There is a trend toward higher bus voltages in various applications, such as electric vehicles, industrial automation, and cloud computing. In contrast, low-power applications, such as actuators, microcontrollers, and sensors, still require low supply voltages. To bridge this voltage gap, a high-voltage DC-DC converter is required to perform a large conversion step. Furthermore, complete integration can significantly reduce the cost and enables compact solutions. This chapter delves into the challenges of handling large input voltages in a single-chip solution. To tackle these challenges, topology and implementation techniques are presented. These principles are applied to a case study of a fully integrated 400 V-to-12 V DC-DC converter in a high-voltage CMOS technology, advancing the input voltage of fully integrated step-down conversion by \(9{\times }\) and the power density by \(270{\times }\) .