I have a very small DC motor (from a Walkman), I would like to control the speed from a micro-controller. For that I would like to connect a MOSFET in series and apply a PWM signal to its gate in order to change the speed of the motor.
I have measured the motor's L & R = 4.7mH, 11.5Ohm (Tao 0.41msec).
From running a few experiments with the motor using a bench power supply I can see that it runs well from a voltage of 0.2V up to somewhere around 0.4V - that is all the range I require.
The power supply I have for this is set to 1.8V (used for the digital part of the circuit) so this makes using standard MOSFETs a bit difficult because I can not supply the voltage required for the gate saturation. I bought a few P-channel MOSFETs like this.
So even though I would have thought this setup would work (Vcc -> motor -> FET -> GND) I can't seem to get good resolution over the control and I don't get as much torque from the motor as I used to get when ran from a DC power supply.
I am not sure what freq. I should be using and not sure what other parameters need to be checked in order to make this work as intended. Any help on this will be appreciated.
* UPDATE * Following Olin's answer I have built the circuit he suggested. I have used a 2N3904 transistor, 180Ohm resistor paralleled with a 4.7nF cap. Attached is the collector voltage when running from PWM code 100 (out of 256). Vcc is 1.8V.