In sensor-node applications, the auxiliary power is drawn from the ac mains to supply the mixed-signal sensor components. There is therefore a need for low-cost, sub-1W, miniature non-isolated ac-dc down-converters. In this work, a new two-chip architecture is proposed to implement a high-voltage quasi-resonant buck converter with peak current-mode control. The current and voltage control loops are implemented in the high-side (HS) and low-side (LS) IC, respectively. Each IC includes a 700V power transistor, sensing and control circuits. The LS IC transmits the digital peak current command, along with mode selection information, to the HS IC using a low-power 20 MHz isolated communication interface. The architecture achieves a peak simulated efficiency of 85.6% when converting from 339 V to 12 V and allows the size of the input filter to be greatly reduced due to the current-mode operation. State-of-the- art UHV BCD technologies offer a unique opportunity to increase the level of integration and reduce the system cost in the targeted sensor node applications, despite the relatively poor figure-of-merit of the HV devices compared to discrete Silicon super-junction or GaN alternatives.