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Troubles with LM-2940 CT12

  1. Sep 17, 2013 #1
    Hello. I use for several months an LM-2940CT12 to supply 3 PCB's. All datasheet rules were respected.Max current is less than 200 mA. A few weeks ago voltage falls down to 8 V ant head voltage which is normally 13.9 V falls to 10.5 V and so regulation was out of order. I replace the component, and after few days, pb appears one more time. I precise that nothing has been changed on PCB's, and sometimes power works and head and then out voltages returns to correct values. How can we explain that erratic function of reguator IC ?
     
  2. jcsd
  3. Sep 17, 2013 #2

    meBigGuy

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    Gold Member

    It could be oscillating. Is it properly bypassed? Does it get hot during normal operation? Did it get hot when the condition occurred? If you powered it down and up again did the condition persist? If you put your finger on the output did it change the voltage?

    If it is oscillating it is probably because you used a cheap electrolytic on the output. You could parallel it with a 0.1 ceramic. There is a section in the data sheet regarding ESR of the output capacitor. (it wouldn't paste well)

    Also, circuit layout can cause oscillation. the filter cap needs to be close to the part and grounds need to be robust.

    What drives the "head" voltage. By head I assume you mean the raw supply is 13.9 DC.

    What package are you using? Is it too small to dissipate the 1/2 watt and the part eventually fries? The SOT-223 is 175C/Watt so it would get pretty warm.
     
    Last edited: Sep 17, 2013
  4. Sep 17, 2013 #3
    Thanks for reply. input cap (filtering) is 4700 µF, output cap 470 µF, distances to IC < 1". IC mounted on heatsink (not really usefull). Absolutely never any oscillation. For the time being it works perfectly ! Anyway I'll place a ceramic cap on output, may be effectively an ESR problem.
     
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