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DRV8843: Using PWM input to drive stepper motor

Caitlin Slayden
Prodigy 80 points
Part Number: DRV8843

Is fPWM an externally applied signal? Or is it internally generated?

The datasheet implies externally applied in table 6.3 recommended operating conditions, but in table 6.5 electrical characteristics, fPWM is referred to as internal current control PWM frequency.

If the PWM frequency is fixed, does modulating the width affect the speed of the motor spin? Or does it affect the current applied to the motor coil?

There is a lot open for interpretation when it comes to the description of PWM in section 7.4.1 Bridge control. I am assuming the PWM signal applied at input AIN1 or 2 depending on slow or fast decay is externally generated. What do the inputs for AINx and BINx look like compared to the output for a full step sequence? Figures 9 and 10 show some information but not enough.

In the case of full step sequence, does the PWM signal get input to the AIN / BIN pins for a duration equal to the full step? and the "on time" before the PWM starts is the blanking time?  or is this just a time set by the user to allow for a "soft start"?  If there are any diagrams or scope captures that show the input signals and output signals for a longer duration of time (to show a few steps), this would be extremely helpful.  

Are there any application notes or documents that would explain implementing the PWM for the stepper motor operation in more detail? 

  • 0
    TI__Guru** 114905 points
    Hi Caitlyn,

    In general, there are two use cases for the DRV8843:

    1) Set the desired current using VREF and a current sense resistor. Set the inputs to 100% and allow the internal current regulation engine to limit the current. Internal current regulation operates at ~50kHz and uses the decay mode selected by the DECAY pin.

    2) Disable the current regulation (or set it at a point where it will regulate as a backup) and control the outputs by providing a PWM signal to the AINx and BINx pins. In this case, the maximum PWM frequency is 100kHz.

    Since you appear to asking about stepper operation, let me answer the questions:

    "If the PWM frequency is fixed, does modulating the width affect the speed of the motor spin? Or does it affect the current applied to the motor coil?"

    The PWM frequency has no effect on the motor speed. The direction of the current and rate of change affects the speed. The current level affects the available motor torque.

    "There is a lot open for interpretation when it comes to the description of PWM in section 7.4.1 Bridge control. I am assuming the PWM signal applied at input AIN1 or 2 depending on slow or fast decay is externally generated. What do the inputs for AINx and BINx look like compared to the output for a full step sequence? Figures 9 and 10 show some information but not enough."

    For a full step, the AIN1 and AIN2 will be high/low or low/high, and the BIN1 and BIN2 will be high/low or low/high. The input values will depend on the desired current direction.

    "In the case of full step sequence, does the PWM signal get input to the AIN / BIN pins for a duration equal to the full step? and the "on time" before the PWM starts is the blanking time? or is this just a time set by the user to allow for a "soft start"? If there are any diagrams or scope captures that show the input signals and output signals for a longer duration of time (to show a few steps), this would be extremely helpful.

    Are there any application notes or documents that would explain implementing the PWM for the stepper motor operation in more detail?"

    Please take a look at www.ti.com/.../slva443.pdf. This is a good starting point. Please let us know if you have additional questions.