This paper presents a SIDO buck-buck converter for separate positive and negative voltage. To provide the gate driving voltage for the dual-mode low voltage output buffer, 0.5 V and 0.3 V are obtained for the first output, -0.4 V and -0.6 V are generated for the second output. With the new structure of the proposed buck-buck converter, only 7 major devices are utilized including 1 inductor, 2 capacitors and 4 switches. The proposed design is implemented with a typical 40 nm CMOS process. The core area is 484.72 × 556.875 μm2. The peak efficiency is 91.282% for the dual output at 0.5 V and -0.4 V, respectively. The cross regulation of the first output is 0.026 mV/mA and 0.022 mV/mA for the modes of 0.5 V and 0.3 V, respectively.