A Fast-Response RBAOT Controlled Buck Converter with Pseudo-Fixed Switching Frequency and Enhanced Output Accuracy

A ripple-based constant on-time (RBCOT) control scheme is excellent in achieving fast transient response during the load current step. However, it suffers from severe electromagnetic interference (EMI) noise and inherent output voltage offset problem. A monolithic ripple-based adaptive on-time (RBAOT)-controlled buck converter is presented to overcome both drawbacks while retaining the advantage of RBCOT. The on-time is adjusted adaptively by detecting the switching node voltage, and thereby ensuring a pseudofixed switching frequency in a steady state, regardless of the input, output voltage, and load current conditions. A correction voltage generated by a coefficient in a virtual inductor current (VIC) circuit is introduced to counteract the offset voltage and enhance the output accuracy. The proposed scheme is simple to implement and suitable for high-conversion ratio applications. Experimental results show that the switching frequency is centered at 1 MHz with less than 6% variation for an output of 5 V and inputs ranging from 8 to 18 V. The output offset voltage is reduced from 16 to 1 mV for an 18 to 1.05-V conversion. For a load current step of 1 A, the output can be settled within $6~\mu \text{s}$ and the undershoot and overshoot voltages are controlled to be within 20 mV.