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Stepper Fundamentals Slide 21

Just as switching regulators have replaced linear regulators in power supplies, switching techniques can be applied to the motor drives as well. When driving a motor, a large inductance is present. So the only additional components needed are those used in the circuit to sense and regulate the current in the motor winding. Common techniques that work well for stepper motors are shown here. In these circuits the comparator senses the coil current by sensing the voltage across the sense resistor and compares it to a reference input. In the fixed frequency implementation, when the current exceeds the set value, the comparator will reset the flip-flop and turn off the transistor allowing the current to decay. The oscillator periodically sets the flip-flop causing the cycle to be repeated, implementing a fixed frequency pulse width modulation (PWM) to regulate the peak current. In the constant off time implementation, the comparator triggers a monostable oscillator that turns the output off for a fixed period of time each time the peak current in the motor reaches the threshold set by the reference input. Although commonly referred to as PWM, it is really a fixed off time frequency modulation since the off time is fixed and the on time varies to regulate the current. Both of these circuits have been implemented in control ICs for stepper motors. In most applications either technique may be used and there will not be any perceivable difference in the operation of the motor. In some applications, particularly when the duty cycle can exceed 50%, the constant off time approach offers some advantages in the stability of the chopping.

PTM Published on: 2011-12-14