Negative voltage PWM?
You can use the following circuit:



If the microcontroller generates a positive PWM signal, the diode will charge Uc to its maximum value (i.e. a constant value of 3.3 V or 5 V depending on your microcontroller). 

Then $$Vout=-Uc + Vin$$ and therefore Vout will swing between 0 and -3.3 (or -5 V) as you want. This assumes a perfect diode with a 0 V drop, I guess that a Schottky diode should be fine for your application.