http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en543956
or the ready made breakout board:
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en544320
I use these with a 9 volt switch mode regulator (http://au.element14.com/xp-power/sr05s09/switching-regulator-9v-0-5a-o-p/dp/1861085) between the solar panel and this unit as the input voltage cannot go above 13 volts. A 12 volt solar panel can go to ~21 volts. You can use a linear regulator but you lose too much power and generate a fair bit of heat.