Field of the Invention
The present invention relates to a touch sensing device and a driving method thereof, and more particularly to a touch sensing device in which touch sensors are embedded in a pixel array, and a driving method thereof.
Discussion of the Related Art
User interfaces (UIs) allow human beings (users) to communicate with various electric and electronic devices to easily control them as intended. Typical user interfaces include a keypad, a keyboard, a mouse, an on-screen display (OSD), a remote controller having an infrared communication or radio frequency (RF) communication function, and the like. UI technologies are advancing toward enhancement of user emotion and operational convenience. Recently, UIs have evolved to a touch UI, a voice recognition UI, a 3D UI, and the like.
The touch UI is often employed in portable information devices such as smartphones and is increasingly applied to notebook computers, computer monitors, home appliances, and the like. Recently, a technique of embedding touch sensors in a pixel array of a display panel (hereinafter, referred to as an “in-cell touch sensor”) has been proposed. According to the in-cell touch sensor technique, touch sensors may be installed in a display panel without increasing the thickness of the display panel. The touch sensors are connected to pixels through parasitic capacitances and signal lines (hereinafter, referred to as “pixel signal lines”). As for a driving method thereof, a period during which pixels are driven (hereinafter, referred to as a “display driving period”) and a period for driving touch sensors (hereinafter, referred to as a “touch sensor driving period”) are temporally divided in order to reduce a mutual influence due to coupling between the pixels and the touch sensors.
In the in-cell touch sensor technique, electrodes connected to pixels of a display panel are utilized as electrodes of the touch sensors. For example, the in-cell touch sensor technique may include a method of dividing a common electrode, for supplying a common voltage to pixels of a liquid crystal display, for use as an electrode of the touch sensors.
As an example of a capacitance-type touch sensor that can be implemented as an in-cell touch sensor, a mutual capacitance-type touch sensor (hereinafter, referred to as a “touch sensor”) has been known.
FIGS. 1 and 2 are a plan view illustrating an electrode pattern of a touch sensor and an equivalent circuit diagram of the touch sensor, respectively.
As shown in FIGS. 1 and 2, a mutual capacitance-type touch screen includes transmission (Tx) lines Tx1 to Tx4 and reception (Rx) lines Rx1 to Rx4 intersecting the Tx lines Tx1 to Tx4 with dielectric materials (or insulating layers) interposed therebetween. A mutual capacitance Cm is formed between the Tx lines Tx1 to Tx4 and the Rx lines Rx1 to Rx4. When a touch driving signal (or a stimulating signal) is supplied to the Tx lines Tx1 to Tx4, electric charges are charged in the mutual capacitance Cm. A sensing circuit senses a touch input based on the amount of change in electric charges of the mutual capacitance Cm before and after the touch.
In FIG. 2, R(Tx) is a resistor of a Tx line, R(Rx) is a resistor of an Rx line, C(Tx) is a parasitic capacitance of the Tx line, and C(Rx) is a parasitic capacitance of the Rx line.
When touch sensors are embedded in a pixel array, a large amount of parasitic capacitances affecting the touch sensors due to coupling between the touch sensors and pixel signal lines is added. The pixel signal lines are signal lines for writing data to pixels. In FIG. 3, pixel signal lines include a data line DL for supplying a data voltage to pixels and a gate line GL supplying a gate pulse (or a scan pulse) for selecting data-written pixels. In FIG. 3, Cfinger is a capacitance equivalently expressing a finger when the finger applies a touch. Clc is a capacitance equivalently expressing a liquid crystal cell. Cdg is a parasitic capacitance between the gate line GL and the data line DL, and Cgs is a parasitic capacitance between a gate and a source of a thin film transistor (TFT).
Parasitic capacitances connected to in-cell touch sensors include Ctd, Ctg, Ctc, Cgc, Cdc, and the like, as shown in FIG. 3. Ctd is a parasitic capacitance between the Tx line and the data line DL, Ctg is a parasitic capacitance between the Tx line and the gate line GL, Ctc is a parasitic capacitance between the Tx line and the Rx line, Cgc is a parasitic capacitance between the Rx line and the gate line GL, and Cdc is a parasitic capacitance between the Rx line and the data line DL. As a size of a touch screen employing in-cell touch sensors increases and resolution thereof increases, the touch sensitivity and touch recognition accuracy declines due to parasitic capacitances connected to the in-cell touch sensors. Thus, in order to apply the in-cell touch sensor technology to touch screens of a large-screen display device, the parasitic capacitance of touch sensors needs to be reduced or minimized.
FIG. 4 is a waveforms illustrating driving signals of a display device, such as that the one illustrated in FIG. 3.
As shown in FIG. 4, in order to drive a display device including in-cell touch sensors, a display driving period Td and a touch sensor driving period Tt are temporally divided. During the display driving period Td, a data voltage Vdata and a gate pulse GP are generated to write data to pixels. The gate pulse GP is swung between a gate high voltage VGH and a gate low voltage VGL. During the display driving period Td, Tx lines and Rx lines of touch sensors serve as a common electrode supplying a common voltage Vcom to the pixels. During the touch sensor driving period Tt, a voltage of the data lines DL is held at the final data voltage of the previous display driving period Td and a voltage of the gate lines GL is held at a gate low voltage VGL. During the touch sensor driving period Td, a touch driving signal Tdrv is supplied to the Tx lines. During the touch sensor driving period Td, the sensing circuit is in synchronization with the touch driving signal Tdrv and senses a variation in electric charges of the touch sensors through the Rx lines.