2.4  PWM Control Input Signal
The following requirements are measured at the PWM (control) pin of the fan cable connector:
PWM Frequency: Target frequency 25 kHz, acceptable operational range 21 kHz to 28 kHz
Maximum voltage for logic low:  VIL = 0.8 V
Absolute maximum current sourced:  Imax = 5 mA (short circuit current)
Absolute maximum voltage level:  VMax = 5.25 V (open circuit voltage)

摘自Intel的PWM specification