An Auto-Selectable-Frequency Pulse-Width Modulator for Buck Converters with Improved Light-Load Efficiency

In order to be useful for mobile systems, the switching frequencies of modern buck converters (BCs) are in the MHz- or even GHzrange to enable the use of compact off-chip inductors and capacitors [1], or even on-chip Ls and Cs [2]. However, such high switching frequencies increase both the switching loss and the gate-drive loss, and degrade BC light-load efficiency since these losses are independent of load current. Pulse-frequency modulation (PFM) is known to improve light-load efficiency by scaling the switching frequency with the load current so that the switching loss and the gate-drive loss are reduced with decreased load current. However, PFM makes the BC output spectrum change with the load current. This unpredictable spectrum causes supplyintegrity to be an issue in mobile systems where different spectrum-sensitive circuits, such as communication ICs, are often used. Recently, the light-load efficiency of BCs operating at a constant switching frequency in MHz range have been improved [3, 4]. However, the methods used only reduce the gate-drive loss, which is the power required to switch the power PMOS and NMOS transistors (MP and MN in Fig. 24.4.1), but not the switching loss, which is the power needed to switch the switching node (VX in Fig. 24.4.1). BC light-load efficiency can only be greatly improved when these two losses are simultaneously reduced.