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DRV8701: Current measurement in DRV8701E

AKSHAY SATPUTE
Prodigy 10 points
Part Number: DRV8701

Dear TI Team,

We are using a DRV8701E IC for driving a brushed dc motor. Our application involves monitoring motor currents if any shoot is observed during any obstruction to the motor. 

We are driving this motor with PWM of 25 KHz .Mosfet used for H-BRIDGE Circuit is IRF1010N N-CHANNEL MOSFET.
We need suggestions for the current measurement from DRV8701E IC.
The Microcontroller adc used is reading from SO (Pin no 11) of DRV8701E IC.
We have kept
reference voltage vref 3 V ,
R-sense shunt resistor = 50 mOHMS
I chop (CHOPPING CURRENT) = 3 Amp.

It is observed while reading the voltage from SO (Pin no 11) of DRV8701E IC.
The output read is found to be not linear compared to current flowing through motors.

How can we read out this current?Do we need to read this value in different way or different algorithm to read the current.

  • 0
    TI__Mastermind 23306 points

    Hey Akshay,

    Thank you for your question.

    When you are spinning the motor and during the stages when the motor is in the brake state based on Table 1 in the DS, we have low side recirculation. And when this happens the current is not flowing through the shunt resistor. Therefore The SO will not be linear compared to the current flowing in the motor. 

    Please read section 7.3.3 of the DS for more info.

    Hope this answers your question.

    Best,

    Akshay

  • 0 in reply to Akshay Rajeev Menon
    Prodigy 10 points

    Dear Akshay,

    In that case ,

    Should microcontroller read ADC from SO Pin of DRV8701E only when SNSOUT Pin is High?

    I tried reading SO Pin through ADC only when SNSOUT Pin is High. But still response was same as in previous case.

    Also PWM frequency to DRV8701E is 25 KHz.PWM Type : Center Aligned PWM.

    What should be the sampling frequency of ADC for such signal to readout SO Pin.

    Can you provide us any reference code of reading SO Pin through adc?

  • 0 in reply to AKSHAY SATPUTE
    TI__Mastermind 23306 points

    Hey Akshay,

    I wanted to get a clarification on what you meant by, "output read is found to be not linear compared to current flowing through motors". Were you referring to the fact that you are not seeing the current during recirculation through SO pin as I assumed above or did you mean something else?

    If you meant the current during recirculation then it will not be possible to see it even if you measure when SNOUT is high since the current is physically not flowing through the resistor. 

    For the sampling rate, you can try measuring at twice your PWM frequency or greater based on Nyquist theorem.

    Best,
    Akshay