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LM6171: LM6171BIMX/NOPB - Possible Failure

Darrell Mayfield
Prodigy 20 points

Part Number: LM6171

Summary from customer:

Error 7 is an error upon self-test. The device's stimulation output is verified before treatment. If it falls outside of a designated range, it returns Error 7. The schematic I attached has the stimulation generation circuit on page 4 and the parameter sensing circuit on page 5.

EWME3525-0035, EWME3525-0002, EWME3525-0016

0035 and 0002 came from the field. The analysis returned the following results:

0035

- Upon visual inspection, capacitor C36 was visibly bulging.

- Both current and voltage were below the threshold on self-test

- Upon installing the PCBA in a test unit and turning it on, U17 rapidly heated up.

- Replacing C36 and U17 fixed the PCBA and made it functional again.

0002

- Upon visual inspection, capacitor C36 was visibly bulging.

- Both current and voltage were below the threshold on self-test

- Replacing C36 and U17 fixed the PCBA and made it functional again.

  • 0
    Prodigy 20 points

    I want to verify that this schematic is not visible to the general public.

  • 0 in reply to Darrell Mayfield
    TI__Genius 17566 points

    Hey Darrell,

    Welcome to E2E, thanks for reaching out. 

    The Schematic was visible to the public forum, though I removed the schematic prints from your post. 

    For help with future designs that are information sensitive, feel free to make your post and ask if you can share directly with us. We will often add you as a Friend on E2E, and we can private message any sensitive design documents to keep them off the public forum. 

    Regardless, I have the documents now, so we can refer to component designators for my review. 

    I think C36 is backwards, no? 

    SN74HCxxx switches a positive DAC voltage into a buffer amp. This is then passed to U19 which inverts the signal with some amount of  BW filtering. This negative DC voltage is then applied to the positive lead of the polarized cap, with the negative lead grounded. 

    Most polarized caps go bad after a few V negative, though this does depend on the electrochemical contents of the cap. As someone who has reverse biased too many electrolytics, even relatively small negative voltages can make them pop or expand. Disappointed

    ECO the board with a flipped capacitor, and we should be all good to go. I figure the capacitor was internally shorting, causing the OPA to drive a low impedance path to GND. Operated continuously, this would cause the OPA to heat up as it is driving high IOUT currents. 

    Please let me know what you think.

    Best,

    Jacob