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DRV8874: Fixed Off-Time Current Chopping, brake vs coast

Andrew Darrow
Prodigy 90 points
Part Number: DRV8874

Hello,

This question may be more related to general motor drive theory, but I am hoping someone can help me understand more:

In fixed off-time current chopping, the DRV8874 enters a brake/low-side slow decay state. 

What was the reasoning behind the decision to enter brake/low-side slow decay state instead of entering coast/Hi-Z state for current chopping?

Thanks,

Andrew

  • 0
    TI__Expert 8985 points

    Hi Andrew,

    In fast decay, the current decays towards zero as fast as possible. This method is also called Coast since the motor coasts to a stop. This behavior is due to the motor speed being proportional to the voltage across the motor. During fast decay, the current will decay quickly because a voltage of greater magnitude but the opposing polarity is applied to the inductive load when driven.

    During slow decay, the current decays slower than fast decay. The motor brakes instead of coasts to a stop. This mode is referred to as Brake. Slow decay occurs by disabling the Forward direction’s high side FET and enabling the Reverse direction’s low side FET. Slow decay is also called active freewheeling.

    As for your question, the reason why the device uses slow decay/brake is that more torque can be applied to the load as the current decay rate is slower.

  • 0 in reply to Hector Hernandez Luque
    Prodigy 90 points

    Excellent, thank you Hector.