A fast fixed-frequency adaptive-on-time boost converter with light load efficiency enhancement and predictable noise spectrum

An integrated fixed-frequency adaptive-on-time (AOT) DC-DC converter with fast transient response and enhanced high light-load efficiency is presented. At both Continuous-Conduction-Mode (CCM) and Discontinuous-Conduction-Mode (DCM) operation, the on-time is adaptively controlled by a frequency locked loop so that the switching frequency tracks the reference frequency with less than $\pm $0.5% error in the whole operating range. At CCM operation, the reference frequency is fixed at 1 MHz. At light load with DCM operation, the reference frequency hops between 1 MHz and 1 MHz/N where ${\rm N} = 2 ^{\rm i} and ${\rm i} = $ 1 to 5 according to the loads without load current sensor. Frequency hopping is introduced to boost the efficiency as well as reduce the EMI noise problem. A glitch-free on-time control circuit is introduced to increase the track accuracy and speed of the frequency locked loop. A mode decision circuit with digital hysteretic window is used to ensure a smooth transition between CCM and DCM operation. An on-time generator with proper logics is used to suppress frequency bouncing during frequency hopping. A prototype with 1.8 V-3.2 V input voltage, 3.0 V-4.2 V output voltage and 0 mA-800 mA load current range has been fabricated in a 0.35 $\mu$m 2P4M CMOS process. Load transient measurements verify that 8 $\mu$s-20 $\mu$s recovery time is achieved for load current increase. Measurement results also show a maximum efficiency of 94.8% is achieved at total output power of 3.269 W, and the light load efficiency is enhanced by the proposed frequency hopping technique with a predictable noise spectrum.