Remember cassette players? 99.9% of them have speed control circuits
that are a voltage regulator that increases the voltage as the current
increases, with a little temperature compensation thrown in as well.
That is likely what the ROHM IC you mentioned is.

They work quite well when the two compensations are set for the
particular motor.

Cheerful regards,

Bob


Philip Pemberton wrote:
> But surely a normal voltage regulator would work as well -- the current 
> through the motor increases, so the output of the regulator increases to keep 
> the voltage constant. Of course, in practice, it seems "constant voltage in = 
> constant speed out" isn't always true...
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