In this paper, we detailed the system electrothermal transient co-design modeling and silicon validation effort that led to the industry’s first highly efficient, highest power density (40A) synchronous step-down converter. The device was designed in an innovative multichip module (MCM) low-profile LQFN packaging technology. By integrating the control and drive circuitry with two discrete N-channel power MOSFETs, significant reduction in system on-resistance, RDS(ON), was achieved. The validity of the co-design electrothermal modeling methodology was assessed by comparing directly to silicon thermal measurements made on an evaluation module (EVM) which comprises the inductor, capacitor, and other essential components. Correlation between simulation and laboratory measurements on the integrated solution will be discussed.