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LM3940 regulator not putting out 3.3V

  1. Apr 8, 2008 #1
    I'm giving it exactly 5 volts directly from a DC Power Supply and the output voltage should be 3.3V but it's not. I don't even have anything connected to the output pin of the regulator (holding it in my hand).

    It seems if
    V_in < 1.5V, V_out = about 0 plus a couple millivolts
    V_in >= 1.5V, V_out = V_in minus a couple millivolts

    I am using an LM3940IT-3.3. I've tried one from a box of them just to be sure I hadn't shorted the old one out. There's only 3 pins: V_in, Ground, and V_out....how can you mess that up?


    I don't know what I am doing wrong as I am using some 7805s in my circuit which have no problem taking my 8volt DC and putting out 5volts.

    Any suggestions? Thanks.
  2. jcsd
  3. Apr 8, 2008 #2
    Have you got the caps like from the diagram in place?
  4. Apr 9, 2008 #3
    That's the last thing I haven't tried yet (ordered some, waiting for them to arrive). But I didn't think the caps not being there would make a difference if I threw it a steady voltage. Could they really be the culprit? If so, why?
  5. Apr 9, 2008 #4

    It's probably singing its head off at around 40MHz...

    I was amazed the first time that happened to me too...

    It needs two 100nF caps, on on the input, one on the output, and probably an electrolytic too.
  6. Apr 9, 2008 #5


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    I don't have time right now to check the datasheet... What are the specs for minimum input voltage, and for minimum output current?
  7. Apr 9, 2008 #6


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    Voltage regulators are feedback circuits, and can only be stable in certain configurations. The capacitors add poles and are necessary for stability.

    Also, many regulators will not function properly unless you are pulling some current from them.

    - Warren
  8. Apr 9, 2008 #7
    Thank you guys for all the responses. I guess I need those caps there. I designed the board with the caps in mind but I guess my preliminary testing before I soldered them in made me feel something was wrong altogether. I'll just wait & see what happens after the caps are in. I'll be honest I'm pretty new at this so this was the first big thing I've learned & will have to keep in mind.

    berkeman, I'm not sure of the minimums but I think the minimum input is 3.3V + dropout[0.31] = 3.61V? And I'm guessing minimum output current is the quiescent current [0.11mA]? Also, 73s to you. Just made General last year.

    The full story is that when the 3940 regulator was connected to the output of the 7805 regulator, the output voltage of the 7805 dropped down to 3.something volts from 5 volts, which baffled me. Feedback is key word I need to remember.
  9. Apr 9, 2008 #8


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    Yes, the minimum input voltage is the desired output voltage plus the drop out.

    The quiescent current is the current drawn by the regulator even when supplying no load. It's basically a small amount of current that's used to bias the transistors in the regulator.

    Many people are surprised to learn that voltage regulators are feedback circuits, but there's really no other way to design them. Basically, the regulator monitors its own output voltage, creates an error signal, and servos the gate voltage of a big power transistor to let more or less current through to the load, regulating the voltage. The result is a feedback loop, and all feedback loops require care to maintain stability.

    - Warren
    Last edited: Apr 9, 2008
  10. Apr 9, 2008 #9


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    FWIW the datasheet I saw listed the dropout voltage as 0.5v.
    Either way it should work with a Vin of 5v.
  11. Apr 10, 2008 #10


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    Are you saying that the output of the LM7805 voltage regulator dropped down to 3V when you connected the LM3940 regulator's input to the 5V rail? If so, there is something wrong with the LM3940, or you have the pinout wrong.

    Keep in mind that the pinout is different for some regulators when they are in different packages. The pinout for the 7805 regulator is different between the TO-92 package and the TO-220 package, for example. Also, if you are using a power transistor package with a metal tab for a heat sink, that tab may not tolerate being grounded (check the datasheet to see what the tab is connected to).
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