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Two-channel audio amplifier drives stepper motor

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 (www.maxim-ic.com) MAX 9715, a tiny, surface-mount, 2.8W Class D audio amplifier, which typically drives 4 or 8Ω 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. 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 electricalnoise 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 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 SHDN input. Figure 2 illustrates the circuit’s input and output waveforms.