Don't forget, a stepper is not a DC motor, it is an AC motor. At full steps the driver delivers 2 pulses that are 90° out of phase. These pulses approximate square waves with minimum pulse width under 1 µs. As micro stepping is used the pulses start to approximate stepped sine waves with the current rising and falling with the steps. Allegro's A4988 datasheet is worthy of study to get an understanding of the workings of a driver.