Ok. From your scope plot:



we know now, that not that:



is the motor current, but this:



So, it seems, that the repetition rate of sync pulses is too slow, so that only a rather small average motor current can establish. This seems to be a consequence of unsuited L to R ratio of stepper motor, means the motor current rises and falls too fast.

The only remedy seems to be the speed-up of repetition rate of sync pulses! Keep the sync low pulse duration as it is up to now, namely 3µsec, but speed-up the repetition rate of sync pulses from now 40µsec to less than 20µsec. This should do the trick.

A look at the according application note shows, that it seems to be impossible to adjust the built-in oscillator of L6506 to fabricate this sync pulse train. So, I would recommend to use an additional CMOS-555 to provide this.


This could also be an eroneous reading due to crosstalk into the probe: Connect the hot tip and ground tip of probe both to the ground terminal of Rsense, so that the signal shown at scope screen should be zero. But due to capacitive crosstalk and magnetic coupling fast switchings in the next neighbourhood of probe can inject interference into the probe resulting in some strange spikes appearing on the scope screen!

Kai