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DRV8881: DRV8881P (R1 and R2 value)

Part Number: DRV8881

Hi,

I am working on schematic of DRV8881P motor driver and I had question about R1 and R2.  I have 250mOhm current sense resistor, 2 Ahm current by using equation (1) of data sheet. How can I find the value of R1 and R2?

Regards,

Farinaz

  • Hi Farinaz,

    R1 and R2 are calculated based on the VREF voltage needed to achieve the desired current with the specified resistor.

    Example: To achieve 2A using 250mOhm resistor, you will need VREF to be 3.3V. VREF = Itrip * 6.6 * .25 Ohm, or 3.3V.

    Once you know what the VREF voltage should be, you then create a resistor divider using R1/R2 to adjust the current. I suggest you use a footprint for both R1 and R2. For 2A Itrip, you can use a 0 Ohm resistor for R1.
  • HI Rick,

    I'm also working on the project and wanted to say that making R1 to 0 ohms makes sense, but I wanted to know if having R2 was even necessary at this point if we don't need the voltage divider. Could we remove R2 and make a straight wire from V3P3 to AVREF and BVREF or if we do need R2 what would be the value of it be, would it have to be hight like 10k ohms?  

    Thank you for the help!

  • Hi Uriel,

    Yes, you can remove R2 if you want to connect directly to V3P3.

    The suggestion to add footprints for both R1 and R2 allows current adjustments if necessary.

    A total R1+R2 of more than 10kOhms would be good. This would minimize the external V3P3.
  • Hi Rick,

    Thank you for the help!

    Regards,
    Farinaz
  • Hi Rick et al -

    For space constrained applications (like ours), it seems to make sense to start with some significant voltage reduction (reduced from 3.3V) at V_ref (for channels A and B) by selection of R1, R2.

    For example, if we put R_sense=47 mOhm, we can put V_ref=0.62V and still have I_trip=2 A.   This can be achieved by taking R1=15 kOhm and R2=3.47 kOhm (about...).

    This allows R_sense to be comfortably implemented in a 0603 size resistor (instead of a massive 1210 size), which can handle up to 0.2W (which easily works because at the 2A peak we will only be running .094W through R_sense).

    Perhaps things would be slightly more noisy this way in terms of implementing I_trip, but really I don't think high precision on I_trip is all that important.

    Does this sound ok to you guys?

    - Tom (& Farinaz)

  • Hi Tom ( & Farinaz),

    Lowering the sense resistor works if you do not care about the current accuracy in your application. If you have an EVM, I suggest you try changing the resistor and evaluate the results.