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DRV8428PRTER Operation Principle

D R
Prodigy 70 points

Hello, 

We are using DRV8428PRTER motor driver to drive DCU17025G18-S006 brushed DC motor. We have a few questions regarding the operation principle and help interpreting the relevant waveforms: 

1. What is the full-scale current that is mentioned in the datasheet? Our understanding is that this is the pk-pk output current, that is given as Ifs=Vref/3. However, our results don't agree with this: 

Orange - Input PWM 
Yellow - Output Duty Cycle 
Purple - Output Current 



Ifs was set to 80mA, but we measured 90-100mApk-pk voltage in our test. We noticed that the measured Irms was around 80mA when load was applied to the motor. Could it be that the Ifs calculation (Ifs = Vref/3) is actually for Irms? Also, what is the guideline to properly measure our output current?

2. Is the motor duty cycle expected to be the same as the input PWM duty cycle, or is it supposed to be regulated by the motor driver? According to the above picture, the output duty cycle is regulated by the motor driver. 
However, in our test where the output current was programmed to 250mA, the duty cycle was set to 30, 70, and 90%. In all three cases, the output duty cycle was equal to the input PWM duty cycle: 

Ifs = 250mA, 30% duty cycle: 
If=250mA, 70% duty cycle:  



90% duty cycle case is similar to the first two.
 
3. How is the acceptable range of the input PWM duty cycle determined? In our tests, at 80mA Ifs, which was achieved by 7% output duty cycle set by the motor driver (first image above), the input pwm duty cycle set up to 50-60% worked well. However, at 80% duty cycle, the output waveform was distorted. Is there a guideline regarding what should the input PWM be set to?  

Thanks!
  • 0
    TI__Mastermind 37683 points

    Hi DR,

    Thank you for posting to the Motor Drives forum.

    We are currently looking into your question and will provide a reply within 24 hours.

    Regards,

    Pablo Armet

  • 0 in reply to Pablo Armet
    TI__Mastermind 37683 points

    Hi DR,

    Apologies for late reply.

    D R said:

    Ifs was set to 80mA, but we measured 90-100mApk-pk voltage in our test. We noticed that the measured Irms was around 80mA when load was applied to the motor. Could it be that the Ifs calculation (Ifs = Vref/3) is actually for Irms? Also, what is the guideline to properly measure our output current?

    The FS current is the peak current at which the current is regulated to in the motor. For example, when FS=80mA, the peak current should be around 80mA. There will be some variation in the peak current due to tolerances and accuracy of the internal current regulation circuitry. What you are measuring is within the acceptable range. Typically at lower currents, the accuracy is worst than at higher FS currents.

    D R said:
    Is the motor duty cycle expected to be the same as the input PWM duty cycle, or is it supposed to be regulated by the motor driver? According to the above picture, the output duty cycle is regulated by the motor driver. 

    It is expected to be the same if the current is below the FS current. Once the current reaches the FS current, the driver will begin to regulate the current and control the output duty cycle to achieve the desired current limit.

    D R said:
    3. How is the acceptable range of the input PWM duty cycle determined? In our tests, at 80mA Ifs, which was achieved by 7% output duty cycle set by the motor driver (first image above), the input pwm duty cycle set up to 50-60% worked well. However, at 80% duty cycle, the output waveform was distorted. Is there a guideline regarding what should the input PWM be set to

    The input duty cycle determines the average voltage at the motor terminals which sets the motor spinning speed (for brushed DC motors). So higher duty cycles means higher speed and average voltage/current in the windings. I think in your case, at 80% duty cycle, the motor current is overshooting the FS current limit causing the ripple to be higher.