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DRV8845: Could I use CC buck power for VM of motor driver?

Allen Huang
Prodigy 130 points
Part Number: DRV8845

Tool/software:

DRV8845 is linear motor driver. If VM is more larger than motor voltage, motor driver will be hot.

Could I add SENSE1 and SENSE2 voltage feedback to CC buck power for supply DRV8845 VM?

  • 0
    TI__Genius 11651 points

    Hi Allen,

    Thanks for your question. My team mate will look into the question and respond.

    Thanks,

    Ibinu

  • 0
    TI__Mastermind 19040 points

    Hi Allen,

    I am a bit confused by your question. 

    Allen Huang said:
    If VM is more larger than motor voltage, motor driver will be hot.

    VM is the "motor voltage". As long as the motor voltage supply is within recommended operating conditions there should be no concern for IC temp. The DRV8845 uses PWM current regulation and smart tune decay to reduce power dissipation. 

    The VSENSEx voltages are used for PWM current control. The following formula can be used to set ITRIP threshold.

    This ITRIP threshold sets the maximum allowable current through the load. 

    "When the voltage across the current sense resistor equals the voltage on the VREFx pin, the current sense comparator resets the PWM latch, which turns off the source driver."

    Allen Huang said:
    Could I add SENSE1 and SENSE2 voltage feedback to CC buck power for supply DRV8845 VM?

    Not sure if this would work. It can be helpful for each new design to first evaluate using one of our EVMs.

    DRV8845EVM Evaluation board | TI.com

    Best,

    David

  • 0 in reply to David Medis
    Prodigy 130 points

    For example:
    DRV8845: VM=24V, Itripmax=1A

    Step motor coil voltage=4V@1A, 2 coil turned on

    DRV8845 will bear 20V*2A power when coils turned on, right?

  • 0 in reply to Allen Huang
    TI__Mastermind 19040 points

    Hi Allen,

    Supplying VM = 24V would exceed the stepper motor voltage rating. It would be recommended to reduce motor voltage to match voltage rating of motor.

    The DRV8845 regulates current through the motor depending on which step setting selected. The full-scale current of the DRV8845 is 1.5A, meaning this is the maximum amount of current that can energize a coil during a full-step configuration. 

    Best,

    David

  • 0 in reply to David Medis
    TI__Mastermind 44235 points

    Hi Allen,

    Modern stepper drivers are based on current mode drivers. For such drivers regulating the current through the winding and maintaining its waveform is important. The voltage is switched with a specific duty cycle which means the average voltage will not exceed the coil voltage. This smoothing of switching happens because of the inductance of the motor coils. So if you measure true RMS voltage across the windings it will be in compliance. With such drivers it is not necessary to use a CC buck power for VM voltage of the driver. The stepper driver is doing this exactly a switching current mode current regulator which actually does the voltage buck by switching on/off, tON and tOFF current regulation. If you use CC buck for VM input the stepper driver will not work as intended. Thanks.

    Regards, Murugavel 

  • 0 in reply to Murugavel4637
    Prodigy 130 points

    Hi

    Your mean is I can control the stepper motor with coil voltage=4V@1A  when VM=24V?

  • 0 in reply to Allen Huang
    TI__Mastermind 44235 points

    Hi Allen,

    Yes this is correct. The stepper coil voltage 4V specification of the motor is for direct drive with no current regulation. 

    Regards, Murugavel 

  • 0 in reply to Murugavel4637
    Prodigy 130 points

    I don't know what "Smart tune" is. It can regulate stepper motor coil voltage is 4V/1A when VM=24V and DRV8845 won't be too hot.
    Right?

  • 0 in reply to Allen Huang
    TI__Mastermind 44235 points

    Hi Allen,

    Allen Huang said:
    I don't know what "Smart tune" is. It can regulate stepper motor coil voltage is 4V/1A when VM=24V and DRV8845 won't be too hot.

    The current mode stepper drivers regulate motor coil current. Coil voltage will be (coil current x coil resistance). 

    Smart tune is part of the current decay operating mode during the tOFF portion of the current regulation. Older stepper drivers used manually tunable mixed decay options. Modern stepper drivers such as the DRV8845 has smart tune feature which automatically tunes the decay ratio for the stepper for proper current regulation for every microstep. These application notes will be helpful, www.ti.com/.../slva321a.pdf, https://www.ti.com/lit/wp/slyy099a/slyy099a.pdf and https://www.ti.com/lit/an/slvaes8/slvaes8.pdf.

    Regards, Murugavel 

  • 0 in reply to Murugavel4637
    Prodigy 130 points

    So it like using coil for inductance, then DRV8845 control switch for CC buck. right?

  • 0 in reply to Allen Huang
    TI__Mastermind 44235 points

    Hi Allen,

    Allen Huang said:
    So it like using coil for inductance, then DRV8845 control switch for CC buck. right?

    At a high level this would be the working theory of switching type current regulation.

    We do not use that CC buck terminology with stepper drivers. CC buck is more of a power management terminology. With stepper drivers it is referred to as current regulation. In microstep mode the target ITRIP current is variable according to the indexer position for microsteps. You could measure the voltage across the stepper motor coil while the motor is running using a true rms meter. Thank you.

    Regards, Murugavel