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DRV8702-Q1: Chopping Current formula

Art Mecina
Genius 16655 points
Part Number: DRV8702-Q1

Hello Guys,

Good day.

Our customer seems to found an error on equation1 page26 of the datasheet. Calculating I(CHOP) using the sampled data gives 2.33A not 3.28 as suggested on the datasheet. Is that an error?

(3.3-(0.05*19.8))/(19,8*0,05)=2.33A

Another issue is, if he calculate with the same data as before the H-bridge current (7.3.5 Amplifier Output), he gets only 1.31A from the equation, where he calculate the Vso parameter with the previous calculated Chopping current .

But here comes another question: If you want to calculate the H-bridge Current from Vso parameter that depends from the bridge current,  the equation is recursive. (Vso=Rsense*Ibridge*19.8)   It can be normal, that the bridge current is weaker than the chopping current, but if he use 0.005Ohm current sense resistor, instead of 0.05Ohm, he get for I chop 23.3A, and for Ibridge -7.69A! How can the bridge current be negative?

Would appreciate it where do you think he got it wrong, and step for step the calculations sequence.

Circuit parameters:
Device= DRV8702-Q1
Mode=HiZ
Vref=AVDD
VDS=HiZ
Idrive=220KOhm to GND
Bridge current must be 12A
Overcurrent can be 12A-15A

Thanks in advance!

Art

  • 0
    TI__Genius 13505 points

    Hi Art,

    It looks like the datasheet is not entirely clear - I found these two threads that should help:

    https://e2e.ti.com/support/motor-drivers/f/38/t/807472?DRV8702-Q1-Is-the-Equation1-in-DRV8702-Q1-datasheet-SLVSDR9D-correct-

    e2e.ti.com/.../686370

    Regards,
    Mike

  • 0 in reply to Michael Erdahl
    TI__Genius 16655 points

    Thanks for looking into this Mike!

    Hopefully that error gets corrected for its creating some confusion.

    Regards,

    Art

  • 0 in reply to Michael Erdahl
    TI__Genius 16655 points

    Hi Mike,

    I have given the links you provided to the customer. However he does have some feedback..

    According to him both equations are wrong. The first is still not giving the same value with the sample data in the datasheet, the second is recursive, therefore does not help.

    Below are his additional responses in verbatim.

    "You try to calculate tge bridge current from a parameter, which depends from the bridge current. Because Vso=Ibridge*Rsense. This is wrong And here is another remark: in the electrical equations the "x" sign means replus eg paralell resistors value is R1xR2=100 Ohm x 27 Ohm=21.26 Ohm. The multiplication sign is therefore a star, or a point on the middle: "*" or :"•" eg.: "3*3=9" or "3•3=9", because "3x3=1.5" .

    I found another error in the datasheet. There is written on chapter 7.3.4 Current regulation: "If the current regulation feature is not needed, it can be disabled by tying the VREF pin directly to the AVDD pin."
    This is wrong. If i connect the vref pin to the avdd pin i get a max 1.2A Current flow through the bridge. This was my main problem. Now i connected the vref to 2.5V, and can drive the bridge out with more Ampers, but i can not try it with a full load, because you can not give me an equation where i can calculate the current, witch can flow through the bridge."

    Please give a valid equation for calculate the bridge current, with a valid sample value set and calculation.

    Also, Please give a valid equation for calculate the chopping current, with a valid sample value set and calculation.

    That's all of it..

    Thanks in advance!

    Art

  • 0 in reply to Art Mecina
    TI__Genius 13505 points

    Hi Art,

    I've asked the design team for clarification on the datasheet equations.  I will update you on Monday if we do not have a response before then.

    Also thanks for the info on the cross operator - agree it is not ideal in this context.

    Regards,
    Mike

  • 0 in reply to Michael Erdahl
    TI__Genius 16655 points

    Hi Mike,

    Any update regarding this?

    Thanks!

    Art

  • 0 in reply to Art Mecina
    TI__Genius 13505 points

    Hi Art,

    Sorry for the long delay - been struggling to maintain power/ internet due to the weather.

    Still waiting on design for data from design and will give you another update this Friday.

    Regards,
    Mike

  • 0 in reply to Michael Erdahl
    TI__Genius 16655 points

    Hello Mike,

    No worries. I might direct the customer to this thread in case of additional inquiries.

    Best regards,

    Art

  • 0 in reply to Art Mecina
    TI__Genius 13505 points

    Art,

    Apologies for the long delay, this fell off my radar.

    For a current I_Sense flowing through R_sense, where V_offset is the amplifier input offset voltage and Av is the amplifier gain:

    V_SO = (I_Sense * R_sense + V_offset) * Av

    and at V_SO = Vref:

    I(CHOP) = (Vref -  Av * V_offset) / (Av * R_sense)

    The datasheet does have a typo - it mentions assuming 50mV for Vio in the ICHOP example, in the and that should be 5mV (typical is 5mV and max is 10mV).

    Regards,
    Mike