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DRV8825 - Vref/current switching circuit

Edson Lopes
Prodigy 30 points
Other Parts Discussed in Thread: DRV8825

Hello,

I have a question regarding the last paragraph of Mr Wilson Zuo's reply:
http://e2e.ti.com/support/applications/motor_drivers/f/38/t/260058.aspx
«For DRV8825, you can achieve the half current by setting and lower the AVref and BVref together.»

Is there any application note or some recommended circuit for toggling the current between the desired high and the low value for vref?

While I'm waiting for some guideline, I've created a digital switch using one transistor and resistors,
I've attached the image to this post (switching from 330mV to 88mV)...
So for Risense = 0.1Ω, I would be changing between 660mA and 176mA.

Best Regards

  • 0
    TI__Expert 8885 points

    Hello,

    Application Report SLOA170, provides the detials you are looking for.

    By the way, your ciruit is also going to work, if you need only two settings.

    Best Regards

    Milan-Motor Application Team

  • 0 in reply to Milan Rajne
    Prodigy 30 points

    I've read the application report and the "GPIO Driven Voltage Divider" seems really good (sparing some components) for switching between two settings. 
    Thank you very much for your answer.

    .

    Another question, in the DRV8825 datasheet (at the end of pag. 5), there is something regarding the values of VREF:

    Recommended VREF [Min = 1V and Max = 3.5V],  
    «Operational at VREF between 0 V and 1 V, but accuracy is degraded»

    It seems that the driver would be more accurate if I increased the Risense from 0.1Ω to 0.82Ω, so instead of changing VREF between [330mV and 88mV], I would be changing it between [2.7V and 0.72V]. Is that ok? Is there any drawback?
    What exactly is degraded, is it the distance between microsteps?

    Best regards,

    Lopes

  • 0 in reply to Edson Lopes
    TI__Expert 8885 points

    Edson,

    You have figured out correctly, for lower Vref values current accuracy suffers, so it would be good to go for higher Vref values by increasing Rsense values. There is not much harm in increasing the Rsense value because you get better current accuracy and control in lieu with little increase in power losses.

    Yes degraded current can lead to different distances between miscro-steps, but it would not be significantly noticed because 90% phase difference between two winding current during micro-stepping. This means when current in one winding is zero with degraded accuracy, other winding current is at its peak with higher accuracy, so over-all impact is nullified.

    It would be very difficult , time conusming and near to impossible, to do a theoretically trade-off between the advantages of currrent accuracy gain vs disadvantage of increased power loss, so I would suggest to follow the data-sheet recommendation of Vref range, if it does not significatly affect your system effieicncy.

    Best Regar

    Milan- Motor application team

  • 0 in reply to Milan Rajne
    Prodigy 30 points

    Again, thank you very much for your awesome answer Mr Milan Rajne. So it is worth using the recommended VREF, then I will search for the right Rsense.

    Best regards,

    Lopes