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DRV8886: current rise time variation depending on Load torque

Takashi Onawa
Genius 13200 points
Part Number: DRV8886

Hi Team,

My customer noticed that current rise time tend to be slightly shorter depending on load torque in DRV8886 evaluation.
Do you know the reason why the current rise time was varied depending on load torque?
I need to explain the reason to the customer.

Motor speed : 500pps
Micro step : NO
Load torque : 50~250mNm(constant during current rise time measurement)

Regards,

Takashi Onawa

  • 0
    TI__Guru 56650 points

    Onawa-san,

    This following picture shows the motor back EMF voltage vs the load torque. When the voltage on the winding terminals is different at different load torque. So, you can see the current rising speed difference.

  • 0 in reply to Wang5577
    TI__Genius 13200 points

    Hi Wang-san,

    Thanks for your prompt response on this.

    I understand that the phase of the BEMF signal changes with torque, but I did't understand the relationship between this phase change and the rise time of the current.
    Could you give me  some more explanation?

    Regards,

    Takashi Onawa

  • 0 in reply to Takashi Onawa
    TI__Guru 56650 points

    Onawa,

    The voltage across the winding terminal could be difference due to the phase change. 

    v=Ldi/dt; => di/dt=v/L; v is the input voltage minus the back EMF voltage. The different phase shift makes v is different at different current level. So, the rise time of the current is different.

    Let us make it simple, in slow decay (two low side FETs are on), only back EMF voltage is applied the winding terminal. Is the current slew rate same if the back EMF is same? You may get some ideas from this application note: https://www.ti.com/lit/an/slvaei3/slvaei3.pdf