Liquid crystal display device including common electrode control circuit

A charge pull-out period is set after power off, and an all-on control signal is set to a high level in the charge pull-out period. A high-level voltage which is output to a scanning line drive circuit formed on a liquid crystal panel is kept at a high level even after power off until a middle of the charge pull-out period. A common electrode control circuit for connecting a common electrode to a ground when the all-on control signal is at the high level and the high-level voltage is at an operation level is configured using two resistors, a NOT circuit, and two FETs, and is provided to a liquid crystal display device. With this, a liquid crystal display device which can lower a common electrode voltage to a ground level when power off and can prevent burn-in after power on is provided.

TECHNICAL FIELD

The present invention relates to a display device, especially to a liquid crystal display device and a drive method therefor.

BACKGROUND ART

Liquid crystal display devices are widely used as thin, light-weight, and low power consumption display devices. The liquid crystal display device includes a liquid crystal panel, a scanning line drive circuit, a data line drive circuit, and the like. On the liquid crystal panel, pixel circuits each including a thin film transistor (hereinafter referred to as a TFT) are formed two-dimensionally. The scanning line drive circuit is also called a gate driver.

In many recent liquid crystal display devices, in order to make the device small, a technology (gate driver monolithic technology) in which the scanning line drive circuit is formed on the liquid crystal panel using the TFTs is adopted. The scanning line drive circuit formed on the liquid crystal panel is also called a monolithic gate driver. The liquid crystal panel on which the scanning line drive circuit is formed is also called a gate driver monolithic liquid crystal panel.

Furthermore, in the liquid crystal display devices, in order to reduce power consumption, a technology in which the TFTs included in the liquid crystal panel are formed using oxide semiconductor may be adopted. For example, a technology of forming the TFTs using indium gallium zinc oxide (hereinafter referred to as IGZO) which is one type of oxide semiconductor comes into practical use. Off leakage current of the TFT (hereinafter referred to as an IGZO-TFT) formed using IGZO is much smaller than that of the TFT formed using other materials. Therefore, according to the liquid crystal display device using the IGZO-TFTs, it is possible to reduce off leakage current of the TFT and to reduce power consumption drastically.

As conventional art, Patent Documents 1 and 2 disclose liquid crystal display devices each having a monolithic gate driver formed using the IGZO-TFTs. The liquid crystal display devices disclosed in Patent Documents 1 and 2 perform a predetermined power-off sequence in order to pull out charge remaining in the liquid crystal panel when power off.

PRIOR ART DOCUMENTS

Patent Documents

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

In the liquid crystal display device, a charge pull-out period for pulling out the charge remaining in the liquid crystal panel is set after power off. In the charge pull-out period, all of the scanning lines in the liquid crystal panel are selected collectively. In the charge pull-out period, it is preferable that a common electrode voltage of the liquid crystal panel be lowered to a ground level (0V). This is because if the common electrode voltage is not equal to the ground level after power off, the charge remain in the liquid crystal panel (specifically, on a pixel electrode and a common electrode), and burn-in occurs in the liquid crystal panel when power on again.

FIGS. 4 and 5are signal waveform diagrams of conventional liquid crystal display devices when power off. InFIG. 4, a power supply voltage VCC changes from 3.3 V to the ground level at time t0. In a charge pull-out period time which is set at time t1to t2, an all-on control signal ALLON for selecting all of the scanning lines becomes a high level. From a start of the charge pull-out period, a common electrode voltage Vcom starts lowering. However, in the conventional liquid crystal display device, it is impossible to secure enough time for controlling the common electrode voltage Vcom after power off. Thus, in the example shown inFIG. 4, the common electrode voltage Vcom cannot be lowered to the ground level in the charge pull-out period.

InFIG. 5, the common electrode voltage Vcom reaches or approaches to the ground level in the charge pull-out period. After that, the common electrode voltage Vcom gets higher in a short time, and then gets lower slowly. Thus, also in the example shown inFIG. 5, the common electrode voltage Vcom cannot be lowered to the ground level in the charge pull-out period.

Especially in the liquid crystal display device using the IGZO-TFTs, since the off leakage current of the TFT is small, the charge remaining in the liquid crystal panel when power off remains undischarged for a long time. Thus, burn-in is likely to occur in the liquid crystal panel when power on again.

