A Spread-Spectrum Mode Enabled Ripple-Based Buck Converter Using a Clockless Frequency Control

Abstract

Current-mode ripple-based dc-dc converters have become a widely adopted topology due to their inherent advantages of improved dynamic transient response along with a simple current sensing scheme. However, the switching frequency of such converters is a function of the converter's duty cycle and inherent circuit delays. In this brief, a novel frequency control of a buck converter is proposed through the implementation of a frequency-to-voltage converter that obviates the need for an external clock, and enables both fixed-switching frequency (F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SW</sub> ) and spread-spectrum modes of operation. A current-mode ripple-based buck converter is implemented in a 180-nm CMOS process for an input voltage of 2.8-4.2 V, output voltage range of 1.3-2.2 V, and load current range of 40-200 mA. In fixed-F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SW</sub> mode, a peak efficiency of 89.4% was achieved at an input voltage of 3.8 V, load current of 200 mA and 1.8 V output voltage, while in spread-spectrum mode, a peak efficiency of 88.2% was achieved for similar conditions. Additionally, in spread-spectrum mode, a 19-dB reduction in peak noise power is observed in the output at the fundamental frequency.