I have created a small test PCB to evaluate the TPA3004D2. My application requires a single (mono) speaker with headphone outputs. I used a slightly modified version of the schematic in figure 36 (page 21) of the TPA3004D2 datasheet. http://www.ti.com/lit/ds/slos407e/slos407e.pdf The main modifications are that I am not using the left channel, so the left channel inputs are AC coupled to ground. The left channel Class D outputs are setup the same as the right channel outputs, execpt instead of driving a speaker, I inserted a dummy load (8 ohm resistor). The left channel VAROUT is terminated with a 10K ohm resistor. The right channel VAROUT output drives both channels of the TPA6110A2 headphone amplifier.
When testing the board with an audio input, the Class D outputs seem to work just fine. However there is no signal coming out of either of the VAROUT outputs. I have tried every possible combination of voltages (within the range of 0-VREF) on VARDIFF, VARMAX, and VOLUME pins, but the VAROUT pins remain constant with only a DC voltage of approximately 2.5V (same voltage as on the V2P5 pin). I used an oscilloscope to probe the VAROUT traces both before and after the DC blocking capacitors. I have also verified that the physical PCB traces were ok and not shorted to ground or any other pin/trace. I thought that maybe the the VAROUT circuit was somehow damaged in the IC, so I replaced the TPA3004D2 with a new chip and the new chip performs in the exact same way.
Do you know of any possible cause for the VAROUT outputs not working?
Sorry for the delay. I'm not sure what's going on here, Figure 36 was actually used by a major TV manufacturer many years ago to product several million TVs, AND, we test the VAROUT functionality....
The fact that you get V2P5 at the outputs is a great sign! This means the op amp is properly biased. I expect the problem is with your volume setting. Please review the d/s where it discusses the volume setting of the VAROUT_x pins and give it another try. It sounds like your VAROUT volume setting is very low...
Audio Applications Engineering Manager
Dallas, TX USA
Unfortunately the project that this was going to be used in was just cancelled, so I have not gone into any more debugging. However, I don't think it was my volume setting. When I first realized that no audio signal was coming out of the VAROUT pins, I adjusted the voltages to give a maximum output (VARDIFF = 0V, VARMAX = VREF, VOLUME = VREF). When that didn't work, I then tried all combinations of voltages (from 0V - VREF) between the VARDIFF, VARMAX, and VOLUME pins. Still I received no audio signal on the VAROUT pins. I also verified that these voltages were in fact getting to their corresponding physical pins on the IC (No shorts or bad solder joints). I guess this will just have to go unsolved. Thanks for your Input.
Have you tried disconnecting VAROUT from the TPA6110 and see if VAROUT works?
Audio Applications Engineer
Dallas TX USA.
Yes, I disconnected the VAROUT pins from the inputs of the TPA6110 and probed them on the oscilloscope (with an audio input to the TPA3004D2) and still nothing but 2.5V DC on the VAROUT pins. I then bypassed the TPA3004D2 and fed the input directly to the TPA6100 and I was able to get audio through the headphones, so I know that the headphone amplifier works.
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