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The fixed-ramp ripple-based constant on-time (RBCOT) control scheme for buck converters has recently been adopted in many dc–dc conversion applications for its simplicity and high light-load efficiency. However, it cannot overcome the problems with feedback instability and excessive load transient response when the converter is required to cover wide working conditions of input voltage, output voltage, and switching frequency in the application of power management integrated circuit (PMIC). In this paper, an adaptive-ramp RBCOT scheme is proposed to solve such a problem. The control law is derived from theoretical analysis of small-signal models. A simple analog circuitry, which can be easily integrated into an integrated circuit, is proposed to achieve invariant Q factor (or Q value, quality factor) under different working conditions. Simulations and experiments were conducted to verify the proposed concept.