Choosing a Stepper Motor Driver IC

Back in my days working in an optics lab, we would typically use stepper motors to drive sensitive translational and goniometer stages for gathering spatially and spectrally resolved measurements. We always used stepper motors for these applications, thanks to their low hysteresis and fine resolution. Any of these stepper motors requires a driver to move a stage in the desired direction.

Whether you are designing sensitive measurement equipment, or you require precise speed and position control for your next electromechanical system, you’ll need to choose the right driver for your stepper motor. Matching a stepper motor driver to a bipolar or unipolar motor is easy if you know which specifications to check in your component datasheets.

Types of Stepper Motors and Drivers

Common stepper motors can be classified as unipolar and bipolar devices, referring primarily to the configuration of the coil windings in each stator. At the most basic level, these motors work in the same way; electromagnets are turned on in a consecutive manner to rotate the shaft to the desired position. These motors are ideal for applications that require precise position control; they should not be used if high speed is required. These stepper motors include variable reluctance, hybrid synchronous, and permanent magnet motors.

Unipolar motors are relatively simple compared to bipolar motors. A unipolar motor uses one winding on each phase, and each winding includes a common tap. Thanks to the common tap, only half of the winding is carrying a current at any given time, producing lower torque than a bipolar motor that runs at the same voltage/current. Two-phase unipolar stepper motors generally require 5 to 8 leads to connect to the driver, depending on how the tap is incorporated into the stator windings. A bipolar motor rotates as the driver applies a specific pattern of forward and reverse current through the two windings, thus the name “bipolar”. These motors require one H-bridge per phase for driving.

Typical motors have two phases in order to reduce the lead count. The rotor can have stacked north and south poles along the rotor shaft (so-called can construction) or fine teeth along the axis of the shaft; the angular separation between these regions on the shaft determines the angular resolution of the stepper motor.

Matching a Stepper Motor and Driver

The most important parameters in the driver specifications for matching to a given stepper motor are:

• Constant current vs. constant voltage drive. The inductance and DC resistance of the windings will cause the stepper motor to exhibit a transient response. This will then influence the rate at which your motor reaches full torque. Constant current drives provide a strong burst of current, which helps the motor reach full torque in a shorter time compared to constant voltage drives. Constant current drives normally include a chopper circuit, which reduces the current in the windings once it exceeds a specified limit.
• Microstepping. Some drivers include an internal interpolation circuit that provides stepping at fractions of a step. The resolution can be decreased by factors of anywhere from 1/2 to 1/16 of the standard step size.
• Number of phases. Stepper motor drivers are designed to drive a specific number of phases. Typical unipolar and bipolar stepper motors use two phases, although a variable reluctance motor uses three phases.

Pay attention to the resonance frequency of the stepper motor and the driver if you intend to drive the motor continuously. If the frequency of your driving pulses matches the motor’s resonance frequency, a strong vibration can occur within the motor housing. This can cause the rotor shaft to come out of sync with the stator windings, effectively causing the motor to stall.

Although the simplest way to build a stepper motor control circuit for a unipolar motor involves a 555 timer and some D flip-flops (or H-bridges for bipolar motors), there are many integrated ICs that provide the same capabilities in a low cost, compact package.

Panasonic AN44069A-VF

The AN44069A-VF stepper motor driver from Panasonic is ideal for driving bipolar stepper motors with 37 V output and 1.5 A constant current sourcing. This IC includes a chopper circuit to limit the current output and a PWM oscillator with two available frequencies for near continuous driving. This driver is ideal for basic stepper motors that do not require extremely precise position control (i.e., microstepping) or high torque.

Photograph of the AN44069A-VF stepper motor driver from Panasonic.

ON Semiconductor STK672-630CN-E

The STK672-630CN-E constant current stepper motor driver from ON Semiconductor is designed for use with 2-phase unipolar stepper motors. This driver provides higher voltage output (46 V) and current output (2.2 A) than the previous component. The motor step rate is controlled with an external clock circuit, providing flexible speed control.

Block diagram of the STK672-630CN-E stepper motor driver. From the STK672-630CN-E datasheet.

Allegro MicroSystems A4983SETTR-T

The A4983SETTR-T constant current stepper motor driver is designed for driving bipolar stepper motors with output of 2 A (2.5 A at <20% duty cycle) at 35 V. This driver comes packaged in a 28-pin QFN package with a thermal pad. This stepper motor driver provides microstepping down to 1/16th of a step, which can be digitally controlled by the user. This stepper motor is a better choice for systems requiring more precise position settings and measurements.

Application diagram with the A4983SETTR-T stepper motor driver. From the A4983SETTR-T datasheet.

There are a large number of stepper motor drivers available on the market, which can make it difficult to determine which driver is best for your particular stepper motor. Octopart gives you access to a range of stepper motors and stepper motor driver options. Try using our Part Selector guide to determine the best option for your next product.

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