A dual-phase peak current-mode control buck converter chip with large load capability was designed. By operating in dual-phase, the chip was guaranteed to hold high efficiency under heavy load. Considering the extra switching loss from two phases during light load, an optimized light load mode was proposed. It used the control voltage generated by the error amplifier in the voltage loop of the current-mode control loop as the load position detector. When the load was stepped down, the active phase could be changed from two to one. As the load turned into ultra-light, the system could also work in discontinuous conducting mode to further improve the efficiency. It was designed in a 0.35 μm BCD process. Simulation results show that under the conditions of 12 V input voltage, 1 V output voltage, 500 kHz switching frequency and 20 A maximum load, compared with the traditional single-phase peak current-mode, the maximum efficiency can be improved by about 3 percentage point under a heavy load of 20 A and can be improved by about 10 percentage point under a light load of 0.5 A.