A quasi-constant frequency AOT applied to buck converter CCM mode

Abstract

An adaptive constant on-time (AOT) module suitable for on-chip integrated BUCK converters is proposed to address the issue of significant switching frequency variations with load current in continuous conduction mode. This involves sampling the voltage drop across the parasitic resistance of the inductor via a resistor-capacitor network, and superimposing it on the output voltage and the voltage of the switching node during the conduction of the rectifying MOSFET via a sampling capacitor. The conduction resistance and voltage drops of the switching and rectifying transistors are also considered to reduce the correlation between switching frequency and load current. Additionally, a method of subtracting threshold voltages is used to achieve voltage-to-current conversion, obtaining charging currents for the capacitor. A low-power comparator is incorporated to ensure a low quiescent current under light-load conditions. The proposed AOT structure is implemented using 0.18\(\mu\)m BCD technology. Simulation results show that the input voltage of the BUCK converter with AOT control ranges from 2.5V to 5.5V, output voltage from 1.5V to 2.0V, with a maximum output current of 3A. Under continuous conduction mode, the load current variation of 2.5A results in a switching frequency variation of 44.9 kHz, with a rate of change of 17.96kHz/A and a variation range of 2\(\%\).