The invention proposes a system and method for extending the maximum duty cycle of a step-down switching converter to nearly 100% while maintaining a constant switching frequency. The system includes a voltage mode or current mode step-down converter driven by a leading edge blanking (LEB) signal, which operates at the desired switching frequency. More particularly, the LEB signal is connected to a slope generator and/or a current sense network. In each switching cycle, the LEB signal forces the slope signal and/or current sense signal to reset, thereby achieving a constant switching frequency. Corresponding methods for how to extend the maximum duty cycle of a step-down switching converter while maintaining a constant frequency are also disclosed.