What is a DC DC controller?
A switching regulator "controller" implements the logic portion of the overall converter inside the IC but does the heavy current switching using one or more external high current and/or high voltage semiconductor switches.

This allows the power able to be handled to be adjusted to suit the application by using more or less capable switches as required, and often achieves overall performance levels which are not practically achieved within an IC of a given size. The overall result is that the task is partitioned into sections which are best handled by components which are usually able to be price and size optimised compared with using a single IC solution.

The LM5085 that you cite Datasheet here - has a 4.5V to 75V input voltage range. Providing a power IC at this voltage rating is usually expensive. 

A typical application circuit (from the data sheet) is shown below. This is rated at about 60 Watts (5A at 12V out, 5B shown on diagram) but by changing Q1 and L1 any sensibly desired power level can be achieved. The ICs cost just under $1 in 1000 quantity at Digikey