Datasheet 2

AN120 – UNDERSTANDING MP6500 CURRENT CONTROL
AN120 Rev. 1.0 www.MonolithicPower.com 2
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INTRODUCTION
Bipolar stepper motors are used in many applications, from driving paper through a printer to moving an
XY stage in industrial equipment. The motors are typically driven and controlled by inexpensive stepper
motor driver ICs. Unfortunately, most of these ICs use a simple current control method that causes
imperfections in the motor current waveforms, which results in less-than-optimal motion quality. In the
MP6500 stepper motor driver, implementing internal, bidirectional, current sensing results in improved
motion quality with lower system cost than legacy solutions.
Bipolar Stepper Motor Basics
A bipolar stepper motor contains two windings. The motor is moved by
driving varying currents sequentially through the two windings. To make the
motor move smoothly, the two windings can be driven with sinusoidal
currents that are 90° out of phase – sine and cosine.
Usually, steppers are not driven with analog linear amplifiers – they are
driven using a PWM current-regulating driver with discrete current values that
break the sine wave into straight segments (see Figure 1). This is called
microstepping. The sine wave may be broken up into any number of
segments, and the waveform approaches a true sine wave as the number of
segments increases. In practice, the number of segments varies from four
to 2048 or more, with most IC stepper drivers implementing between four and
64 segments. Since one sine wave generates four mechanical states in a
stepper motor called steps, a 32-segment sequence is called a ⅛-step
operation.
Why Current-Control Accuracy is Important
The position of a bipolar stepper motor’s rotor depends on the magnitude of the currents flowing thorough
the two windings. Normally, if a stepper motor is used, there is a requirement for accurate mechanical
positioning or accurate speed control of some mechanical system, so it is only logical that the accuracy
of the motion is determined in part by the accuracy of the winding currents being used to drive the motor.
There are two problems that inaccurate current control causes in the mechanical system. First, at slow
speeds or when a stepper motor is used in a positioning application, the motor steps a different amount
at each microstep, causing an error in positioning. Second, at higher speeds, the nonlinearities cause
short-term speed variations within a single rotation of the motor, adding undesired components to the
torque that increase noise and vibration of the motor.
PWM and Decay Modes
Most stepper motor driver ICs rely on the inductive nature of the stepper motor windings to implement
pulse-width modulation (PWM) current regulation. Using an H-bridge arrangement of the power
MOSFETs for each winding, the supply voltage is applied to the winding at the beginning of a PWM cycle,
causing the current to build through the inductance of the winding. Once the current reaches the desired
level, the H-bridge changes state to reverse the current buildup. After a fixed period of time, a new PWM
cycle begins, and the H-bridge drives current through the winding again.
I
A
I
B
I
A
I
B
8 Segments
½-step
32 Segments
1/8-step
Figure 1: Stepper Motor
Current Waveforms