Unless you know what you are doing, the 5116 may not provide enough headroom for you. Though it is rated at 100V, if you are starting at 85V, you will need to be careful to ensure transients are not high enough voltage to damage the 5116. Switching transients over 100V would be easy. This is not necessarily a novice design.
Semiman
The design spec was rather loose, so I just threw something out that could potentially be useful. For a solar panel, I'd be a bit skeptical that 84vdc was the absolute max output (full sun, cold temps, no load, etc.). Without knowing the full spec, it's not possible to produce a real solution. I was just trying to prompt some thought about what the spec really was... but the LM5116 is still an interesting IC for a switcher. I don't have a lot of spare time to play with projects, but I'd like to see someone else do it.
edit: as a secondary consideration, do most solar power systems design for EN61000, which is the EMC spec for stationary applications? I'm mostly thinking about the burst and surge specs which include some rather high transient voltages (1000 volts or so). I'd hate for a nearby lightning strike to take out the system.
