Fully Integrated Buck Converter With Fourth-Order Low-Pass Filter

Fully integrated buck converters are typically operated with a second-order LC low-pass filter and a switching frequency beyond 100 MHz. The motivation for such design choices is to reduce the size of passive components in the LC low-pass filter required for small output voltage ripple. However, in a buck converter with on-chip planar spiral inductors, a fourth-order filter can deliver better performance characteristics without area penalty. This paper presents a comparative study of a fourth-order LC low-pass filter versus a second-order LC low-pass filter with on-chip planar spiral inductors and on-chip capacitors. A fully integrated buck converter is then designed with a quasi-V-2 controller to demonstrate the benefits of a fourth-order LC low-pass filter. The prototype chip, which is implemented in a 65-nm CMOS process, produces a nominal voltage of 0.7 V from a 1-V supply. The fourth-order LC low-pass filter uses a total inductance of 1.8 nH and a total capacitance of 4 nF. Measurement results demonstrate fast transient response on the order of nanoseconds. A peak efficiency of 76.1% is achieved, and the output voltage ripple is kept below 15 mV over the entire range of load current from 40 to 180 mA.