Linear robust controller design based on Uncertainty and Disturbance Estimator theory for nonlinear uncertain single-input single-output systems with external disturbances is discussed in the paper with application to the output voltage control of a unidirectional universal multimode buck/boost/buck-boost converter. The converter is characterized by different nonlinear static and dynamic behaviour in each of the operating modes. The proposed controller forces the system to maintain nearly nominal performance through the whole range of operation space by appropriately estimating and cancelling the terms including nonlinearities, parametric uncertainties and external disturbances. In addition, measurement noise is included in the analysis to demonstrate the trade-offs of the proposed method. Despite the non-triviality and relative complexity of the method, the resulting controller structure is simple, allowing low-cost, low-part-count analogue implementation. Extended simulation results are presented to demonstrate the effectiveness of the proposed control approach.