An output buffer and a controlling method are disclosed. The output buffer comprises an upper buffer and a lower buffer. In the controlling method, at first, a first voltage (V1) and a second voltage (V2) are applied on the upper buffer, and a third voltage (V3) and a fourth voltage (V4) are applied on the lower buffer, wherein V1>V2, V1>V4, V3>V2, and V3>V4. Then, the upper buffer is operated to output data to a plurality of pixels thereby operating the liquid crystals of the pixels over an upper supply range, wherein the upper supply range is from V1 to V2. Thereafter, the lower buffer is operated to output data to the pixels thereby operating the liquid crystals of the pixels over a lower supply range, wherein the lower supply range is from V3 to V4.