• State
  • Locked Locked
  • Replies 5 replies
  • Subscribers 65 subscribers
  • Views 546 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM2940IMP-5.0 component failures

John Schimandle
Intellectual 390 points

I've been working on this problem for about 2 weeks and we have finally traced it down to a failing LM2940IMP-5.0 regulator. Looking at the data sheet it seems like it's almost impossible to kill this part. It has output short circuit protection, thermal protection and input reverse voltage protection and also mirror image installation protection. The parts almost always fail open circuit on the output but in some no load tests there is a very small voltage of about 100 mV that seems to follow the input voltage. We did have one part that failed shorted so the input voltage appears on the output.

The part provides clean 5V power to some analog circuitry on the board. Typically the current draw is less than 50 mA. This part is rated at 1A so there should be plenty of head room.

The problem was hard to troubleshoot because another component on the board was connected to the 5V digital and 5V analog supply. This component then sourced the 5V analog when the regulator wasn't working through the 5V digital supply. The board appears to work but the part that was sourcing the 5V analog was functioning intermittently with very strange behavior.

This is a very old design at 7+ years and many units have been shipped. This is the first time my client has seen the LM2940 part failing.

I would like to get some idea of how this part can fail. I also have 4 failed units if they are needed for failure analysis. Back of part is labeled MALAY 9534. All parts are labeled with LM2940IMP-5.0 on front.

  • 0
    TI__Guru 72545 points

    I will try to direct your questions to the correct person.

    Frank D

  • 0
    TI__Mastermind 22610 points

    Personally, I don't know of any way to stay within the operating specs and still blow up the LM2940.

    What's the operating input voltage? Any indication of overheating? Any possibility that the LM2940 could be reverse biased (i.e. Vout>Vin)?

    If you have samples of failed and good parts from the same lot then I suggest that they be submitted for formal PQA failure analysis. That way there's a formal record of the problem and any discoveries.

    I'm going to forward this thread to the Product Engineer and I'll try to track down the QA/Rel person for this device.

  • 0 in reply to Donald Jones
    Intellectual 390 points

    I would totally agree with you from reading the data sheet. This part looks almost indestructable. This design is 7+ years old with 1000s of working units in the field. The typical load on this LDO is around 50mA with some boards pulling 100mA if all DAC outputs are used and are all driving maximum load with maximum signal. Not a typical configuration.

    Input voltage is 12-15 volts depending on the type of supply used in the product.

    When you power down the unit you could get Vout > Vin for a brief period of time. I just took a measurement with the regulator very lightly loaded and there is a period of time during turn off where there is a reverse voltage of about -700mV for about 200ms. Actually its more like a ramp from -700mV to -180mV over 200 ms.

    Just let me know where to send the failed components. You can reach me at john_schimandle@hotmail.com.

  • 0 in reply to John Schimandle
    TI__Prodigy 10 points

    Hi John,

    I am the prodcut engr for this device. You are correct  that this design almost indestructable. With the worst case of 15Vin, 100mA for this 1A part, it should able to handle this. What is the failing temp? And we will need to see the good and bad parts on our bench when we receive the units. If you have the plot for the ramp during the turnoff, appreciated that you can send along with the PQA return. All other application informations such as input & output caps would be helpful too.

    thanks, Luther