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LM3409: Failure during test

Dominic Leone
Prodigy 120 points
Part Number: LM3409

I was running a thermal cycle test on my circuit. 300kHz switching and 1.5A drive current with 12VDC input and 3.1VDC output.

The thermal cycle test 

Thermal Cycle Endurance Test:

  1. Input Voltage: 12VDC
  2. Low: -10°C
  3. High: 85°C@85%RH
  4. Ramp time: 1 minutes or at discretion of tester if test chamber cannot achieve 1 minute rise/fall time for temperature requirements.
  5. Dwell time: 10 minutes

After 72 hours the chip failed....kind of. The inner workings are a little mysterious, but i figure a diode check should give some useful information. Using a Fluke 85V i measured the diode with the black lead on Pin 6 and the red lead on Pin 7. on a working chip, I got 1.1V same when i put the red lead on pin 8, red lead on pin 9 i got 0.5V and a diode good beep. 

On the bad chip I was reading 0.5V across all those points. Then i read short when i check the red lead to pin 7. Then, i read 1.1V and the chip worked again and i threw it back in oven.

Do you know what caused this to happen internally with the chip? is there some sort of re-settable fuse internally? or is this some weird anomaly that occurred?

  • 0
    Prodigy 120 points
    Just more info, after it failed in the chamber i had it sit on my desk doing nothing for like an hour before it fixed it self somehow
  • 0 in reply to Dominic Leone
    TI__Mastermind 25500 points

    Hello Dominic,

    That is pretty odd. It isn't anything we have seen with the IC before and there isn't any sort of internal fuse. There is a thermal shutdown function that could explain the difference. Do you know what temperature the IC reaches during this time? If it is this it should cool sufficiently with the power off withing a minute or two at most at elevated ambient temperatures.

    But from you description and from other issues I have seen it sounds like it could also be mechanical if it "fixes itself" over time. The most common is an insufficient solder joint from the DAP to the PCB ground plane. As the board expands and contracts you could have contact issues and the IC could behave strangely. Just as well it could be a cold solder joint on another connection on the board. Thermal cycling is great for catching cold solder joints. So I'm leaning towards that explanation considering how thoroughly tested this device has been.

    However, if you verify the manufacturing and solder joints and still have the issue with multiple boards then we can look into it further. 

    Regards,

    Clint