Output capacitor voltage regulator problems

Discussion in 'The Projects Forum' started by leonhart88, Aug 21, 2012.

  1. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    Hi All,

    I'm encountering a strange problem where my voltage regulator works without a certain output cap, but with the cap, it doesn't output the correct voltage.

    I've attached a picture of the schematic. I have a really large output tantalum cap (10uF). I've discovered (through a lot of scratching of my head), that the voltage regulator does not work when I have this large cap in. It outputs something like 4.7V instead of 12V like it's supposed to. I also notice that as time goes on, the voltage slowly drops. If anything, I would expect the voltage to slowly rise as the cap gets charged up.

    Is the cap too large? The datasheet recommends 0.1uF, but I didn't think a very large cap would cause problems. Or is it because I'm using a tantalum capacitor and has something to do with the ESR?

    Thanks.
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,136
    1,786
    If there is no other load in the system then I would suspect the capacitor. With No DC load on the output it should output the 12V plus or minus a little bit. It should also not be drawing any current and remain at room temperature.

    A dropping output of 4.7 volts sounds like the output has a heavy load, is drawing way too much current, and is probably is finger burning HOT. In short -- find the short -- so to speak.
     
  3. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    Hi Papabravo,

    There is currently no load, as I'm putting together parts of my PCB and I'm doing the power circuits first. Actually, the only load is a 4.75K resistor with an LED.

    I also encountered another large cap which was causing problems for another switching power supply on my board. I removed a 100uF cap and it works as expected now. The 100uF on that circuit and the 10uF on the 78L12 are much larger than what the datasheets suggest. Can the problems I'm encountering be caused by having too large of an output capacitance?

    Would having no load on the supplies also be a culprit? I did not see anything in the datasheets which talk about minimum loads. I have attached the other circuit as well.
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,136
    1,786
    I'm not saying there is a problem with the schematic. Linear regulators of the 78Lxx type are not likely to care about the size of the capacitor on the output. There are some regulators that require a minimum amount of capacitance for stability. More should not be a problem. As always RTFDS.

    Look for non-obvious faults on the board or in the surrounding environment. An ohmmeter can tell you if a cap is shorted. If the cap is not shorted have a resistor (470-680 ohms) and an alligator clip handy to discharge it.

    Your problem is unlikely to be the size of the capacitor. Let us know what you find.
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    scope it and see if the output is oscillating. This part is a switching converter:

    http://pdf1.alldatasheet.com/datasheet-pdf/view/265022/LINER/LTM8021.html



    From the data sheet:

    Tantalum caps have a self resonant frequency of a few hundred KHz, they can not be used on this part. You must use ceramic only. Tantalums will act like resonant tank circuits.
     
    Last edited: Aug 21, 2012
  6. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    Hi All,

    Thanks for your suggestions and help. It turns out I just had the polarity of the capacitors wrong (doh!). My silkscreen was confusing so I was putting the caps the wrong way...I guess next time this will be the first thing I double check.

    I've also opted to get rid of the 100uF tantalum on the LTM8021. It was the only place I was using that part and it is kind of unnecessary and would reduce my parts count too.

    Thanks again!
     
  7. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    As I said above: the LTM8021 is a 1.1MHz switcher. A tantalum capacitor is beyond it's usable frequency at 1.1MHz, it actually looks inductive not capacitive.

    I read the data sheet for the LTM8021 and it has extensive information on the ceramic caps you are supposed to use with it.

    From the data sheet:

    I believe you could use Tants only if there was enough ceramic capacitance in parallel to swamp out the resonant effect.
     
    Last edited: Aug 21, 2012
  8. ramancini8

    Member

    Jul 18, 2012
    442
    118
    When you put large caps on regulator outputs you run the risk of blowing the regulator during shutdown if the input voltage goes to ground. the solution is to bypass the regulator with a reverse diode.
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Since this is a buck regulator switcher IC, it probably has an inherent reverse diode as part of the NPN pass transistor used for the switch.
     
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