Even in the liquid crystal display devices described in Patent Documents 1 and 2, since a power supply voltage supplied from a power supply control circuit turns off before the charge pull-out period, it is difficult to lower a voltage of a common electrode to the ground level in the charge pull-out period.

Therefore, providing a liquid crystal display device which can lower a common electrode voltage to a ground level when power off and can prevent burn-in after power on is taken as a problem.

Means for Solving the Problems

The above problem can be solved, for example, by a liquid crystal display device including: a liquid crystal panel having a common electrode and formed using oxide semiconductor; a power supply control circuit configured to output a control signal which becomes an active level in a period set after power off, and a power supply voltage which is kept at an operation level even after power off until a middle of the period; and a common electrode control circuit configured to connect the common electrode to a ground when the control signal is at the active level and the power supply voltage is at the operation level.

The above problem can be also solved by a drive method for a liquid crystal display device including a liquid crystal panel having a common electrode and formed using oxide semiconductor, the method including steps of: setting a control signal to an active level in a period set after power off; keeping a power supply voltage at an operation level even after power off until a middle of the period; and connecting the common electrode to a ground when the control signal is at the active level and the power supply voltage is at the operation level.

Effects of the Invention

According to the above-described liquid crystal display device and drive method therefor, in a part of the period set after power off, the control signal becomes the active level, the power supply voltage becomes the operation level, and the common electrode is connected to the ground. Thus, a common electrode voltage is lowered rapidly after a start of the period and reaches the ground level in a short time. Therefore, it is possible to prevent charge from remaining in the liquid crystal panel after power off and to prevent burn-in after power on.

MODES FOR CARRYING OUT THE INVENTION

FIG. 1is a block diagram showing a configuration of a liquid crystal display device according to an embodiment. A liquid crystal display device10shown inFIG. 1includes a power supply control circuit11, a level shifter12, a liquid crystal panel20, and a common electrode control circuit30. Typically, different IC chips are used for the power supply control circuit11and the level shifter12. Hereinafter, a field effect transistor is referred to as an FET.

A power supply voltage VCC is externally supplied to the liquid crystal display device10. In the following, the power supply voltage VCC is assumed to be a voltage of 3.3 V. Based on the power supply voltage VCC, the power supply control circuit11outputs various control signals and power supply voltages required for an operation of the liquid crystal display device10. Specifically, the power supply control circuit11outputs a gate high voltage VGH2, an all-on control signal ALLON, a common electrode voltage VCOM, and the like. Hereinafter, an output terminal of the gate high voltage VGH2is referred to as T1, an output terminal of the all-on control signal ALLON is referred to as T2, and an output terminal of the common electrode voltage VCOM is referred to as T3.

FIG. 2is a diagram showing details of the liquid crystal panel20. As shown inFIG. 2, the liquid crystal panel20includes scanning lines21, data lines22, pixel circuits23, a scanning line drive circuit24, and a common electrode25. The scanning lines21are arranged in parallel with each other. The data lines22are arranged in parallel with each other so as to intersect with the scanning lines21perpendicularly. The pixel circuits23are arranged two-dimensionally corresponding to intersections of the scanning lines21and the data lines22. The pixel circuit23includes a TFT26and a pixel electrode27. The common electrode25is disposed so as to oppose to all of the pixel electrodes27included in the liquid crystal panel20. A liquid crystal capacitor28is formed in the pixel circuit23by providing a liquid crystal layer (not shown) between the pixel electrode27and the common electrode25.

The scanning line drive circuit24is formed on the liquid crystal panel20using TFTs together with the pixel circuits23(monolithic gate driver). The level shifter12outputs a drive signal C1to the scanning line drive circuit24formed on the liquid crystal panel20, based on the gate high voltage VGH2, the all-on control signal ALLON, and the like. The scanning line drive circuit24drives the scanning lines21based on the drive signal C1. The data lines22are driven by a data line drive circuit (not shown). The common electrode voltage VCOM which is output from the power supply control circuit11is applied to the common electrode25.

