Phill Leyva
EDN
Although relatively expensive, monofilar-wound, bipolar stepper motors provide strong torque for a given physical size. However, each of the motor's two windings requires eight driving transistors connected in groups of four in an H-bridge configuration. Each transistor must withstand and quickly recover from overloads and short-circuit conditions, and a driver must consequently include complex and large discrete-component protective circuitry.
As an alternative, Figure 1 shows a motor-driver circuit based on Maxim's MAX9715, a tiny, surface-mount, 2.8 W Class D audio amplifier, which typically drives 4 or 8 Ω speakers. Each of IC1's two outputs consists of a MOSFET H-bridge that drives a pair of output lines, OUTR+ and OUTR– and OUTL+ and OUTL–, that connect to the stepper motor's A and B windings, respectively. Each pair delivers a differential-pulse-width-modulated signal with a nominal switching frequency of 1.22 MHz. The circuit's low-interference design eliminates the requirement for output-line filters.
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| Figure 1. | A single surface-mount circuit and a few passive components can drive a bipolar, monofilar-wound stepper motor. |
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Capacitors C1, C3, C4, and C6 provide bypassing for IC1's power input and bias pins, and C5 and C7 provide bulk-holdup capacitance for the Class D power amplifiers' outputs. Capacitors C8 and C9 limit the amplifiers' input bandwidth to 16 Hz, and L2 and L3 suppress electrical-noise pickup by the long input cables. Comprising C1, C2, and ferrite bead L1, a pi-section noise filter suppresses noise on IC1's power-supply input. A suitable controller feeds digital pulses to IC1's A_Step and B_Step inputs, which respectively drive the motor's right and left channels. Internal short-circuit and thermal protection guards the amplifier against overcurrent and short circuits caused by the stepper motor or its connecting leads.
| Table 1. | A_Step and B_Step pulse sequence. | ||||||||||||||||||
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Table 1 illustrates the A_Step and B_Step pulse sequence that rotates a typical stepper motor in one direction by continuous application of steps 0 through 4. Step 4 returns the motor's shaft to its starting position and completes its 360° rotation. To reverse the motor, begin at the bottom of the table to reverse the pulse pattern and work upward. You can disable both of the amplifier's channels by applying a logic-low signal to Pin 8, IC1's active-low input. Figure 2 illustrates the circuit's input and output waveforms.
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| Figure 2. | Waveforms from the circuit in Figure 1 include the A_Step input (Channel 1), B_Step input (Channel 2), outputs OUTR+ (Channel 3) and OUTR– (Channel 4), and the signal that arrives at the motor’s windings (OUTR+ minus OUTR–, middle trace), which the oscilloscope’s math function computes. |
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