This article describes the design of an observer based robust linear output feedback controller for the regulation and output reference trajectory tracking tasks in switched 'buck' converter circuits feeding a completely unknown time-varying load. The state-dependent perturbation effects of the unknown load resistance are on-line estimated by means of a generalised proportional integral (GPI) observer, which represents the dual counterpart of GPI controllers introduced in Fliess, Marquez, Delaleau and Sira-Ramirez (Fliess, M., Marquez, R., Delaleau, E., and Sira-Ramirez, H. (2002), 'Correcteurs Proportionnels-integraux Generalises', ESAIM: Control, Optimisation and Calculus of Variations, 7, 23-41). The reconstructed perturbation complements the controller in a cancellation effort which allows the core of the feedback controller to become a traditional proportional derivative (PD) controller. The designed average feedback controller is then implemented via a sigma-delta-modulator, which effectively translates the designed continuous average feedback control input signal into a discrete valued switched input signal driving the converter's input switch and preserving all relevant features of the average design. The Appendix collects some generalities about GPI observers.