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LMP8646: Inaccurate output voltage

Part Number: LMP8646

I wanted to use this IC as a current sensor so just for preliminary testing I used a sense resistor(Rsesne) of 0.5 ohms and gain resistance (Rg) of value 49.9K. The supply voltage given to the IC was 12V and the common mode voltage (Vcm) to the input terminals was 6V. I passed a current of 0.4 A. From this I calculated that the output voltage (Vout) would be 1.996 V however I was getting an output voltage of just 0.13 V.

I tried changing the currents to different values however I never got the expected value. The value that I got was always less than what I was expecting.  

  • Atharva, can you share your schematic? We'll look a little more in depth in the morning, but if you could share just that bit around the current shunt device, we'd appreciate it.

    Best Regards,

    Jason Bridgmon, TI Sensing Products Applications Support

    Current Shunt Monitor Video Training Series

    Hall Effect Sensor Video Training Series

    TI makes no warranties and assumes no liability for applications assistance or customer product design. You are fully responsible for all design decisions and engineering with regard to your products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning your designs.

  • Hi Atharva,

    Thanks for using TI E2E Forums. I did some calculations with your application parameters and you are correct. With Rg=49.9k ohm, Rshunt=0.5 ohms, Vs = 12V and Load = 0.4A you should measure 1.996V in the output (Vout). I also ran a simulation on TINA and got the same results, see below. 

    For troubleshooting, please measure the  voltage on the input pins of the device (between IN- and IN+) to make sure you are getting 200 mV. You can share your schematic with me, I am happy to review it for you. 

    Mayrim Verdejo

    TI Current Shunt Monitors Applications Support

  • In reply to Mayrim Verdejo:

    I have attached the board and schematic files of the PCB that I made. It has 2 more ICs on it DRV 8412 and LP 38512-ADJ.
  • In reply to Atharva Karaguppi:

    I have made the PCB on EAGLE software.
  • In reply to Atharva Karaguppi:

    Hi Atharva,

    Please send me the schematic on pdf format. The Altium file you send me seems to be empty. Also, please measure the voltage between the inputs of the device (as I mentioned on previous reply) and let me know if it is 200mV or not. Refer to this training video for debugging and tips as of where to probe. 

    Mayrim Verdejo

    TI Current Shunt Monitors Applications Support

  • In reply to Mayrim Verdejo:

    Hi Atharva,

    I was able to get your schematic using Eagle. My apologies, Eagle and Altium have same extensions.

    I will need further clarification from the circuit. Why is there 2 headers for load? The headers pins are not labeled so I am not understanding the circuit. Also, is the IN- tied to  Rsense? There is no nodes on the schematic so it is not clear to me if there is a connection.

    Our typical circuit  should look like the image below. Can you please confirm that your Rsense and Load are connected this way? Do you have a block diagram? Please confirm the Vsense value as well.

    Mayrim Verdejo

    TI Current Shunt Monitors Applications Support

  • In reply to Mayrim Verdejo:

    The load+ and load- are actually tied to IN+ and IN- respectively.
    Yes, the IN- pin is tied to the sense resistor and to the load- pin as well.The load shall be present between load+ and load-.And before load+ I shall place the battery or the voltage source.

    Yes I have confirmed the value of Vsesne as well. 

    In the board is there presence of parasitic resistances in the path of the sense resistor which might be affecting the effective value of the sense resistor and hence resulting into inaccurate outputs ? 

  • In reply to Atharva Karaguppi:

    Atharva,

    Thanks for the reply. Did you get to collect the measurements? In order for me to detect if the problem is with our INA we need to make sure that you are measuring the expected shunt voltage.

    Mayrim Verdejo

    TI Current Shunt Monitors Applications Support

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