LTC3119 is a synchronous current mode monolithic buck-boost converter that outputs up to 5A of continuous output current in buck mode from a wide variety of input sources, including single- or multiple-cell batteries, unregulated wall adapters as well as solar panels and supercapacitors. By Graham Prophet @ edn-europe.com:
The device’s 2.5V to 18V input voltage range extends down to 250 mV once started. The output voltage is regulated with inputs above, below or equal to the output and is programmable from 0.8V to 18V. User-selectable Burst Mode operation lowers quiescent current to 35 µA, improving light load efficiency and extending battery run time.
18V, 5A buck-boost DC/DC delivers 95% efficiency – [Link]