• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 1 answer
  • Subscribers 29 subscribers
  • Views 438 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DRV8881: Formula (1) for the chopping current

Hirotaka Matsumoto
Guru 27525 points
Part Number: DRV8881

Hi all

Would you mind if we ask DRV8881E?

There is formula (1) for the chopping current on the datasheet P16.


At the our customer's circuit, they configured as follows;
-Vref=0.49V
-TRQ=00(100%)
-RISENSE=0.2ohm±1%

Actually(result), current limit was 250mA～310mA.ISENSE pin voltage was 0.05V～0.06V. 
Using formula(1), calculation value is 370mA.
There is some difference between  calculation value and result data.

So, using Figure 9. xISEN Trip Voltage over VREF Input on the datasheet P9,
in case of VREF=0.49V, trip voltage is 0.05V.
It seems that the customer's result is correct in case of Figure 9.

We would like to confirm formula(1).

Is this formula(1) for current limit in case of Vref=3.3V only?
In case of Vref=0.49V,  formula(1) is not suitable, right?
And then, AV(gain amplifier) is not linearity, right?
As the background of this question, our customer would like to calculate limit current in case of VREF=0.5V.

Kind regards,

Hirotaka Matsumoto

  • 0
    TI__Guru 56650 points
    Hirotaka-san:

    How do you measure the 250mA～310mA current limit? Do you use a scope to capture the peak current value? Would you share the waveform?

    The formula should be good for 0.49V Vref. TRQ * Av is constant.
  • 0 in reply to Wang5577
    Guru 27525 points

    Wang5577 san

    Thank you so much for your support always!

    And then, we appologize that our explanation makes you confuse.

    How do you measure the 250mA～310mA current limit?
    Do you use a scope to capture the peak current value?
    Would you share the waveform?
    ->Please refer to the attachment file.
    20190514_DRV8881E.pdf

    The formula should be good for 0.49V Vref. TRQ * Av is constant.
    ->Using this attached file, Itrip will be as follows;
       Itrip=(0.46×1(TRQ=00))/(0.2×6.6)=0.348A
       ・RISENSE=0.2Ω±0.1%
       ・VREF=0.46V
       ・TRQ=00(100%)
      So, it seems that actual limit value is 364mA, there is bit difference, however we got that the formula should be good for 0.49V Vref. TRQ * Av is constant.
      OK, we got that your saying is correct.
      
    But, we would like to confirm "Figure 9. xISEN Trip Voltage over VREF Input".
    In case of VREF=0.46V(≒0.5V), Vtrip=0.05V.
    So, using formula(1),  
    Itrip=VREF×TRQ(100%)/(6.6×RSENSE)
    Vtrip=Itrip×RSENSE=VREF×TRQ(100%)/(6.6)=0.46/6.6=70mV

    As the result, there is the difference "value using formula(1)" and "Figure 9. xISEN Trip Voltage over VREF Input" in case of VREF=0.5V.
    Could you give us the advice for it?


    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Guru 56650 points

    Hirotaka-san:

    From the attached pdf file, the ISENSE pin voltage (rectangular waveform) doesn't match ISENSE current waveform (triangular waveform). I would think some measurement issue.

    a. the winding current is same as the ISENSE resistor current

    b. ISENSE current passes a resistor. The voltage waveform should match the current waveform.

    Also, are you drive a stepper motor? Would you zoom out the ISENSE current waveform? I want to see a full step cycle current and find the peak value.

    The  formula 1 doesn't show the offset when the VREF is near zero. If you use a small VREF,

    a. please use Figure 9 as reference.

    b. increase the sense resistor value to increase the accuracy with the formula 1.

  • 0 in reply to Wang5577
    Guru 27525 points

    Wang5577 san

    Thank you so much for your support always.

    I would think some measurement issue.
    ->We found that the customer has some measurement issue.
       We apologize that the uncorrect wave form makes you confuse.
       Please refer to the attachment file which is correct wave form.
    20190515_DRV8881E.pdf

    Also, are you drive a stepper motor?
    ->Yes, they use stepper motor.
    Would you zoom out the ISENSE current waveform?I want to see a full step cycle current and find the peak value.
    ->Peak current is 316mA.

    The  formula 1 doesn't show the offset when the VREF is near zero. If you use a small VREF,
    a. please use Figure 9 as reference.
    b. increase the sense resistor value to increase the accuracy with the formula 1.
    ->OK, we got it.

    We have some additional questions.

    <Question1>
    On Figure9, it shows the value(VREF and VTRIP) which is typical. 
    How much is the tolerance of these value?(as the reference, could you give us the Min/Max value?)

    <Question2>
    If the customer uses formula(1) to calculate Ifs correctly, should they use Vref=3.3V?
    How much is the Vref which is filled with formula (1)?
    More than 1.0V?

    Kind regards,

    Hirotaka Matsumoto


  • 0 in reply to Hirotaka Matsumoto
    TI__Guru 56650 points

    Hirotaka-san:,

    1. Figure 9 xISEN Full-Scale Trip Voltage value is below formula 1 result. I would think the figure 9 value is the typical value.

    From the formula 1, the Amplifier attenuation error is related to DRV8881 which can be found in spec table: Av. I would think this Av error should be at xVREF = 3.3 V.

    2. When the customer uses formula(1) to calculate Ifs, they should use the voltage on xVREF pin voltage.

    It is about +/-5% error at 3.3Vref and Torque=100%, the estimate error is about 5%*3.3V/1V=  16.5% at one volt VREF.

    Those numbers are estimation numbers because I don't have any production  test data to confirm it.