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78L05: How hot should it get

John Mountcastle
Prodigy 20 points

I have a small radio frequency generator/counter, a NorCal FCC-1/2 if it helps, that suddenly quit working. In trying to troubleshoot it. I happened to touch a 78L05 and found it quite hot to the touch. Almost too hot to touch. Is this within the realm of normal or likely the cause of my failure?

Thanks for reading,

John

  • 0
    TI__Guru**** 152566 points

     John,

    The 78L05 may be bad, however it is more likely that the output current is too high because of a damaged component located elsewhere.
    Have you measured the VIN and VOUT voltages?

  • 0 in reply to Ron Michallick
    Prodigy 20 points

    Thanks Ron, I'm nowhere near the device now but if VIN and VOUT mean voltage from the battery and to the device, They were about right, VIN just under 12V, VOUT just around 5. I know something I can do. I'll put an ammeter between the battery and the device and see what it's drawing. If it's way higher than the spec, I guess I can conclude that some device has failed and created a short because it worked fine for several weeks then one day just quit.

    Regards

    John

  • 0 in reply to John Mountcastle
    TI__Guru**** 152566 points

    John,

    If VOUT is 5V then the uA78L05 did its job. The heat will be set by power dispassion which is (VIN - 5V) * IOUT.

  • 0 in reply to Ron Michallick
    Prodigy 60 points

    Hi Ron,

    I also have a question regarding the uA78L05ACLPR (TO-92 package) operation at its maximum output current (~97mA) and regulating at about 4.85V.  Given the input voltage of 12V (also regulated), and based on the maximum case temperature of 73C, are we over stressing the the linear regulator?  

    We know that ambient temperature of our application should not exceed 40C and based on the listed thetaJ-C, virtual junction temperature should be more than 15C below the maximum 150C.   However, we did have several 78L05 failures (failed components would output 8 -9V instead of 5V after about 1 year of being deployed in the field), and we're very much concerned about the long term reliability of our application.  What is, if at all, possible failure mechanism of continuous operation at max output current?  Is there a higher current drop in (same package) replacement for uA78L05ACLPR?

    Thank you for your help in advance.

    Best Regards,
    Zoran

  • 0 in reply to Zoran Mihajlovic
    TI__Guru**** 152566 points

    Zoran,

    I suspect long term usage at high current and high temperature would have potential issue with electro-migration. This is the metal in the circuit traces on the die moving which leads to shorts or opens.

    We do have higher current version with wider metal traces (less prone to EM) but they are not LP. Like uA78M05