Hey guys, will this work?

[Darrel Taylor]

At 200khz, you only get a Duty range of 0-40.
Not going to be a very precise regulator with that.

Your previous examples were at 32.767khz, which PicMultiCalc shows 0-244 duty range (better). But like mister_e said, you'll need to limit the range.

With a 20mhz crystal you can get 0-610 @32khz, or 0-100 @ 200khz.
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[jmgelba]

Guys thanks for the ideas and snippets. I have not been around today to work on this. I need to up the frequency to 300KHz now to reduce inductor size and value. I was going to use a 20mhz resonator but just wanted to get something up quick and dirty so I used the internal osc at 8mhz.

My schematic is a scribble on a piece of paper at the moment, but its essentially a classic buck converter design that could be found anywhere on the web. I think some of the problem is ripple voltge on the 5v line. This changes the internal vref of the adc.

I'll mill or get the boys in the shop to etch me a pcb in the morning so I can load up on the capacitance and cut down on the noise.

Oh, last night, I killed 12 Osram Dragon infra red led's that are not mine. Oops. I think they are something like $15 a piece

[Ioannis]

Oh, last night, I killed 12 Osram Dragon infra red led's that are not mine. Oops. I think they are something like $15 a piece

1 or 2 is OK. But 12..????

Next time put a Dummy load first.

Ioannis

[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.