I built this driver using this schematic. scroll down to page28 6 to 12Vdc Input 5W LED Driver Schematic. http://www.st.com/stonline/products/literature/an/11777.pdf It is running at 12V supply voltage and the led specs are White led star, 7-7.5v forward, 700ma. The output of the board to the led is about 11.85v. How do I get it to run at 7v @ 700ma to get full brightness? Thanks for your help.

 

(The OP originally stated that his input voltage was 7.5v, the output was 7.3v @ 20mA.)
Increase the input voltage. It's likely that you'll have to increase it to 8.5v or more.

The driver you built is a buck-type regulator, not a buck-boost regulator. Even though it will be fairly efficient, there will be a voltage drop from the input to the output. Even lightly loaded, you already have a 0.2v drop from input to output. Expect that to increase somewhat as the current output increases.

 

From a PM:

I am now running the supply voltage at 12V to my buck type regulator like you said. Now the output is about 11.85V. Now how do I get it to drop to safe voltage 7-7.5V @ 700ma for my star 5w LED?

Sorry, I don't help via PM. It defeats the purpose of the board (sharing knowledge), besides jamming up my in-box, and depriving you of input from other people that would like to help you.

Please also note that the board is not interactive; it may take hours (or even days) for you to receive (a) response(s).

Getting back to your problem at hand, I suggest that you immediately stop using the circuit to power your LED until the problem with the circuit is determined and corrected! In place of the LED, use a 12v automotive tail light; these are usually rated for around 27 Watts, and are very cheap - it's not likely you'll hurt one while experimenting with it, either.

I'm suspicious of your Rs (0.24 Ohm sense resistor) - is it the correct value?
Also, are R1 and R2 the values as shown? Are you certain they are connected properly? Any chance that C4 is shorted or open? Is D2 good?

Review pages 9/10 in the PDF you linked to about the current feedback circuit.

 

All the components check out ok. I just built what the schematic said to do. I dont know if this is specific for my led or not. I would say not. But anyways I do not know what I have to do next. I looked a the current feedback section, I understand it a little but do not understand the math. I need the output to be about 7v @ 700ma. how do I get that? Right now the output to the led is 11.85v

 

What is the current through the LED (or taillight bulb) now?

Can you take a picture of how you've wired it up and post it?

 

11.85 Volts .020 or 20ma into the car light like you said to do. I hooked up the meter in series with the light to the board for amps and parallel for volts. Can the wrong choke cause some voltage or amp problems? I have a choke that is small like the size of a 1/4 watt resistor but it is 68uh like the schematic says to use? On other boards I see large coils used??

 

It is the inductor you're using that is the problem then; it is wound with very fine wire that is not capable of handling the current necessary.

The recommended inductor is a Coilcraft unit; Mouser.com sells that exact part number:
http://mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMueR%2blcXtRMwWSB8T95hmG31OcBuynOZio=

It's rated for 1.4A, however it is a surface-mount package that you may have problems dealing with. You might use this inductor instead:
http://mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMsAOuv9ANsF2Gu4JcKpiu4y
That one's rated for 1.3A, but is a radial lead package.

It looks like you have some pretty long wires from the board to your LED. You should try to keep the wires fairly short.

It's very hard to tell from the fuzzy photo (it's not well focused), but it looks like pin 7 of the L59730 isn't connected to anything. It MUST be connected to GND.

 

Does this choke have any effect on the voltage output or amperage?

 

Does this choke have any effect on the voltage output or amperage?

Of course. From the looks of things, your 68uH inductor may actually be a 62 Ohm resistor instead, which would give you just about the current that you're reading.

There may be other problems, too; like pin 7 of the L59730 not being connected to ground. Your photo of the board is so fuzzy that it's really difficult to tell much.

 

sorry about the pic. no matter how hard i tried i could not get a clear one. well I am connected to ground. the trace runs under the chip. cant see it. Thanks for all your help so far. you have been very kind. I will change things around on monday.