A good explanation of the difference between a CLC and an LC filter follows. Critical inductance applies to an LC filter. But a CLC filter in which the input C is small starts to move towards an LC situation. Oddly the LC filter, if properly designed, gives better regulation
There is a derivation of the formula Lc > Rc/(6pif) and a good general description here
http://www.r-type.org/articles/art-144.htm