In view of the Large Hadrons Collider experiments upgrade, where the inner front-end electronics will require higher supply power, a new power distribution scheme has to be designed. This thesis presents a new and more efficient scheme based on step-down DC-DC converters, which will be placed close to front-end electronics where it will be exposed to intense radiation (250 Mrad) and magnetic field (4Tesla). Other than tolerance to these fields, additional constrains are small volume and mass and low noise emission not to affect the sensitive front-end electronics. Since no commercial component can meet these requirements, an Application Specific Integrated Circuit (ASIC) DC/DC converter has been developed and is presented in this work. The chosen converter topology is the buck because it presents the best compromise in terms of efficiency and low mass among the different compared topologies. Radiation hardness can be achieved with appropriate choice of the semiconductor technology combined with layout modifications. Magnetic field tolerance is attained with the use of an air core inductor whose shape can be toroidal for limiting the magnetic field emissions. All the ASIC building blocks are presented in this thesis likewise their design methodology: their schemes and the transistors sizing are discussed as well as the AC analysis to grant the stability of the output voltage over the selected range of frequencies. Three different prototypes had been integrated in the two chosen semiconductor technologies (mainstream and backup). Tests showing high efficiency (up to 85%) and the required level of radiation hardness are presented and discussed.