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For DRV-8824-Q1, Decay mode setting's suggestion?

ChungChing Chiu
Prodigy 110 points
Other Parts Discussed in Thread: DRV8846, DRV8880, DRV8885, DRV8824-Q1

Dear Sir,

Please help to suggest the suitable usage of three decay mode setting.

Can we just define decay mode as below?

Slow mode : full or half stepping , low speed

Fast Mode : for high level microstepping, fast speed

Mixed mode : for high level microstepping, fast speed

can we use Fast/Mixed mode for Full or Half stepping, what will happen?

(we just know if slow mode with high level microstepping will results in Current distortion)

Finally, what's the suggestion of how to choose suitable decay mode for different usage?

Thanks you very much for your support!!!!!!!!!!!!!!!

  • 0
    TI__Guru** 114905 points

    Hi ChungChing,

    Can you provide scope captures of the current waveforms in each mode?

    Do you need the Q1 version of the device? If not, please look at either the DRV8846 (18V max recommended VM), DRV8880 (45V max recommend VM), or DRV8885 (37V max recommended VM).

    The DRV8846 and DRV8880 automatically adjusts the decay mode to avoid the current distortion you are seeing. For more information on the feature, click LINK

    The DRV8885 has integrated current sensing, eliminating the need for external sense resistors.  There is a 30% mixed decay setting for both increasing and decreasing steps that should avoid the current distortion.

    EVMs of all three devices are available for evaluation.

  • 0 in reply to Rick Duncan
    Prodigy 110 points

    Hi Rick,

    We just want to learn more about how to set the decay mode for our applicaiton.

    Before we use slow mode for high speed and high level microstepping application.

    We found that there is distortion on current scope.

    Our customer need more information about how to use these three decay mode in different case.

    If fast mode can also use to full or halt stepping,

    Can we just set fast mode for all application?

    no matter what level microstepping we use?

    and what's the advantage of these three mode.

    Now I just know slow mode can't use for high level microstepping.

    Please help to explain more about these decay mode.

    Thanks a lot~

  • 0 in reply to ChungChing Chiu
    TI__Guru** 114905 points

    Hi ChungChing,

    The overall goal of setting the decay mode is to minimize current ripple at each step. This provides higher average current (torque) to the motor. Setting the decay mode usually requires trial and error. There are many variables involved, such as VM voltage, motor inductance, and motor speed.

    Please refer to LINK1  and LINK2

    The decay modes discussed in LINK1 are applicable to the DRV8824-Q1.

    The answers below are generic. Having the motor may change the answers.

    If fast mode can also use to full or halt stepping <- Yes, fast decay is best mode in general. It is unlike to create current distortions when the motor is stopped.

    Can we just set fast mode for all application? <- You can try setting fast decay for all applications. Depending on motor speed, you may see some distortion.

    What's the advantage of these three mode

    Fast decay -- advantage (prevents loss of current regulation), disadvantage (higher current ripple, lower torque)

    Slow decay -- advantage (less current ripple, higher torque), disadvantage (possible loss of current regulation)

    Mixed decay -- ideally it is a compromise between fast and slow.

    For the DRV8824-Q1, mixed decay is not always mixed decay. Please note the last paragraph of section 8.4.1 of the datasheet.

    Mixed decay mode begins as fast decay, but at a fixed period of time (75% of the PWM cycle) switches to slow decay mode for the remainder of the fixed PWM period. This occurs only if the current through the winding is decreasing (per the indexer step table); if the current is increasing, then slow decay is used.

    I hope this helps.