Contents I. Introduction
In recent years, portable electronic products, such as mobile phones, notebooks, and audio players, have entered into human life. It is desirable that these devices have small size, light weight, and long battery life. DC–DC buck converters are widely used in portable electronic devices for their low-voltage power supplies and high-power conversion efficiency [1]. However, the periodic switching frequency of the dc–dc buck converter will produce switching noise, which leads to spectrum noise tones at the fundamental switching frequency and its harmonics. These spectrum spikes often degrade the performance of noise-sensitive circuit, such as analog/RF circuits [2]. The spread-spectrum technique is an effective method to reduce the switching noise or the electromagnetic interference (EMI). This technique can spread some of switching frequency noise energy to a wider frequency band so as to decrease its interference [3], [4]. Several spread-spectrum techniques, such as frequency hopping [5], [6], random switching frequencies [7], and – modulation [8], have been reported, which improve EMI in switching-mode power converters. The pulsewidth modulation (PWM) [5], [7], [9] control technique or the pulse-frequency modulation (PFM) [10] control technique employs frequency hopping to decrease the switching noise, which requires high hopping rate, because the high hopping rate is excellent for spur reduction [5], [6]. Low hopping rate is ineffective in reducing the switching noise; however, the high hopping rate is difficult to design due to duty-cycle errors. If the hopping clock and the switching frequencies were asynchronous while hopping between different frequencies, the hopping frequency will not be the same as the desired switching frequency and cause duty-cycle errors. The duty-cycle error will increase the output voltage ripple, which can be observed in the output spectrum.
