Boost Converter Trouble

Discussion in 'The Projects Forum' started by kjprag01, May 29, 2009.

  1. kjprag01

    Thread Starter New Member

    May 29, 2009
    I am building a DC-DC boost converter off of the LT1172. This boost converter is supplied by a 9v battery and has an output of 40v, which is what i wanted, however. (There is always a however)... When I hook the boost converter to my 145 5mm LED array and each led is 3.3V at 20mA the voltage begins to fall until my leds no longer light up, when I unhook the array of LEDS the voltage jumps right back to 40V. Why does this happen and how do I keep this from happening?
  2. Mike Mandaville

    Active Member

    May 27, 2009
    Wow, are you building a real boost converter? Wht's that?
  3. kjprag01

    Thread Starter New Member

    May 29, 2009
    Yes, it is a real boost converter. The circuit takes an input voltage and outputs a voltage larger than the input. In my case 9V to 40V. The basic topology i am using can be found here and searching for the LT1172, the URL dosen't work... Sorry
  4. steinar96

    Active Member

    Apr 18, 2009
    hmm, it sounds like your load is draining the capacitor storing energy between pulses faster then it can recharge. Are you sure your booster can supply the load the wattage it needs ?
  5. mik3

    Senior Member

    Feb 4, 2008
    Are you sure you inductor and output capacitor have the right values?
  6. AchMED

    Active Member

    Aug 5, 2008
    Your trying to drive 145 LED's that each drop 3.3V at 20mA? Your Boost converter outputs 40Vdc. You would have to string them in parallel. If your just hooking 145 LED's up in series at your converters output it wont work unless your boost converter outputs at least 3.3 *145= 478.5V.

    Power out would be about 10W.A nine volt Alkaline if that is what your using would have to supply about 12W it wouldn't last to long probably on the order of microseconds.The large peak currents would kill it pretty quick.

    For LED's you typicaly regulate the current. Somthing like the NCP3065



    Google "constant current sources" or "LED drivers+ boost" etc.

    Here is an example LED driver useing the TPS40211.The data sheet is here.

    Last edited: May 29, 2009
  7. nrg2009

    New Member

    Jan 19, 2010
    Thanks for the schematic. It seems like the TPS40211 is a perfect driver for 3W LED's. I'm a newbie, and interested in using maybe 5 of the 3W LED's that I can drive from my 12VAC (5A) landscape lighting. Could anyone help me build a driver? I have one LED working on a LM317 but it doesn't seem all that efficient.

    Thanks again!