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LP2985-N: Failure

Part Number: LP2985-N

I am using an LP2985IM5-5.0 with the recommended circuit from the data sheet. I have 10V input, 5V out, about 10mA load. I have seen 2 failures where the output voltage is about 6.5V on both failures. There is no oscillation on the failed unit, the 6.5V output is very clean. On a failed unit I measure about .95V on the bypass pin. What are possible failure modes that can cause this type of failure?

  • Hi Alan,

    Typically when an LDO has an output higher than its set voltage, there is an unintentional leakage path biasing the output high. Since a traditional LDO does not sink current, the pass element will turn off until the load can pull the output voltage back within regulation. Please help answer a few questions that will help us debug your application.

    Do you have any scope shots that you can share of the failure?

    Do the LDOs ever output 5V or do they always output 6.5V?

    Is there a way to isolate the LDO from its load on your board? If so, what is the isolated output of the LDO?

    Is there a 6.5V rail on your board near your load? Some loads have multiple input rails.

    Very Respectfully,
    Ryan
  • No scope shots, output was a clean 6.5V.

    Yes, output was 5V, unit ran for about 40 hrs.

    I did isolate the LDO from the load, output voltage decreased by about 50mV, but still about 6.45 and ref pin measured .95V.

    There is no other supply near the load, there was definitely a failure of the LDO. I replaced the bad LDO with a new one and the unit worked properly.

    There is a possibility of some transients being coupled into the circuit. What I would like to find out is what type of transient if any could cause this type of failure, for example transient on input, transient on output, transient causing reverse polarity?

    Knowing this would assist my troubleshooting. Assuming there was a transient that caused the failure, what type of transient would be most likely to cause the LDO to have 6.5V output. I have had 2 units so far both with 6.5V output. It seems a bit unusual to have both failures produce the exact same high output voltage. The part could fail and have any output from 0V to the rail. I am hoping the 6.5V output gives some sort of clue regarding the failure mode.

  • Hi Alan,

    Thank you for the additional information.  I agree that it is interesting that two units failed to have the same output voltage, especially since you do not have 6.5V anywhere else.

    The most likely transient to damage a device is one that causes reverse current to flow through the LDO.  An external Schottky diode placed in parallel with the LDO as shown in section 8.2.2.4 of the datasheet would help protect the LDO from damage in these events.

    Very Respectfully,

    Ryan

  • I'm sorry I failed to mention that I do have a Schottky diode in the circuit.

    Is there anything else you can think of that would cause this type of failure?

  • Hi Alan,

    Thanks, that should rule out reverse current. Do you have any inductors on the input or output of the LDO or any long wires in your setup?

    Very Respectfully,
    Ryan