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Stepper Fundamentals Slide 30

The first anomaly that will effect the microstepping drive or the holding current control is the minimum current that the chopping drive can regulate. For any peak detect chopping control, like the controls shown earlier, there is a minimum duty cycle that can be achieved due to limitations in the control circuit. The minimum ON time may be set by several factors. One factor is the propagation delay through the loop. This includes the delay through the comparator, the propagation delay through the logic and the turn off delay time through the base or gate drive circuit and the power transistor. Basically it is the delay from the time the voltage across the sense resistor exceeds the reference voltage at the input to the comparator until the power transistor can be turned off. In some controls, like the L297 and L6506, the oscillator basically sets a minimum on time which is the width of the sync pulse that is clocking the RS flip flop. Some control ICs have a blanking time during which the output of the comparator is ignored. This is done intentionally to mask the switching noise but effectively increases the minimum on time. For a fixed off time control, the off time set by the monostable and thus the minimum on time and fixed off time, establish a minimum duty cycle. For fixed frequency control, the period is set by the oscillator and the minimum on time effectively sets the duty cycle directly. In either case the result is that there is a minimum duty cycle that the control circuit can regulate.

PTM Published on: 2011-12-14