A Spurious-Free Switching Buck Converter for Portable Applications

DC–DC buck converters are essential to extending the standby time and maximizing the battery life of portable electronic devices. In a portable environment, load conditions vary drastically from heavy-load to light-load levels, depending on whether the device is in active or standby mode. These devices run in standby mode most of the time. The output spectrum of DC–DC buck converters in such an environment must also be spurious-free (i.e. of low tonal content) for them to be able to supply sensitive circuits, such as ICs in mobile communication systems.;Among the various buck-converter architectures, switching converters offer the highest power conversion efficiency over a wide range of conversion ratios. Increasing the switching frequency of the buck converter enables the use of compact off-chip components and enhances the converter's integrability. However, operating at high switching frequencies increases switching losses and reduces the power conversion efficiency, especially at light loads. The ultimate goal in designing a switching buck converter is to resolve the conflict between operation at a high switching frequency and efficient power conversion, while maintaining a regulated output-voltage with low tonal content.;This thesis proposes a spurious-free switching buck converter with enhanced light-load efficiency for use in noise-sensitive portable electronic devices. The proposed converter achieves low output noise by using a delta-sigma-modulator (ΔΣ) controller. Its light-load efficiency is enhanced by: 1) scaling the switching frequency of the buck converter (i.e. the sampling frequency of its ΔΣ-modulator controller) according to the load current; 2) switching its operation from continuous conduction mode (CCM) to discontinuous conduction mode (DCM) at light loads; and 3) using a new low-power current-sensing circuit. The ΔΣ modulator is designed with an input-feedforward architecture, which enables the switching frequency of its controller to be scaled without disturbing the stability of the feedback loop of the buck converter, and also reduces the controller quiescent current.;The proposed switching buck converter was fabricated in a 0.13-µm digital CMOS process with a maximum switching frequency of 10 MHz, enabling the use of compact off-chip components. The results measured demonstrate that this buck converter achieves a spurious-free output with a noise floor below - 60 dBm and voltage ripples below 70 mV across its full loading range (2 mA to 800 mA). Furthermore, it achieves a power efficiency of greater than 70% over this entire range, with a peak efficiency of 95.12%.