This paper presents a high efficiency, fixed- frequency waveform Buck-Boost converter which converts a 2.7V to 4.5V Li-ion battery voltage to the regulated voltage featured high efficiency and low electromagnetic interference (EMI) for brain computer interface applications. A smoothly switching peak-valley current control mode, enabling this converter to switch freely from valley-current-mode control in Buck mode to peak-current-mode control in Boost mode, is adopted to avoid sub-harmonic oscillations. And a fixed- frequency control technique based on cycle alternation in Buck-Boost mode, which fixes the frequency of the inductor current as opposed to random-mode-control, is adopted to obtain high efficiency and low EMI. The proposed Buck-Boost converter is implemented in 180nm BCD process. It can achieve and keep the efficiency of above 92.33%, of which the peak efficiency is 98.97%, over the load current ranging from 100mA to 400mA and the input voltage ranging from 2.7V to 4.5V.