Trouble using buck-boost driver for constant-current LED source!

[SittingBear]

Hello EEVBlog community,
I'm having a lot of difficulty in properly getting a constant-current buck topology circuit going for 2 white LEDs with a forward voltage of ~3.5V.

The buck circuit I am using is pretty simple and is found in page 1 of the datasheet for the buck-boost driver I am using found here: http://www.onsemi.com/pub_link/Collateral/NCP3066-D.PDF

The datasheet is not very descriptive in my opinion, and I have looked at other datasheets of near identical drivers. I was able to properly figure out and work a constant voltage setup, but that isn't what I want. Before you ask and or find it, yes there is a design tool for both buck and boost topologies. However, after extensively playing with the values from this design tool I have come to the conclusion it is not at all good at predicting values for an accurate or clean outcome. I believe the formulas make a lot of incorrect assumptions in what actually happens.

Any help on this would be great! I also have a copy and have started to read, along with skimming through more relevant chapters, the "Fundamentals of Power Electronics - Second Edition" book which goes into extreme detail about buck, boost, and other regulating circuits.


Thanks!
-Bear