 in^2 per watt of power dissipated for a 40C rise:
But that does me no good as far as calculating PCB size if I don't know how much power the chip is actually dissipating. I also have a PCB size calculator, but it needs JC not JA and this datasheet doesn't seem to supply that, so that's no good to me.
Then there's another snag On page 8 of the amplifier's datasheet there's a graph for 12V into 4ohms, where it shows efficiency (%) & power dissipation (mW) vs output power, which doesn't make any sense. If the amplifier is 85% efficient at 16W output, how is it the power dissipation is only 5mW? If I assume 15% of 16W is wasted, that comes to 2.4W that I'd expect the amplifier has to dissipate. Is the power dissipation axis labeled wrong? Because while 5mW seems absurdly low compared to what I calculate it should be, it wouldn't surprise me if it was double what I calculated.
On a second look, I'm pretty sure they mean Watts, because the graph next to it for an 8 ohm load lists the power dissipation in Watts, and at 10W of output power the dissipation is 1.5W. Which kinda meshes with the dissipation being 3.75W @ 10W output with the 4 ohm load.
But is that power dissipation for both speakers together? Or just one? If it was both together, than my calculation of 16W @ 15% efficiency dissipating 2.4W would make sense, since if you double that it's almost 5W, which is what that first graph says is the power dissipation at 16W into a 4 ohm load with a 12V supply.
But regardless, that brings us back to the first statistic I found in the datasheet:
Continuous Power Dissipation (TA = +70°C)
44-Pin Thin QFN (derate 27mW/°C above +70°C, single-layer board) ...................................................2162mW
If the amplifier can only handle ar ~2W of continuous power dissipation, what of that 5W of power dissipation? Whether that's one or two speakers doesn't really matter, it's well above that 2W limit either way. And a 4 layer PCB only gets you to 3W.
Yet here Adafruit is, selling the same chip on a tiny board, with up to a 12V input and stereo output:
What gives? Will that Adafruit amp melt if run continuously at 12V into a 4 ohm stereo load? I highly doubt it or they'd warn about it. And I've used smaller 10W boards. So something must be up with my math here.