A Pseudo-Constant Frequency Constant On-Time Buck Converter with Internal Current Ripple Injection and Output DC Offset Cancellation

Abstract

A constant on-time buck converter with internal inductor current synthesizer and output DC offset cancellation technique is proposed. A fully integrated inductor current synthesizer based on valley voltage detector is employed to avoid system instability for constant on-time converters with low-ESR output capacitors. Moreover, the valley voltage detector could shift the emulated inductor current signal to zero valley level. In this way, the output DC offset due to ripple injection variation could be eliminated. An on-time circuit with comparator delay compensation is presented to alleviate the switching frequency variation. The small-signal model and design criteria are derived for system stability design. The circuit has been implemented with 0.18 $\mu\text{m}$ BCD process. The measurement results show that the transient response is about $15.4~\mu\text{s}$ and the overshoot or undershoot voltage is less than 36 mV when the load transient between 1 A and 5 A for input voltage of 12 V and output voltage of 1.05 V. The switching frequency across load variation $\Delta f_{SW}/\Delta I_{LOAD}$ is 4.6 kHz/A at the nominal frequency of 700 kHz. The output DC offset is less than 10 mV in case of 4 A load current change.