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Original question:

DRV8845: I find an error from DRV8845 datasheet.

DRV8845: DRV8845RHHR

kiran surve
Prodigy 170 points
Part Number: DRV8845


Hi,

We have used DRV8845RHHR in our project to drive stepper motor having following specifications.

1) No of phase - 2

2) Phase voltage - 6.6V

3) Phase current - 1A

4 ) Phase resistance - 6.6ohms

5) Size : NEMA 17

The applied voltage on VM1 and VM2 pins is 11.5V.

The VREFX reference voltage pins are connected to PWM outputs of microcontroller for which the VOH logic level is 3.3V. 

We have used voltage divider ckt and adjusted VREF voltage of 1.47V on all VREFX pins.

All sense pins have 0.47 ohms 1 watt resistor to GND.

The maximum itrip calculated is ITRIPMax = VREF / (3 × RSENSE)

                                                                    = 1.47/(3 x 0.47) 

                                                                    = 1.04A

The maximum power to stepper motor is 6.6W as mentioned in the above motor specifications.

How much PWM duty cycle should be set on Vrefx pins to limit the maximum power of 6.6W to motor with above stepper motor specifications?

Also provide calculations for the same.

Regards,

KIRAN SURVE

  • 0
    TI__Mastermind 23451 points

    Hey Kiran,

    kiran surve said:
    How much PWM duty cycle should be set on Vrefx pins to limit the maximum power of 6.6W to motor with above stepper motor specifications?

    These should be a reference voltage, not a PWM signal.  You could set up a RC-filter on your PWM signal to generate a fairly smooth analog voltage on the pin if you don't have a DAC pin available on your MCU.  For testing purposes many people just use a potentiometer.  

    See the PWM Current Control section for the equations about the I_TRIP value:

    Do you have a DRV8845EVM you can test with?  It has a very convenient web GUI interface that you can type in a VREF voltage and that voltage will be output to the device. 

    Best,

    Jacob 

  • 0 in reply to Jacob Thompson
    Prodigy 170 points

    Hi Jacob,

    Thanks for the answer. 

    My confusion is if I am using stepper motor with following specifications:

    1) Phase voltage - 6.6V

    2) Phase current - 1A

    3) Phase resistance - 6.6ohms

    But instead of 6.6V, I apply 12V to the motor, what should be the phase current I should set for the motor?

    Regards,

    KIRAN SURVE

  • 0 in reply to kiran surve
    TI__Mastermind 23451 points

    Hey Kiran,


    Stepper motors are typically operated at much higher voltage than the rated voltage. We run steppers with similar specifications at 24V.   I'm reading online that steppers can typically be driven up to 20x their rated phase voltage.  

    The key is regulating the current to the recommended level. As long as the current is regulated, the voltage across the windings is within tolerance.

    The benefit of operating the stepper motor at higher voltages is that the stepper motor speed can be increased - you get more torque at higher speeds. 

    So in your case, 12V will give you more torque at a given current level.  I would start with the phase current around 1/3 of the max rating, so 0.3A is a good start.  Try that, and increase or decrease as needed - you might get the best performance at 0.5A or 0.7A for your given speed.  It just depends on the motor.  And I recommend using at least 1/32 microstep for the smoothest operation, up to 1/256 microstepping.  

    Best,

    Jacob 

  • 0 in reply to Jacob Thompson
    Prodigy 170 points

    Hi Jacob,

    Thanks for the detailed information.

    Just to clear my confusion, if I apply 12V with 0.7A, the power would be 8.4W, which is more than the rated 6.6W for motor. 

    Means to keep the power wattage within 6.6W for motor and operating with 12V, I need to apply maximum 0.55A to motor. Increasing current above 0,55A would operate the motor out of specifications of 6.6W.

    Would you please elaborate the above.

    Following are the Stepper motor specifications:

    Regards,

    KIRAN SURVE

  • 0 in reply to kiran surve
    TI__Mastermind 23451 points

    Hey Kiran,

    I recommend you test it and find in your system with this motor and a given load how much current you need, and how much the motor heats up with that given current.  I think you'll find the motor capable of >6.6V at 0.7A without issue.  Motors are not like an IC - the max wattage rating is not Phase Voltage * Phase Current ratings.   

    Feel free to order our EVM to speed along your development: https://www.ti.com/tool/DRV8845EVM

    Best,

    Jacob