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DRV8889-Q1: DRV8889-Q1: Driver PWM waveform

LEE JaeYong
Prodigy 20 points
Part Number: DRV8889-Q1

i did a circuit design with DRV8889-Q1 (12V-step motor).

Generally PWM waveform 

but our board waveform, why our waveform first full step after PWM (Micro step _Full step with 100% current)

our setting is miss or DRV8889-Q1 is normal operating.

  • 0
    TI__Guru 56650 points

    Lee,

    The motor driver regulates the winding current to give a good sinusoid current by adjusting the PWM duty cycle. If the duty cycle is 100%, that means apply full input voltage to the winding to change the winding current.

    Vin=Vbemf+I*R+Ldi/dt;  Vbemf is the motor back EMF voltage which is related to the motor speed; I is winding current; L is winding inductance.

    Would you increase the input voltage or reduce the motor rotation speed and check the PWM signal again?

    Also, the winding current change is also related the decay mode setting. A fast decay can make the winding current quickly drop. Would you try dynamic decay mode or fast decay mode and check the waveform again?

  • 0 in reply to Wang5577
    Prodigy 20 points

    Dear Wang Li.

    Thanks your answer.

    I check again.

    And I want to know, DRV8889-Q1 driver output waveform is correct or not..??

    because only SPI PWM (From MCU) & current set & Micro step set, our result is that picture.

    Full - PWM-reset

    Thanks.

  • 0 in reply to LEE JaeYong
    TI__Guru 56650 points

    JaeYong,

    1. The motor driver’s PWM controller regulates the winding current to follow each micro-step winding current desired value to get smooth motor control, for example: the DRV8889 datasheet table 3’s value.

       

    2. To get the table 3 current value and maintain that current at each micro-step position, the PWM controller adjusts the PWM duty cycle. Different motor drive may have different PWM control algorithm and decay mode setting.

      I have checked DRV8889 on bench today. You can see no switching and output voltage free drop period. In that period, the PWM controller tries to regulate the winding current at zero. So, PWM controller doesn’t turn the switching FET on or off unnecessary. But, different company or different decay mode setting may have different control algorithm to maintain the zero winding current. The first picture in this post keeps doing switching to get zero winding current.  I would say DRV8889 has less noise and loss to maintain zero winding current with the smart ripple control decay mode.

       

    3. Overall, the winding current is the end result. The PWM duty cycle control is a method to get the desired winding current. Different decay mode or PWM algorithm could have different waveform. Let us check and focus on the winding current.

       

    4. DRV8889 offers eight decay mode and up to 1/256 micro-step to refine the winding current. Customer can study their motor to get best winding current waveform.

    1. The motor driver’s PWM controller regulates the winding current to follow each micro-step winding current desired value to get smooth motor control, for example: the DRV8889’s table 3’s value.

       

    2. To get the table 3 current value and maintain that current at each micro-step position, the PWM controller adjusts the PWM duty cycle.

      Different motor drive may have different PWM control algorithm and decay mode setting. For example: the first circle in each test waveform:

      I have checked DRV8889 on bench today. You can see no switching and output voltage free drop period. In that period, the PWM controller tries to regulate the winding current at zero. So, PWM controller doesn’t turn the switching FET on or off unnecessary. But, different company or different decay mode setting, we have different control algorithm to maintain the zero winding current. On-semi part keeps doing switching to get zero winding current. 2nd circle in each waveform is similar: different way to regulate zero winding current. I would say DRV8889 has less noise and loss to maintain zero winding current with the smart ripple control decay mode.

       

    3. Overall, the winding current is the end result. The PWM duty cycle control is a method to get the desired winding current. Different decay mode or PWM algorithm could have different waveform. Let us check and focus on the winding current.

       

    4. DRV8889 offers eight decay mode and up to 1/256 micro-step to refine the winding current. Customer can study their motor to get best winding current waveform.