Unsolved M308 "linear-sensor" for DC-motor torque control?

[o_lampe]

Hi gents,
I want to use a hall-based current sensor on a DC-motor and control torque by PWM'ing a heater output. The feedback loop is the aforementioned linear sensor on a thermistor header.

Different to a usual heater-cartridge I want to be able to set a minimum PWM-duty to keep an external motor driver initialized.
How can I achieve this?
I also want to use a high PWM frequency and a slow feedback loop, but I'm not sure what the default ratio is?
(thermistor read once per main-loop and heater PWM frequency is xxx-times faster?)