fungus:
Why would you need 32kHz PWM for a fan? A mechanical device like a fan isn't going to respond any better to 32kHz than 100Hz.
Because of annoying noise while driving fan at lower speeds. I read that low PWM frequency produce that noise.
Quote from High Frequency PWM Fan Controller
"The PWM controllers usually generate acoustic noises, when the PWM frequency is within the acoustic spectrum (20Hz to 20KHz). A high frequency PWM controller usually operates above the 20KHz, thus the human ear cannot hear this sound. Moreover, PWM controllers can achieve very stable and low speeds without the possibility of a fan stall."
Ok, now that's funny. I set frequency to 30 Hz and annoying noise disappeared! 