A ramp generator for a constant on-time DC-DC converter, wherein the ramp generator is configured to reduce DC offset and smooth transitions between conduction modes. The ramp voltage generator includes a common voltage generator suitable for generating a common voltage; a first ramp voltage generation block suitable for generating a first ramp voltage responsive to a first switching signal and a control signal, wherein the first switching signal resets one or more valley points of the first ramp voltage to one or more valley points of the common voltage; and a second ramp voltage generation block suitable for generating a second ramp voltage responsive to a second switching signal, the first ramp voltage, and the control signal.