Simple circuit controls stepper motors

Analog Devices AD9833

Noel McNamara

EDN

Stepper motors are useful in many consumer, industrial, and military applications. Some, such as personal-transportation systems, require precise speed control. Stepper-motor controllers can be simple (Figure 1), but they require a variable-frequency square wave for the clock input. The AD9833 low-power DDS (direct-digital-synthesis) IC with an on-chip, 10-bit DAC is ideal for this task, because you need no external components for setting the clock frequency (Figure 2). The device contains a 28-bit accumulator, which allows it to generate signals with 0.1-Hz resolution when you operate it with a 25-MHz MCLK (master clock). In addition, the circuit can easily stop the motor if you program a 0-Hz output frequency.

The AD9833 DDS IC generates frequencies with 0.1-Hz resolution.
Figure 1. A stepper-motor controller requires only a few logic circuits.
 
The AD9833 DDS IC generates frequencies with 0.1-Hz resolution.
Figure 2. The AD9833 DDS IC generates frequencies with 0.1-Hz resolution.

Figure 3 shows the complete system. The most significant bit of the on-chip DAC switches to the VOUT pin of the AD9833, thus generating the 0-to-VDD square wave that serves as the clock input to the stepper-motor controller. Writing to the frequency-control registers via a simple, three-wire interface sets the clock frequency. Writing a 0 to the frequency register stops the clock, thereby stopping the stepper motor. When you are not using the DAC, you can power it down by writing to a control register. This power-down action results in the AD9833's drawing only 2 mA from the supply. Reducing the MCLK frequency can further reduce the supply current. The AD9833 is available in a tiny, 10-lead package, so you can assemble the complete control system on a very small pc board.

The complete stepper-motor controller uses a DDS IC to generate the variable frequencies for the circuit in Figure 1.
Figure 3. The complete stepper-motor controller uses a DDS IC to generate the variable
frequencies for the circuit in Figure 1.

Materials on the topic

  1. Datasheet Analog Devices AD9833
  2. Datasheet Texas Instruments SN74HC86
  3. Datasheet Texas Instruments SN74LS76A

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