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A fully-integrated step-down DC-DC converter with LDO mode to reduce the sleep power consumption and improve efficiency at light loads is presented. The converter can switch adaptively between the buck converter and the LDO mode by continuously sensing the load current. Besides, lossless current sense technique is implemented for the mode selection circuits. An improved LDO is also proposed to achieve good stability under various loads. The chip was implemented in 0.13 μm CMOS technology. The measured peak efficiency reaches 80% at 90 mA load, and efficiency at 10 mA load increases by 22.6% compared with forced buck converter mode. Furthermore, the overall power consumption is reduced by 12X at the sleep mode, from 10 mW without the hybrid mode to 0.84 mW.