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LMP8640HV: Current sensing error (actual load 30A but display 20A)

Gary Zeng
Intellectual 1145 points
Part Number: LMP8640HV
Other Parts Discussed in Thread: LMP8640

Tool/software:

Hi Ti experts,

My customer uses LMP8640 in OBC for current sensing. They found it has wrong current sensing value. When the actual load current is 30A, it can only get the wrong value 20A.

They have done the ABA experiments. It passed. New one has no such issue. Issue rate is 1%.

  • 0
    TI__Mastermind 32280 points

    Hello Gary,

    Normally we recommend lower than 10Ω at the input due to the input bias.  Also is R66 51Ω?  I do not think that is correct.  Is there anything in their circuit that could be causing our inputs to go beyond the absolute max ratings, even during transients?

    Regards,

    Javier

  • 0 in reply to Javier Contreras4
    TI__Intellectual 1145 points

    Hi Contreras,

    Thans for your reply.

    What is the effect if customer use 33ohm rather than <10ohm?

    And R66 is indeed 51ohm. But it is not the shunt resistor. Shunt resistors are R191 and R192 as below. Both are 0.2mohm. Which means actual value of shunt resistor is 0.1mohm. Current is 30A. Differential voltage should be 3mV(0.1mohm x 30A=3mV). Common voltage is 28V. Is it ok?

  • 0 in reply to Gary Zeng
    TI__Mastermind 32280 points

    Hello Gary,

    Why is R66 installed?  What purpose does it serve?  This will cause additional current running on the sense line beside the IB needed for the LMP8640.

    I would confirm that the voltage on the bad device and good device are the same at the inputs of the LMP8640.  TP43 and TP44 are not good enough as the 33Ω resistor will impact the measurement.  Having different IB could cause differences in output voltages but I would not assume to the amount you are seeing.

    The layout of the parallel 0.2mΩ can impact measurements as well but I would not expect and ABA swap to be much different.

    Regards,

    Javier