Here's a pretty simple variant of a PDM modulator I implemented years ago. (I think I already posted it but can't find the thread anymore.)
A benefit is that it's low part count, and very easy to analyze formally. (Guess you could say it's basically a free-running, first-order 1-bit sigma-delta modulator.)
As it is, it's free-running - will yield PDM frequencies in ~10-20MHz range with the components shown, but you can always add a fixed clock source and a flip-flop to make the output synchronous if required. The benefit of a free-running design, aside from its simplicity, is that it yields sort of a "spread spectrum", which could be a slight benefit EMI-wise I guess.
Note that it's powered with a 3.3V supply in this example, but if you up it to 5V, you'll get much better performance - average PDM freq will be a bit higher as well. At 5V, FFT analysis of the output with a 1kHz, 1Vpp input signal: worst harmonic relative level is lower than -100dB. Of course, the average freq at 15-20Mhz is a bit high to drive a power stage here - losses could be a problem. But for transmitting audio with a digital signal, it's extremely simple and still pretty good I think (that was my application.)