The relationship between flicker and a common voltage (Vcom), which directly affects the image sticking problem of liquid crystal displays, is explained. A simple symmetry of flicker with respect to Vcom is introduced. A new circuit utilizing a digital interface to generate Vcom is proposed. The circuit easily controls Vcom through the digital interface, enabling automation of the Vcom optimization. The development of an automatic Vcom-tuning system plus the Vcom optimization algorithms using the symmetry of flicker is presented. The total tuning time was 0.35 seconds regardless of screen size.