Mobile terminals such as smartphones, tablet computers and notebook computers have been prevalent. A mobile terminal includes a flat display device using liquid crystal or organic electro-luminescence (EL) elements. The display device is connected to a host device which outputs image data, commands, and the like. The display device includes a display panel and a driver which processes commands and drives the display panel.
In the display device, pixels two-dimensionally arranged on the display panel include a common electrode and a pixel electrode. The liquid crystal or organic EL elements are arranged between the common electrode and the pixel electrode. When the driver writes a pixel signal to the pixels on the display panel, the liquid crystal or organic EL elements arranged between the common electrode and the pixel electrode are controlled and an image is thereby displayed.
Display devices capable of detecting an inputting object such as a finger and a touch pen (also called a stylus) approaching or contacting the screen have been widely employed. The operation of allowing the inputting object to approach or contact the screen is called a touch operation or a touch, and the detection of a position of the inputting object is called touch detection. Examples of the touch detection include various types such as an optical type, a resistive type, a capacitive type, and an electromagnetic induction type. The capacitive type is the detection type utilizing a feature that the electrostatic capacitance between a pair of electrodes (called a drive electrode and a detection electrode) is varied by approach or contact of the inputting object, and has benefits that the structure is comparatively simple and that the power consumption is small.
The display device equipped with the touch detection function includes an on-cell type (also called an external type) in which the display device and the touch panel implementing the touch function are produced separately and the touch panel is bonded to the screen of the display device, and an in-cell type (also called a built-in type) in which the display device and the touch panel are integrated. In the in-cell type display device, for example, the detection electrode is formed between a color filter and a polarizer, and a common electrode formed on a thin film transistor (TFT) substrate is also used as a drive electrode. Since the in-cell type display device includes no external touch panel, the display device is entirely slim and lightweight, and visibility of the display is also improved. In the in-cell type display device, however, the display period and the touch detection period need to be set separately and achievement of both the display drive and the touch detection is a problem to be solved.
Recently, touch panel-equipped display devices have increased in size. For example, elongated display devices have been developed similarly to center information display (CID) units provided on dashboards of vehicles to function as display devices for displaying road guidance information, and the like in a car navigation system. The CID unit displays gauges such as a speedometer, a tachometer, a fuel gauge, a water temperature gauge, and a range finder, and the information similar to the gauges, in addition to the road guidance information.
An in-cell type display device includes a number of wiring layers, much parasitic capacitance and much parasitic resistance, a large time constant of CR, and the drive electrode can hardly be driven with a desired waveform. For accurate touch detection, the drive electrode needs to be driven with a desired waveform. For this reason, the touch detection period is required to be longer in the in-cell type display device. To maintain the display frame rate, however, the touch detection period cannot be freely made longer.
Thus, the conventional display device including the in-cell type touch detection function is influenced by the parasitic capacitance and parasitic resistance, and the drive electrode can hardly be driven appropriately.