The TFT26included in the pixel circuit23and TFTs (not shown) included in the scanning line drive circuit24are formed using a same material by a same manufacturing process. These TFTs are oxide semiconductor TFTs each having an oxide semiconductor layer formed using oxide semiconductor. In this manner, the liquid crystal panel20is formed using oxide semiconductor. The oxide semiconductor TFTs will be described later.

The common electrode control circuit30includes two resistors31,32, a NOT circuit33, and two FETs34,35. The FETs34,35are N-channel type transistors. The resistors31,32are connected in series. One end (upper end inFIG. 1) of the resistor31is connected to the terminal T1of the power supply control circuit11, and the gate high voltage VGH2is applied to the one end of the resistor31. Another end of the resistor31is connected to one end of the resistor32. Another end of the resistor32is grounded. Hereinafter, a node to which a connecting point of the resistors31,32is connected is referred to as Na.

An input terminal of the NOT circuit33is connected to the terminal T2of the power supply control circuit11, and the all-on control signal ALLON is supplied to the input terminal of the NOT circuit33. Based on the all-on control signal ALLON, the NOT circuit33outputs an inverted signal (hereinafter referred to as a signal XALLON in the description) of the all-on control signal ALLON.

A drain terminal of the FET34and a gate terminal of the FET35are connected to the node Na. A gate terminal of the FET34is connected to an output terminal of the NOT circuit33, and the signal XALLON is supplied to the gate terminal of the FET34. Source terminals of the FETs34,35are grounded. A drain terminal of the FET35is connected to the common electrode25. More specifically, the drain terminal of the FET35is connected to a wiring which connects the terminal T3of the power supply control circuit11and the common electrode25.

When the all-on control signal ALLON is called a control signal, the gate high voltage VGH2is called a power supply voltage, and the node Na is called a first node, the common electrode control circuit30includes a first resistor (resistor31) having one end to which the power supply voltage is supplied and another end connected to the first node, a second resistor (resistor32) having one end connected to the first node and another end connected to a ground, the NOT circuit33for outputting the inverted signal of the control signal, a first transistor (FET34) having a first conduction terminal (drain terminal) connected to the first node, a second conduction terminal (source terminal) connected to the ground, and a control terminal (gate terminal) to which the inverted signal is supplied, and a second transistor (FET35) having a first conduction terminal connected to the common electrode25, a second conduction terminal connected to the ground, and a control terminal connected to the first node. As shown below, the common electrode control circuit30connects the common electrode25to the ground when the control signal is at an active level and the power supply voltage is at an operation level.

In the liquid crystal display device10, a charge pull-out period for pulling out charge remaining in the liquid crystal panel20is set after power off. The gate high voltage VGH2is a high-level voltage supplied from the power supply control circuit11to the scanning line drive circuit24on the liquid crystal panel20. The gate high voltage VGH2is kept at a previous high level (hereinafter, referred to as an operation level) even after power off until a middle of the charge pull-out period.

FIG. 3is a signal waveform diagram of the liquid crystal display device10when power off. InFIG. 3, at time t0, the power supply voltage VCC changes from 3.3 V to a ground level. The charge pull-out period is set at time t1to t2after power off. The all-on control signal ALLON becomes a high level in the charge pull-out period, and becomes a low level otherwise. The gate high voltage VGH2is kept at the operation level (high level) even after power off until the middle of the charge pull-out period. Hereinafter, a time at which the gate high voltage VGH2starts lowering is referred to as ta. Furthermore, a voltage of the common electrode25is referred to as Vcom, distinguishing from the common electrode voltage VCOM which is output from the power supply control circuit11.

Before the time t1, the gate high voltage VGH2is at the operation level, the all-on control signal ALLON is at the low level, and the signal XALLON is at the high level. Thus, the FET34becomes an ON state, a voltage of the node Na becomes the ground level, and the FET35becomes an OFF state. Before the time t1, the common electrode voltage VCOM which is output from the power supply control circuit11is applied to the common electrode25. Therefore, the voltage Vcom of the common electrode25is equal to the common electrode voltage VCOM which is output from the power supply control circuit11.

