How to choose the resistors for UC3906
Look at the spec for the BQ24450. It's the exact same IC, same die - just changed the name for some reason - much better explanations. Also, if you have a problem initially getting the IC into the right state when first powering on (i.e bulk charge ), try putting a 10uf cap or so across the sense resistor that goes to pin 13 (the bottom resistor in the output voltage divider). Or you can put a reset switch across that same resistor - just press the switch after powering on to start the the bulk charge mode. Took me a while to figure that out - the circuit was going directly into the float state - skipping over bulk charge and over voltage. Hope this helps someone!