SMPS buck converter trouble - very low current

Discussion in 'The Projects Forum' started by b1rdjx88, Apr 12, 2011.

  1. b1rdjx88

    Thread Starter New Member

    Jan 9, 2010
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    Hi, I'm working on a datalogger project using a Pic 18f4550 to save water quality sensor readings to an SD card.

    I've put together an SMPS buck converter to supply 3.3V to the micro and SD card, but it seems to be limiting the output current to a maximum of 60uA. I am using a L4971 smps IC, and I've built the typical application circuit from the datasheet (http://www.st.com/stonline/books/pdf/docs/5598.pdf). I'm wondering if it is stuck in soft-start mode somehow. Any ideas?

    I realize it's way oversized for my application. I'm reading through a couple of the pages for buck converter design linked to from earlier forum posts, and I think I'll design an appropriate one in the future. For now I'd just like to get what I have working.

    Thanks.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    Did you use the suggested PCB layout? Or are you trying to construct it on a breadboard?
    Are you certain that C5 is good (not shorted)?
    Are you certain that C6 is good (not shorted or open)?
    Is the output voltage correct?
     
  3. b1rdjx88

    Thread Starter New Member

    Jan 9, 2010
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    It's a breadboard I'm using. Stray capacitance or RF interference issues? Is there a way to compensate for this or am I just out of luck with the breadboard?

    It is putting out the right voltage - charges the capacitor across the output up to ~3.37V and holds it there.

    I've checked all the connections with a multimeter and can't find any problems.

    Thanks for the help, and apologies for trying to short cut figuring this out from fundamentals.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, circuits like this don't usually do very well on breadboards. The breadboard itself has lots of parasitics (L and C). Breadboarded projects tend to have long loopy wires running everywhere, and that'll kill the performance of this circuit.

    With the values shown, you're running at 100kHz. Square wave outputs will need about 1MHz bandwidth to look halfway decent.

    Did you use the exact core (MicroMetals KoolMu 77120) and 65 turns of AWG 25 or 0.5mm magnet wire for the inductor? Slapping in just any old inductor won't do.
     
  5. b1rdjx88

    Thread Starter New Member

    Jan 9, 2010
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