AN120 Understanding MP6500 Current Control Understanding MP6500 Current Control Application Note Prepared by Pete Millett April 2017
AN120 – UNDERSTANDING MP6500 CURRENT CONTROL 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.
AN120 – UNDERSTANDING MP6500 CURRENT CONTROL This process is repeated, so the winding current goes up and down with the peak current programmed by a state machine and a DAC that sets the desired current for each segment. When the state machine advances to the next segment, the regulated peak current changes accordingly. After the desired peak current is reached, the H-bridge can drive the winding current down in one of two ways.
AN120 – UNDERSTANDING MP6500 CURRENT CONTROL In general, it is desirable to use slow decay, since it causes much less current ripple and allows the average current to more accurately track the peak current. However, as the step rate increases, slow decay is not able to lower the current through the winding fast enough to maintain accurate current regulation.
AN120 – UNDERSTANDING MP6500 CURRENT CONTROL Average current These should be the same level Figure 4: Conventional Current Regulation Waveform In the waveform above, the motor moves more in the step after the peak current than in the step before the peak current. This results in a position error and instantaneous speed variation. A similar jump occurs as the current waveform crosses zero.
AN120 – UNDERSTANDING MP6500 CURRENT CONTROL VIN MP6500 VIN VCP Charge Pump VG VREF Int. VCC LS Gate Drive V CPB UVLO OVP OTS Gate Driver AOUT1 nFAULT AOUT2 STEP µC Curr. Reg. DIR nENBL MS1 MS2 ISENSE VIN Control Logic nSLEEP ROSC VIN CPA BOUT1 Gate Driver PWM Timer Motor BOUT2 Curr. Reg. ISENSE ISET GND Peak Current Set Resistor Figure 5: Block Diagram The MP6500 can drive peak currents of up to 2.5A (depending on package and PCB design) at supply voltages ranging from 4.
AN120 – UNDERSTANDING MP6500 CURRENT CONTROL IOUT tOFF Fast Decay ITRIP Change ITRIP Level tOFF Step Pulse Slow Decay Slow Decay During tOFF Unless IOUT > ITRIP at end of tOFF Figure 6: MP6500 Automatic Decay during Step Transition In cases where the supply voltage is high, the inductance is low or the desired current regulation level is very low, so it is possible that the current will increase above the desired regulation level significantly.
AN120 – UNDERSTANDING MP6500 CURRENT CONTROL An advantage of the control method used in the MP6500 is that no user adjustments are needed for different motors or supply voltages; the decay function is fully automatic. With conventional stepper motor drivers, the decay mode (and sometimes even the off time) needs to be tuned in each application to maximize the motion quality.
AN120 – UNDERSTANDING MP6500 CURRENT CONTROL To make measurements of the motion, a frequency-to-voltage converter (Coco Research KAZ-723) was used to process the output of the optical encoder with its output observed on an oscilloscope and FFT analyzer. This output is a voltage that is representative of the motor speed updated at a very high rate. This setup is shown in Figure 9 and Figure 10.
AN120 – UNDERSTANDING MP6500 CURRENT CONTROL Figure 11: Analog Current Waveform Motion Quality Using the same setup and identical test conditions, the motor was then driven by a popular bipolar stepper driver, which uses conventional peak current control using external sense resistors. This driver uses slow decay when increasing the current and mixed decay when decreasing the current.
AN120 – UNDERSTANDING MP6500 CURRENT CONTROL The MPS MP6500 stepper driver IC, which uses internal current sensing and the current regulation scheme described above, provides considerably better motion quality. The speed variation (see Figure 13), while not quite as small as it is when driven with an analog sine/cosine current waveform, was much lower than that attained with the conventional driver IC. That translates into a smoother, quieter operation, and more accurate positioning.
AN120 – UNDERSTANDING MP6500 CURRENT CONTROL Stall Resonances Figure 14: Conventional Driver Speed Ramp Using the same setup and winding current, the MP6500 shows the ability to drive to a significantly higher speed (see Figure 15). A stall occurs at a measurement voltage of about 10V, which corresponds to around 600RPM. This is due to improved current regulation at high step rates. Stall Resonances Figure 15: MP6500 Speed Ramp AN120 Rev. 1.0 4/12/2017 www.MonolithicPower.