At the time t1, the all-on control signal ALLON changes to the high level. Accordingly, the signal XALLON changes to the low level, and the FET34becomes the OFF state. A voltage Va of the node Na in the charge pull-out period is given by a following equation (1). In the equation (1), R1, R2 respectively represent resistance values of the resistors31,32.
Va={R2/(R1+R2)}×VGH2  (1)

The resistance values of the resistors31,32are determined so that the FET35becomes the ON state at the time t1. Therefore, at the time t1, the FET35becomes the ON state, and the terminal T3of the power supply control circuit11and the common electrode25are connected to the ground. This state continues at least by the time ta.

After the time ta, the gate high voltage VGH2is lowered. When the resistance values of the resistors31,32are determined as described above, the FET35keeps the ON state even after the time ta until the gate high voltage VGH2is lowered to some extent. Therefore, even after the time ta, while the FET35is in the ON state, the terminal T3of the power supply control circuit11and the common electrode25are connected to the ground as before.

In this manner, after the time t1, while the FET35is in the ON state, the common electrode25is connected to the ground. Thus, charge remaining in the common electrode25at the time t1is discharged through the FET35, and charge remaining in the pixel electrode27is also discharged accordingly. Therefore, the voltage Vcom of the common electrode25is lowered rapidly after the start of the charge pull-out period and reaches the ground level in a short time. The voltage Vcom of the common electrode25reaches the ground level at latest by the time t2when the charge pull-out period finishes.

As described above, the liquid crystal display device10includes the liquid crystal panel20having the common electrode25and formed using oxide semiconductor, the power supply control circuit11for outputting a control signal (all-on control signal ALLON) which becomes the active level (high level) in a period (charge pull-out period) set after power off, and a power supply voltage (gate high voltage VGH2) which is kept at the operation level (high level) even after power off until a middle of the period, and the common electrode control circuit30for connecting the common electrode25to the ground when the control signal is at the active level and the power supply voltage is at the operation level.

The liquid crystal panel20has the scanning lines21, and the control signal is the all-on control signal ALLON for selecting all of the scanning lines21. The scanning line drive circuit24for driving the scanning lines21is formed on the liquid crystal panel20, and the power supply voltage is a high-level voltage (gate high voltage VGH2) which is output to the scanning line drive circuit24. The above period is the charge pull-out period, and the all-on control signal ALLON becomes the high level in the charge pull-out period.

Therefore, in the liquid crystal display device10, in a part of the period (charge pull-out period) set after power off, the control signal becomes the active level, the power supply voltage becomes the operation level, and the common electrode25is connected to the ground. Therefore, the voltage Vcom of the common electrode25is lowered rapidly after the start of the period and reaches the ground level in a short time. Thus, according to the liquid crystal display device10, it is possible to prevent charge from remaining in the liquid crystal panel20after power off and to prevent burn-in after power on.

In the following, the oxide semiconductor TFT will be described. The oxide semiconductor included in the oxide semiconductor layer of the oxide semiconductor TFT may be an amorphous oxide semiconductor or a crystalline oxide semiconductor having a crystalline portion. As the crystalline oxide semiconductor, a polycrystalline oxide semiconductor, a microcrystalline oxide semiconductor, and a crystalline oxide semiconductor in which a c-axis is aligned so as to be substantially perpendicular to a layer surface, and the like can be mentioned. The oxide semiconductor layer may have a layered structure including two or more layers. When the oxide semiconductor layer has the layered structure, the oxide semiconductor layer may include an amorphous oxide semiconductor layer and a crystalline oxide semiconductor layer. Alternatively, a plurality of crystalline oxide semiconductor layers having different crystal structures may be included. Or, a plurality of amorphous oxide semiconductor layers may be included. When the oxide semiconductor layer has a two-layered structure including an upper layer and a lower layer, an energy gap of the oxide semiconductor included in the upper layer is preferably larger than an energy gap of the oxide semiconductor included in the lower layer. However, when a difference in the energy gaps between these layers is relatively small, the energy gap of the oxide semiconductor in the lower layer may be larger than the energy gap of the oxide semiconductor in the upper layer.

A material, a structure, and a film forming method of the amorphous oxide semiconductor and each of the above-described crystalline oxide semiconductors, a structure of the oxide semiconductor layer having a layered structure, and the like are described, for example, in Japanese Laid-Open Patent Publication No. 2014-7399. For reference, all the disclosure contents of Japanese Laid-Open Patent Publication No. 2014-7399 is incorporated herein by reference.

