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LMP8645 on high side sense

Dario Lopez
Prodigy 30 points
Other Parts Discussed in Thread: LMP8645, LMP8640

Hello,

     I'm unsuccessfully using an LMP8645 to current sense on the high side of a DC circuit.  I'm supplying an amplifier with 12V DC whose current draw range is 60±10mA.  I'm interested in monitoring that current draw.  I'm using a 10 ohm shunt resistor in front of the amp, and a gain resistor of 21K.  In+ is seeing 12V and In- is seeing ~12.4V nominal.  My calculations of 21k/5k = 4.2V/V suggest that I should see 2.5V at the amplifier's output.  I'm powering the amp with a single 5V source.  However, I'm measuring 4.9V at the amp output.  I chose this amplifier because it was advertised as accepting high common mode voltages w.r.t. the source.  Did I misinterpret the spec?  Does anyone see what I'm doing wrong?

 

Dario

  • 0
    TI__Mastermind 49766 points

    Hi Dario,

    The resistor value is okay. You should be seeing around 2.52V on the output at 60mA.

    But you say the voltage on -IN is 12.4V. The -IN voltage should be lower than +IN. The LMP8640 is not bi-directional and cannot handle "negative" sense voltages. The +IN should be on the supply side of the sense resistor.

    To work properly. +IN should be 12V and -IN should be 11.4V.

    Do you have the inputs backwards? Is the motor kicking-back EMF?

    And, yes, the common mode voltage is separate from the supply voltage and they can be different. No problem there.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 30 points

    Paul,

         Thank you.  It seems I also need help with writing posts... That 12.4V was a typo.  In+ is 12V and In- is 11.4V.  I just ran another experiment and unhooked everything  but the power to the chip and I'm reading 4.9V on Vout.  Do I have a busted chip?

    Dario

  • 0 in reply to Dario Lopez
    TI__Mastermind 49766 points

    Hi Dario,

    "Floating" the inputs can give random results due to the bias currents. I don't recommend it...

    Remove the motor load and power it up (so that both inputs see 12.00V). The output should be around zero.

    If it reads 4.9V...then something is wrong. The voltage on the output and the gain set resistor should be the same (the output just buffers [x1]) the voltage across the resistor.

    It is better to measure *at* the device pins, rather than across the components since it is easy to have open pins on these small devices.

    Also make sure the 21K resistor is the value you think it is...they are easy to miss-read. Trust but verify...

    This s a fairly simple device...not much to go wrong.

    If everything measures out...then it could be a damaged device. Make sure the motor is not kicking back large EMF voltages. You may want to put clamping diodes across the supply and the motor.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 30 points

    Paul,

         Great advice.  Thanks.  I'll try that.  I'll post the results.

    Dario

  • 0 in reply to Dario Lopez
    Expert 1580 points

    I am working on a project where I need to monitor a current drawn by a 48V BLDC motor. I only have access to the 3 phases of the motors. I am using the evaluation board , which I have modified with a resistor of 20mOhm and a Rg of 47Kohm and the HV version of the part.  The chip is powered with 5VDC. Everytime I run the motor for a while, the chip get damaged. Am I missing something ? Is this the right part for this project ?