An Accurate, Low-Voltage, CMOS Switching Power Supply With Adaptive On-Time Pulse-Frequency Modulation (PFM) Control

Integrated switching power supplies with multimode control are gaining popularity in state-of-the-art portable applications like cellular phones, personal digital assistants (PDAs), etc., because of their ability to adapt to various loading conditions and therefore achieve high efficiency over a wide load-current range, which is critical for extended battery life. Constant-frequency, pulsewidth modulated (PWM) switching converters, for instance, have poor light-load efficiencies because of higher switching losses while pulse-frequency modulation (PFM) control in discontinuous-conduction mode (DCM) is more efficient at light loads because the switching frequency and associated switching losses are scaled down with load current. This paper presents the design and integrated circuit prototype results of an 83% power efficient 0.5-V 50-mA CMOS PFM buck (step-down) dc-dc converter with a novel adaptive on-time scheme that generates a 27-mV output ripple voltage from a 1.4- to 4.2-V input supply (battery-compatible range). The output ripple voltage variation and steady-state accuracy of the proposed supply was 5 mV (22-27 mV) and 0.6% whereas its constant on-time counterpart was 45 mV (10-55 mV) and 3.6%, respectively. The proposed control scheme provides an accurate power supply while achieving 2%-10% higher power efficiency than conventional fixed on-time schemes with little circuit complexity added, which is critical during light-loading conditions, where quiescent current plays a pivotal role in determining efficiency and battery-life performance

I. Introduction

The widespread demand of handheld portable devices with voice, data, imaging, and multimedia all rolled into one requires increasingly efficient power-saving solutions powered from widely variable battery voltages. State-of-the-art power management circuits are therefore used to generate these fixed and dynamic voltage rails in a system from battery supplies (e.g., NiCd and NiMH: 0.9–1.8 V, Li-ion: 2.7–4.2 V) [1]. What is more, fuel-cell technologies, because of their superior energy density [2], are also vigorously pursued in research for military, space, and possibly consumer electronics. Direct methanol fuel cell (DMFC) systems, for example, exhibit a 0.2- to 0.7-V terminal voltage variation [3]. All this implies that both currently available and future portable power supplies have a wide variation in input supply voltages, which the power management system must comprehend in its generation of a constant and/or dynamically adaptive output voltage.