Monolithic Low-EMI CMOS DC–DC Boost Converter for Portable Applications

This brief presents the design of a novel low-electromagnetic interference DC-DC step-up (boost) switching converter for portable applications. The converter can switch between pulsewidth modulation and pulse-frequency modulation modes for different load conditions, and uses a new ultralow-current spread spectrum frequency modulator to reduce the harmonic noise peak. The stability of the converter system with a spread spectrum frequency modulator is first analyzed in this brief. The harmonic peak reduction for the switching frequency and the second harmonic is 14 and 18 dB, respectively. A new two-stage soft-start circuit is also implemented to greatly reduce the start-up current. The start-up current of the boost converter is effectively limited below 500 mA. This brief also investigates the effects of the spread spectrum on conversion efficiency and output ripple voltage. About 93% maximum conversion efficiency can be reached for both operation modes. The chip was fabricated using a TSMC 2P4M 0.35- μm CMOS process.