> the low side is an FET that is PWM controlled to Gnd

Varying the duty cycle would make a variable power resistor. For
example, if FET Ron is 1 ohm, then a 1:59 duty cycle would average
to 60 ohms. 1:599 would average to 600 ohms. To avoid very low
on:off ratios, use a series fixed power resistor. eg 60 ohms (including
FET Ron), 1:1 for 60 ohms, and 1:9 for 600 ohms. 300mA is nothing
to a $2 power FET wih low Ron and sharp drive (Russell has a cheap
3-transistor driver)

Joe
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.