Solid-state LED lighting, as the fourth-generation lighting technology, has drawn great popularity due to its long lifetime, high luminous efficacy, high energy efficiency and sustainability. Technical advancement and rapid cost-down in LED technology bring entirely new possibilities in lighting applications and stimulate a significant transition from conventional lighting in the global lighting market. Considering huge amount of conventional lighting fixtures in current use, a complete modification for LED lighting at system level can be costly, complex and time-consuming. Efforts are therefore made to develop retrofit LED lamps that can directly fit into the existing lighting systems with electrical compatibility. However, a typical multi-lamp luminaire for 2ft linear fluorescent lamps usually contains one or several branches where two lamps are connected in series to a common electro-magnetic ballast of that branch. The unique wiring scheme forms an input-series system and inevitably causes stability problems with conventional LED drivers, which are usually based on closed-loop switched-mode power supplies. Besides, operation conditions are flexible for 2ft retrofit LED tubes: they are randomly installed in either the two-lamp or one-lamp branch in the luminaire and can be connected with or without electro-magnetic ballast, which accordingly addresses additional design requirements. This thesis work aims at developing an electro-magnetic ballast compatible LED driver for 2ft LED retrofit tube applications. A modulated peak current controlled self-oscillating buck-boost converter is proposed to achieve a positive dynamic input resistance, thereby realizing stable series operation. The validation can be generalized to other input-series systems. On the basis of proposed approach, a complete designing process of the LED driver, which is electrically compatible with the existing fluorescent lighting fixture, is presented. Key points of designing to fulfill all application requirements (include automatic mode selection) are discussed in detail. Two prototypes provide experimental results to verify the effectiveness of the proposed design.