Current sensor is an absolutely needed component for over-current-protection, and for loop stability control in the current mode control buck system, in which, the protection ability and the stability of the system depend on the accuracy of the current sensor. Among several lossless current sensing methods, the MOSFET-based current sensing and inductor-based current sensing are standout as simple and convenient current sensing methods, especially for on-chip current sensor design. In this paper, several up-to-date reported designs are reviewed and discussed, especially about their speed, accuracy and power consumption. In addition, for the MOSFET-based current sensing method, “A 5 MHz Power Saving Current Sensor with Adaptive Bias and Forced Slew Rate Providing Sensibility For Duty Range from 20% to 80%” is proposed and verified by simulation. For the inductor-based current sensing method, a detailed analysis of the recent current sensor state of the art in  is performed to reduce its unnecessary structure. Finally, design considerations for correctness and transient enhancing are discussed.