This work presents a novel architecture for extraction and on-line tracking of maximum efficiency point for generic integrated DC-DC converters; basically, this optimal condition can be obtained by adapting the size of the power switches for the desired operation, in order to find an optimal value for which the total losses are minimal [1]. The proposed technique allows to monitor constantly the operating conditions and track automatically and instantly the optimal number of active switches which minimize the total losses for every load current. In order to validate the theoretical results, proposed method is implemented inside a 6A DC-DC fully-integrated buck converter realized in 180nm technology with 5V input voltage rating and switching frequency up to 4MHz.