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DRV8872: Does the signal ISEN can be used for current monitoring into the load?

Bruno Simard67
Prodigy 240 points
Part Number: DRV8872

Hello,

I would like to have your point of view of the way to use the voltage level on the ISEN pin to monitor the current level into the motor. The objective will be to monitor the current in all conditions (below the current limit and at  the current limit.

R_ISEN will be from 6ohms to 23 ohms depending the current limit set. Current limit range 58mA -100mA

Do you think that this way to proceed will be adequate and precise for the current level evaluation into the motor?

Thank you

  • 0
    TI__Mastermind 23431 points

    Hey Bruno,

    See section 7.3.3 Current Regulation.  We recommend choosing your sense resistor based on Equation 1 for your ITRIP level.  If your ITRIP is 100mA, then RISEN = 0.35V/0.100A = 3.5Ω.

    Regards,

    Jacob

  • 0 in reply to Jacob Thompson
    Prodigy 240 points

    Hello Jacob,

    Thank you for this reply. I realize that I did not put enough details of my circuit. Also I know well the equation I_TRIP= 0.35V/R_ISEN which were based my calculations below

    Here are my calculations: 

    Finally please review my first sentence of my first question introduction.

    My question was related to the possibility to use pin 7 (voltage on ISEN) of the driver to read the current level in all time into my motor in all conditions (when driver is in current limited condition but also during normal operation or simply when the motor is not current limited but voltage limited %PWM)

    Again thank you for your feedback

    Regards

    Bruno 

  • 0 in reply to Bruno Simard67
    TI__Mastermind 23431 points

    Hey Bruno,

    Ahh I see!  Yes, you should be able to measure the voltage at ISEN to read the current.  I don't foresee a problem with this.  Though the voltage will be very low (0V - 0.35V) , so you might need an op-amp to measure it depending on your MCU analog input accuracy.  

    Regards,

    Jacob

  • 0
    Genius 5040 points
    Bruno Simard67 said:

    R_ISEN will be from 6ohms to 23 ohms depending the current limit set. Current limit range 58mA -100mA

    Do you think that this way to proceed will be adequate and precise for the current level evaluation into the motor?

    Hi Bruno,

    With R_sen=6Ohms, at 100ma the voltage at the pin is 0.6v, no good, as the ITRIP protection can happen at 0.32v according to the datasheet so the driver will never can drive 100mA load. Obviously 23 ohms is even worst. 

    So select a resistor so that Rsen* I should be always less than 0.32v in normal operation. Of course you can use ADC converter to monitor the voltage at Isen to find out the load current anytime. 

    P.S. I see on the schematic that it may have 3 resistors in parallel to have the total value from 6 ohms to 3.5 ohms. This can work but I suggest to target a trip voltage of 0.32v, not 0.35v as the driver can trigger  ITRIP at 0.32v and higher. So yours resistors need to be changed.

    Brian