A controller includes a first timer for setting a turn-on time of a first high-side switch of a buck-boost converter, wherein the turn-on time of the first high-side switch is determined by an input voltage of the buck-boost converter, an output voltage of the buck-boost converter and a first predetermined bias voltage, a second timer for setting a turn-on time of a second low-side switch of the buck-boost converter, wherein the turn-on time of the second low-side switch is determined by the input voltage of the buck-boost converter, the output voltage of the buck-boost converter and a second predetermined bias voltage, and a valley current mode control device for setting a turn-on time of a first low-side switch and a turn-on time of a second high-side switch of the buck-boost converter.