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DRV8824 sense resistor

DanR
Prodigy 240 points

Other Parts Discussed in Thread: DRV8824, DRV8821

Hi, my customer is considering using the DRV8824 for his stepper motor control application and is asking about the sense resistor. What power rating do you recommend for the sense resistor?

Thanks!

Dan

  • 0
    Prodigy 240 points

    Hi,

    It's been a couple of weeks and no one has responded to my customer's question. I looked for a schematic in the user guides. But it states " A schematic is available in the form of a PDF file (“CPG004_Schematic.pdf”) inside the EVM_Related folder on the supplied CD".

    Can someone send the schematic to me?

    Thanks!

    Dan

  • 0 in reply to DanR
    TI__Expert 7455 points

    Hi Dan,

    Sorry for the delay in answering this question. The power rating of the SENSE resistor will depend on how much current you will be driving. Typical sense resistors are in the 100 to 400 milliohns range. For the DRV8824, maximum current per phase is 1.6A so I would assume the 400 mOhm resistor would be used.

    ITRIP = 1.6A = VREF / (5 * SENSE) = 3.2V / (5 * 0.4) 

    At 1.6A, the 400 mOhm resistor would be dissipating 1.024W if the current was DC. However, notice the 1.6A is the peak of the sine wave being generated as the stepper is microstepped. Hence the most accurate equation would be to use the RMS current of 1.13A, which in turn gives a power dissipation of 512 mW. In this case a 1W 2512 power resistor would do the job.

    Do note that if you plan on holding torque at full current and without making VREF a smaller value (the way I would do it as there is no reason to full blast the stepper motor when it is not moving, and if there is it may be the wrong motor for the job), then 1.024W will in fact be the power dissipation if you happen to stop the motor on a microstep in which the current is regulating to the 1.6A peak. If you foresee this happening, you may want to utilize a 2W resistor just to cover your bases.

    2512 resistors are fairly expensive when compared to the smaller footprints. So it is not rare to see two or more smaller size resistors in parallel. Per example two or three 1206 resistors in parallel, or even four or five 0805 resistors in parallel. Just mentioning this as an option, not necessarily that I recommend it.

    Hope the info helps.

    Best regards,

    JIQ

  • 0 in reply to Jose Quinones
    Prodigy 100 points

    Hi Jose,

    I was wondering what should be my Sense Resistor if I'm using a DRV8821 to supply only 0.8A with a Vref of 3.3V?

    Should I use a 800 mOhm resistor with a Power dissipation of 1W (528 mW)? I'm asking because it's above the typical values you mentioned.

    Thanks!

    Andres

     

  • 0 in reply to Andres Bergeret
    Prodigy 100 points

    Hi!

    I still haven't received any reply regarding this.

    Thanks!

    Andres

     

  • 0 in reply to Andres Bergeret
    TI__Expert 7455 points

    Hi Andres,

    The range I proposed back then is based on the typical values we use on the great majority of our EVM's and application notes. However, there should be no penalties for using larger (or smaller) values, as long as the VREF and SENSE input voltages are not compromised.

    That being said, to obtain 800 mA sine wave peak on the DRV8821, you can use a 0.825 ohm resistor. I found a few while searching on Digikey.

    Your total power dissipation on the resistor will be (0.8)(0.8)(0.707)(0.825) = 373 mW, which is quite below the 1W rating. Notice I multiplied by 0.707 as what you want to use is the RMS value of the current, not the sine wave peak. If you are using full step, the chip automatically uses a factor of 0.707 of the ITRIP equation.

    Hope the info helps. Best regards,

    Jose I Quinones 

  • 0 in reply to Jose Quinones
    Prodigy 100 points

    Jose,

    Thank you so much for your reply!

    Regards,

    Andres