Hello all,

I'm trying to design an analog loop for a Class-D audio amplifier output stage to increase PSRR. My input is already a digital PWM stream and i want to implement an analog-integration-of-digital-signals type of loop, which I've seen in several articles, one is given in the circuit attached (classd-1.jpg).
It consists of 2 integrators and a comparator. both the input and output are digital PWM streams at ~400KHz.
I've played around with it for a while and I'm having hard time to figure out what time constants should i set for the integrators for this concept to work properly.

If i set the F3dB of the first integrator too low - it integrates over the audio band, outputting a saw-tooth-like waveform averaged around the audio itself. After a few cycles the average increases to a point where the output clips, losing the switching of the circuit.

On the other hand, if the first integrator knee frequency is higher than the audio band - i get something similar to the square wave at the input, and the second integrator is now averaging over the audio band, this time THIS one clips after a few cycles.

I think i'm missing something here and i'd appreciate your thoughts.

Thanks
Tomer