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INA240: Inline motor current sensing and gain confusion

Cloe Gibbons
Prodigy 30 points
Part Number: INA240
Other Parts Discussed in Thread: TMCS1100

Hi,

I have read through the forum but have been unable to find the answers to my questions. I am hoping I can find some help by posting here.

I am trying to achieve inline current sensing of a BLDC motor. I am planning to use a 0.82mOhm shunt resistor (9W & 70*C TCR). My motor/power source is 12V. I will supply the INA240 with 5V. I have wired it for Vs/2 voltage reference with ref1 to 5v and ref2 to ground, yielding 2.5V. The output of the INA240 is going to an ADC (analog) pin on the ATMega328p. I will be sensing between 0A to 30A. From my calculations I have got Vdiff = 25mV and PrSense = 0.747W. IN- is connected to the motor phases and IN+ is connected to the mosfets (sources (high-side)/drains(low-side)), either side of the shunt resistors.

I have read through the datasheet but I have a few questions.

  1. Is one shunt resistor on each motor phase, each connected to the one INA240 a correct configuration? (3 shunt resistors & 1 INA240 total) Or do I need 3 INA240's?
  2. I was originally planning to use the INA240A4 with 200V gain, but I am since questioning that perhaps that is too much of a gain? I am a little confused about the gain. Could you advice me on a suitable gain for my project?

I hope you have all the information you need, I look forward to replies! Thank you in advance.

  • 0
    TI__Mastermind 30600 points

    Hi Cloe,

    Welcome to the Forum. To answer your questions:

    1. Three shunt resistors and three INA240’s are needed, one set for each phase.
    2. Given the shunt resistors and supply, the suitable gain would be 50 (INA240A2). You could also choose 20 (INA240A1) but the output range will be smaller. You’re correct that gain of 200 is too much and the output will be clipped when the motor current exceeds 15A.

    Regards, Guang

  • 0 in reply to Guang Zhou
    Prodigy 30 points

    Hi Guang,

    Thank you very much for your help! You've answered my questions perfectly and cleared everything up.

    However, since posting yesterday, I have also found your TMCS1100 hall-effect current sensor. Since I am very tightly constrained to a 19mm circular PCB, I think this might be a better fit as you do not need three current shunts and only need three TMCS1100s. Do you think this would be a better alternative? I know it's current sensing limit is 30A, I will be looking to measure up to 20A constant with 30A peak.

    Thank you again,

    Cloe.

  • 0 in reply to Cloe Gibbons
    TI__Mastermind 30600 points

    Hi Cloe,

    Given your situation, TMCS1100 might be a good alternative. Based on the SOA curve, a 20A RMS current should be safe.

    Regards, Guang

  • 0 in reply to Guang Zhou
    Prodigy 30 points

    That's great thank you.