Hybrid Buck Converter With Constant Mode Changing Point and Smooth Mode Transition for High-Frequency Applications

To achieve high frequency and high efficiency over a wide load range, a monolithic voltage-mode dc-dc buck converter with advanced burst mode (ABM) and pulsewidth modulation (PWM) is presented in this paper. The load current is detected by estimating the currents flowing through the high-side and low-side switches, which maintains a near constant mode changing point. A counter-based scheme is used to achieve seamless and smooth transition between PWM and ABM. Both operating modes share the same control blocks, and no additional zero current detecting circuit is needed. In addition, the output stage switch size can be adjusted to reduce switching loss and improve efficiency under ultra-light load conditions. Experimental results show that the integrated 3 MHz converter achieves high efficiency from 80 to 91%, over a wide load range from 0.001 to 5 A. When the switch size is set to 1/3 of the maximum value, an additional 5% efficiency improvement at under 0.1 A load can be realized. The mode changing point is set at 0.4 A with less than 10% variation when the input voltage changes from 3 to 5 V. The load transient response is also improved significantly.