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DRV8870: Current Formula in Fast Decay Mode

Hector Jing
Prodigy 70 points
Part Number: DRV8870

Hi team,

When the device works in slow current decay, both low-side FETs are enabled, and the current circuit loop is approximateto a LR discharge circuit，and the current decay formula related to the off-time and load is:

where I0 is the current before current decay, that is, peak current attainable.

And what I confused is, when the device is working in fast decay mode, what is the current formula related to the off-time and load? In the fast current decay,  the current flows back into the supply voltage. Does the current decay just as the speed of current rising?

Another problem, Is there any relationship between the output voltage (|VOUT1-VOUT2|)and the power supply VM? For example, when VM = 30V, I measured the output voltage as 24V. I guess there may be a proportional relationship between the two.

Thank for your help.

  • 0
    TI__Mastermind 37683 points

    Hi Hector,

    when the device is working in fast decay mode, what is the current formula related to the off-time and load?


    - Let me do some research on this and I will get back to you. You should expect are reply by Friday (US time).

    Does the current decay just as the speed of current rising?


    -I suggest reading this appnote to learn more about fast decay. 

    Is there any relationship between the output voltage (|VOUT1-VOUT2|)and the power supply VM?

    -Typically, the higher VM, the higher the current through the load and the larger the voltage drop across the load. So the voltage across the outputs (|VOUT1-VOUT2|) will become larger as VM increases.

  • 0 in reply to Pablo Armet
    Prodigy 70 points

    Hi Pablo,

    Thank for your support. Is there any results about the research on the current formula in fast decay mode?

    Best Regards,

    Hector Jing

  • 0 in reply to Hector Jing
    TI__Mastermind 37683 points

    Hi Hector,

    Sorry for late reply.

    During fast decay (toff), the opposite FETs will turn on which will flip the Vm polarity across the motor winding. This will force the winding current to quickly decay before the current in the winding changes direction. During Toff, the motor will continue to rotate in the same direction and so the BEMF needs to be taken into consideration for calculating the current formula. Taking all considerations described above, the current through the winding can be expressed as follows: 

    Where ton<t<T (T: period of PWM cycle), Io is peak current at t=ton, and R is equal to R_L (winding resistance)+2*R_DS(on). 

    Keep in mind that at t=T, the next PWM cycle will begin and so the current will never go negative.