These drivers appear to be nice for quickly getting a stepper motor application up and running. I'm running into an issue with the Rsense resistors. I'm trying to build a board that will let me vary the drive current between ~0.2 to ~2.0 amps. The IChop equation (datasheet, p9) is a nonlinear equation that requires low value resistors. Additionally, the Rsense must be set individually for each of the 2 motor windings. Values needed are from 0.33 ohms (2 amps) to 3.3 ohms (0.2 amps).
Accurately setting max current, especially at the high end of the scale, appears difficult. Using 3.3V Vrefx, a 2A limit requires a 0.33 ohm resistor and a 1.8A limit requires a 0.3666 ohm resistor; a delta of 0.03 ohms. Not only is the required precision high, 33mohm resistors are expensive.
Can anyone recommend a strategy for having adjustable current on my circuit boards? I'd want to ensure that RsenseA and RsenseB are the same value, so I'm not sure potentiometers would work. I'd like to be able to have decent resolution in choosing a current between 1A and 2A, which appears to mean a handful of odd, low value resistors, which I'd like to avoid. Ideally a resistor ladder with a jumper would work, but given the nonlinearity of the current equation I'd need some weird resistor values.
There's gotta be a better way, right?
