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INA138: Need help - I've ruined 3 parts now and I don't know why

Richard Cooke
Expert 1530 points
Part Number: INA138
Other Parts Discussed in Thread: INA168,

Hi Folks,

I need to measure the charging current going to a 8S3P LiFePO4 battery pack and so far I've broken two INA138 chips and one INA168.  The max current is <11A and my sense resistor is 0.01 ohms.  The max  charge voltage is 30V and min is 27V.  Pin 3 is connected to the + end of the sense resistor.  I've measured the voltage at the sense resistor and it doesn't exceed 0.110 volts.  The gain is approximately 27.

Is there a recommended protection scheme for the inputs?  I haven't put a scope on the sense resistor but do you think that maybe a spike is getting to the '168 and killing it?  The leads from the sense resistor are about 12 inches long.

Any suggestions are greatly appreciated.

Thanks,

Richard

  • 0
    TI__Mastermind 31815 points

    Hey Richard,

    Thanks for choosing TI and reaching out about this issue. Would you mind answering a couple questions to help me and the team?

    What exactly is the failure with the INA138 you are seeing? In what way are they failing?

    What is causing the failure? Does the failure happen immediately? Does it occur when turning on the battery charger or when connecting J20 to the sense resistor with the battery charger already on?

    Is your charger at a voltage of 30V or is it greater than 30V?

    Sincerely,

    Peter Iliya

    Current Sensing Applications

  • 0 in reply to Peter Iliya
    Expert 1530 points

    Hi Peter,

    The failure is a complete loss of output. The battery charger never goes above 29.2V.

    But I think we've figured out what went wrong.  The battery we're using isn't supposed to be removable so we never put any input protection in the circuit.  Turns out that the proper procedure wasn't being followed so the battery connector was plugged/unplugged while the current sense leads were connected to the board.  During the battery connection there was a visible arc so that was sent down the leads to the INA138/INA168 chip.

    Now that we've learned our lesson (don't assume the procedures will be followed) we'll add protection to the inputs.  Other than adding TVS diodes to the sense resistor lines do you have any other suggestions?

    Thanks,

    Richard Cooke

  • 0 in reply to Richard Cooke
    TI__Mastermind 31815 points
    Hey Richard,

    There are other components you can use than TVS diodes such as ferrite beads and RC filters. For transient protection you can refer to one of our reference design here:
    www.ti.com/.../TIDA-00302

    For sustained overstress conditions (e.g., a battery is plugged in the reverse polarity, which could break the part with a >2V differential input voltage) you can refer to this FAQ where links are provided to various educational sources.
    e2e.ti.com/.../3610.dealing-with-device-protection-when-you-cant-exceed-0-3v

    Please post back if you experience more problems.

    Sincerely,
    Peter Iliya
    Current Sensing Applications