The oxide semiconductor layer may include, for example, at least one kind of metal elements selected from In (indium), Ga (gallium), and Zn (zinc). The oxide semiconductor layer includes, for example, an In—Ga—Zn—O based semiconductor (for example, indium gallium zinc oxide). The In—Ga—Zn—O based semiconductor is a ternary oxide of In, Ga, and Zn, and a ratio (composition ratio) of In, Ga, and Zn is not especially limited, and may be, for example, In:Ga:Zn=2:2:1, In:Ga:Zn=1:1:1, In:Ga:Zn=1:1:2, and the like. Such an oxide semiconductor layer can be formed using an oxide semiconductor film including In—Ga—Zn—O based semiconductor. The In—Ga—Zn—O based semiconductor may be amorphous or crystalline. As the crystalline In—Ga—Zn—O based semiconductor, the crystalline In—Ga—Zn—O based semiconductor in which the c-axis is aligned so as to be substantially perpendicular to the layer surface is preferable.

A crystal structure of the crystalline In—Ga—Zn—O based semiconductor is described, for example, in the above-described Japanese Laid-Open Patent Publication No. 2014-7399, Japanese Laid-Open Patent Publication No. 2012-134475, Japanese Laid-Open Patent Publication No. 2014-209727, and the like. For reference, all of the disclosure contents of Japanese Laid-Open Patent Publication No. 2012-134475 and Japanese Laid-Open Patent Publication No. 2014-209727 are incorporated herein by reference. Since a TFT having the In—Ga—Zn—O based semiconductor layer has a high mobility (more than 20 times when compared with the amorphous silicon TFT) and a small leakage current (less than 1/100 compared when compared with the amorphous silicon TFT), it can be suitably used as drive TFTs (for example, TFTs included in a drive circuit provided on a same substrate as a display area, in a surrounding portion of the display area including pixels) and pixel TFTs (TFTs provided in the pixel).

The oxide semiconductor layer may include other oxide semiconductor in place of the In—Ga—Zn—O based semiconductor. For example, an In—Sn—Zn—O based semiconductor (for example, In2O3—SnO2—ZnO; InSnZnO) may be included. The In—Sn—Zn—O based semiconductor is a ternary oxide of In, Sn (tin), and Zn. Alternatively, the oxide semiconductor layer may include an In—Al—Zn—O based semiconductor, an In—Al—Sn—Zn—O based semiconductor, a Zn—O based semiconductor, an In—Zn—O based semiconductor, a Zn—Ti—O based semiconductor, a Cd—Ge—O based semiconductor, a Cd—Pb—O based semiconductor, a CdO (cadmium oxide), an Mg—Zn—O based semiconductor, an In—Ga—Sn—O based semiconductor, an In—Ga—O based semiconductor, a Zr—In—Zn—O based semiconductor, a Hf—In—Zn—O based semiconductor, and the like.

By providing the common electrode control circuit30to the liquid crystal display device including the liquid crystal panel formed using the oxide semiconductor TFTs, such as the IGZO-TFTs, it is possible to lower the common electrode voltage to the ground level after power off and to prevent burn-in after power on.

Note that although the power supply voltage VCC is assumed to be 3.3 V in the above description, the power supply voltage VCC may be equal to or more than 5 V. In this case, the effect that the burn-in after power on can be prevented becomes more apparent.

This application claims a priority based on Japanese Patent Application No. 2017-118260 filed on Jun. 16, 2017, and entitled “Liquid Crystal Display Device And Drive Method Therefor”, which is incorporated herein by reference in its entirety.

DESCRIPTION OF REFERENCE CHARACTERS

10: LIQUID CRYSTAL DISPLAY DEVICE11: POWER SUPPLY CONTROL CIRCUIT12: LEVEL SHIFTER20: LIQUID CRYSTAL PANEL23: PIXEL CIRCUIT24: SCANNING LINE DRIVE CIRCUIT25: COMMON ELECTRODE30: COMMON ELECTRODE CONTROL CIRCUIT31,32: RESISTOR33: NOT CIRCUIT34,35: FET