Hey guys, will this work?

[jmgelba]

It was a mistake. I have a test box where I can push a few buttons and switch between different types, numbers and colors of LED's. I use it to test drivers that are new and about to be installed into a product. I also have some power resistors in there which are my dummy loads. I pressed the wrong switch and connected 12 IR led's instead of the dummy load. I put 23v through 2 strings of 6. The have about 1.8vdrop each typically.

It was not a good night.

[jmgelba]

OK, I found some time to try Bruce's code and I got the same results. At a very high and very low feedback signal the PWM signal disappears and the output goes 100% high. If the feedback signal varies rapidly the PWM goes 100% low.

I took off all the switching components to remove any chance of noise and just used a pot to vary a fake feedback voltage and got the same results.

I am going to try the code and ciruit used in the AN1130 app note. My schematic is different to this - I have the inductor on the positive side of the LED's and use a PFET.

My requirements are also different.

Input = 24v
LED forward V = 21V (6x blue LED's with 3.5vdrop each)
LED I = 700mA
Current Ripple = 10%
Switiching Frequency = 200KHz

I calculate the inductor value to be 131uH. I will have to lower the gain on the feedback opamp from 11 to maybe 5 or something. I'll figure that out later - just a simple resistor value change.

I'm going to spend a bit of time trying to convert the code from C to PICBasic. I've havent used C before so that will take me a while to actualy figure out what the heck void and so on means.