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?

 

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?

Wow, are you building a real boost converter? Wht's that?

 

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 www.linear.com and searching for the LT1172, the URL dosen't work... Sorry

 

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 ?

 

Are you sure you inductor and output capacitor have the right values?

 

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

http://www.onsemi.com/PowerSolutions/product.do?id=NCP3066

or

TI
http://focus.ti.com/docs/prod/folders/print/tps61161.html

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

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

http://www.ti.com/lit/gpn/tps40211

 

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!