How to generate 30 PWM outputs?

Question

I need to generate 30 PWM outputs with a frequency of about 20 kHz. I need to control the duty cycle of each PWM output.

It is a bit too much to be reasonably done on a MCU (I usually work with STM32 microcontrollers) because the MCU has to communicate with other components as well. I was thinking there would exist multi-channel PWM drivers, but the ones I have found (LT8500, PCA9685) cannot go as high as 20 kHz and I cannot accept a frequency lower than 20 kHz.

One solution would be to add a second microcontroller so that I could handle all the outputs with 2 MCU. But that means I need to develop two firmwares and a communication protocol between the 2, which is a bit of work.

What other options could I consider in such a situation?

Thanks. :)

Answer 1

Probably the simplest approach (in terms of hardware) would be to use an FPGA. Something simple like SPI to send the PWM duty cycles.

But if you're concerned about having a second (similar) MCU and some simple communication, you may not much like the learning curve. The code itself would be pretty simple once you're up to speed.

Answer 2

A small FPGA would be the simple solution, but if you want to avoid it here is an analog alternative, which would work if you need to update the duty cycles only rarely / slowly:

Use some DACs, which can be controlled by the MCU. Then you would have 30 DC output voltages. Afterwards you can convert those 30 voltages to 30 PWMs using 30 relaxation oscillators which are simple to build using one comparator each:

^{simulate this circuit – Schematic created using CircuitLab}

With these components and a 3.3 V supply, the toggle frequency would be approximately 100 kHz.