After reading datasheets and application notes I was still no closer to setting up this chip. I then decided to use three 1K trimpots as voltage dividers across the power rails and pin 4 , 6 and 7 to set the lower , upper and LED brightness. And IT WORKED beautifully, I can adjust the voltages by putting a MM across the relevant pin and adjust the pot until I get the voltage I need. I've even put an LDR across the pin 7 pot and now have an autodimming feature. This seems too easy, am I doing something that will come back to bite me or have I just got lucky doing it this way?

 

It depends, if this is strictly for personal edification you'll be absolutely fine. OTOH if this is the first of 10,000 units to be made just like it, it'll bite big time

Punch Imlach, the great hockey coach once said, "If I had a choice between luck and skill, I'd pick luck every time."