Context: I am working on an STM32 project and am now trying to add audio output to the project.
I am planning on using the PAM8302A Class-D audio amplifier to drive the speaker, which has a differential audio input.

Converting the 250 kHz PWM from the MCU to analog audio is simple- just an RC filter, differential PWM requires two filters - to provide a differential audio signal.

Now the tricky part: I need to control the output volume from a DC Control voltage.

I sketched up an idea that seems workable- but always I seek the perspective of others.
Using two small signal MOSFETs, I convert the PWM to open-drain, with matched pullups. The pullup resistors are tied to a variable DC voltage- the voltage on the drains are now a variable-amplitude square waves.
An RC low-pass removes the carrier, leaving a differential analog signal, this gets AC coupled into the two inputs of the class-D amp.

I was trying to imagine if this could be done simpler with some switching diodes, need to ponder this...
The diodes are fast- I am concerned that the 20-50 nS propagation delay of the MOSFET might introduce some nasty distortion, especially near the ends of the duty cycle range?

I don't need great control linearity, and the volume does not need to go all the way to zero.
Clicks and pops when switching this on-and-off are not wanted, it needs to be low power when OFF as well.

Comments invited!

 

Why can't you read the control voltage with the MCU's A/D then multiply the signal that is generating the PWM by the control voltage before it is sent to the timer?

If that doesn't work, get a pair of SPDT analogue switches, connect the PWM to the control input of the analogue switch.
Buffer the control voltage with an op-amp and connect to the N/O side of the analogue switch, and connect the N/C side to ground.

Must the PAM8302 have a differential input? Can't you connect one side to ground and drive the other?

 

Why are you converting PWM to analog audio and then having the PAM8302A convert that back to PWM?

 

Must the PAM8302 have a differential input? Can't you connect one side to ground and drive the other?

Both inputs are internally biassed to Vdd/2. Grounding via series R & C might work?