A 2.5 MHz, 6 A ripple-based adaptive on-time controlled buck converter with pseudo triangular ramp compensation

Abstract

This paper introduces a ripple-based adaptive on-time controlled buck converter with pseudo triangular ramp compensation. Ripple-based control has its advantages over traditional PWM control of fast response and simple structure without frequency compensation network. Constant on-time (COT) control is one of the conventional ripple-based control methods, which needs large output capacitance with large ESR to guarantee the system stability, yet leading to large output ripple. In order to strengthen system stability with small output ripple and fast transient response, a novel pseudo triangular ramp compensation is proposed. Also COT control has its drawback of switching frequency variation. Therefore an adaptive on-time generator is adopted to fix switching frequency, in which VO/N2RS and Vin/N2RS are used as current sources to eliminate the effect of input/output voltage. A voltage outer loop is also adopted to improve DC precision. The power MOSFETs are integrated inside the chip. The proposed converter was implemented in 0.18 μm 5 V CMOS process. Experimental results show that the output ripple is around 20 mV. The switching frequency can be fixed at 2.5 MHz with only 1% variation. Also with 330 nH off-chip power inductor, this converter can output up to 6 A load current. Measured peak efficiency is 85%.