Ultimate integration of a power converter either means embedding all components in silicon or stacking them closely in a 3D arrangement. High switching frequency is a straightforward approach for reducing the values of passive devices. Silicon integration is the natural approach for switched-capacitor converters but still fails for inductive converters because of the magnetic devices, especially when a magnetic material is involved, what is the case for high power density. These latter components may be fabricated in an integrated approach but the fabrication process is not compatible in essence and cost with the silicon process. Heterogeneous integration is then a cost-effective solution. The article details an example of such an heterogeneous fabrication of a step-down converter. A suitable architecture is first selected for high switching frequency and high efficiency operation. Using an advanced silicon node complicates the architecture if the input voltage is much higher than the technology nominal voltage. Experimental measurements are presented that corroborate the simulation. 91 % peak efficiency is demonstrated for a 100 MHz switching frequency buck converter at 3.3 V input voltage, 1.2 V to 2.4 V output voltage and 0–300 mA load current.