Driving a brushless DC motor (with a UC3625?)
It appears that speed control is implemented by a DC level on "Speed In" on pin 7.

They show this connected to a signal from the tachometer signal on pin 20.
It appears that if you take the 10k off pin 20 and feed a pwm signal into it that this will be smoother tp DC by the 10k and 100 NF and will apply speed controlling DC to pin 5 - which seems to be what you want. Making the capacitor or resistor larger than the 100 NF shown will smooth the DC and so stop the system hunting with PWM but will slow response to changes in control voltage level. T present time constant is 10k x 100 NF = 1 mA so you'd want say 10 kHz+  PWM frame rate to avoid jitter. Check data sheet for max allowed Rin.