JP2000194304A - Protective circuit and liquid crystal display device having the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable maintaining a normal display state without imparting damage on a TFT element and gate line or data line by inputting an input signal to an electrode line while converting it into voltage equal to or lower than a reference voltage when the voltage of the input signal is higher than the reference voltage. SOLUTION: When a signal S is inputted to a comparator 60, the comparator 60 compares it with a reference voltage to output the result as a signal S' and a switching element 62 receives this to change over a signal route to either of a route X or a route Y. At a normal time, this device drives pixels by turning a TFT 44 ON/OFF by receiving a signal selecting the route Y. When high voltage of static electricity or the like is applied on the signal S, the comparator 60 compares the signal S with the reference voltage and it instantly transmits a signal S' making a switching element 62 select the route X being the route of a safty side of the element 62 and then a signal voltage level converting circuit 64 converts the signal S' into a voltage level signal S" which is best suited for driving the TFT and outputs it.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is to prevent a display failure due to damage to pixels of a liquid crystal display device when a high voltage such as static electricity is applied to the liquid crystal display device and to apply a high voltage during display. The present invention also relates to a protection circuit for a liquid crystal display device capable of maintaining a normal display state without disturbing an image without damaging a TFT, a gate line or a data line of a pixel, and a liquid crystal display device using the circuit.

[0002]

2. Description of the Related Art FIG. 1 shows a conventional TF as a liquid crystal display device.
It is a figure showing the schematic structure of a T liquid crystal display. Note that TF
T is an abbreviation for Thin Film Transistor (thin film transistor). As shown in FIG.
CD (Liquid Crystal Display, liquid crystal display)
It has a signal control board 10, a power supply section 12, a gate line drive circuit board 14, a data line drive circuit board 16, and an LCD panel 18. The LCD panel is also called a liquid crystal display panel. The components other than the LCD panel are constituted by a printed circuit board.

The LCD signal control board 10 generates various signals necessary for displaying an LCD panel from video signals and synchronization signals input from the outside, and generates a gate line drive circuit board 14 and a data line drive circuit board 16. To send that signal. The power supply unit 12 supplies necessary power to each unit. The gate line drive circuit board 14 drives the gate lines of the LCD panel by a signal from the LCD signal control board 10 via a driver described later, and the data line drive circuit board 16 receives the signal from the LCD signal control board 10 via the driver. Drive the gate line of the LCD panel by the signal of. LCD panel 1
Reference numeral 8 generally comprises two glass plates in which a liquid crystal material is sealed, and the glass substrate on which the TFT is mounted is called a TFT array substrate. Gate lines and data lines are arranged in a matrix on the TFT array substrate, and TFTs are arranged near intersections thereof.

FIG. 2 is a sectional view of a general TFT array substrate. As shown in FIG. 1, a TFT portion 22 as an active element, a display electrode 24 made of a transparent conductive film (ITO film), and a storage capacitor electrode 26 for holding a data signal are arranged on a glass substrate 20. The TFT section 22 includes a gate electrode 28, a gate insulating film 30 made of a silicon oxide film or the like,
A-Si (amorphous silicon) which is a semiconductor active layer
A layer 32 includes an a-Si layer 34 in which N ⁺ impurities are mixed, a source electrode 36, and a drain electrode 38. In this example, the source electrode 36 is connected to the data line, and the drain electrode 38 is connected to the display electrode 24. Further, it has an alignment film 40 necessary for a liquid crystal display.

FIG. 3 is a diagram showing an equivalent circuit of a unit pixel of a conventional TFT liquid crystal display device. As shown in the figure, the data line 42 is connected to the source of the TFT 44 and the gate line 4
6 is connected to the gate of the TFT 44, and a liquid crystal portion 48 (capacitor) is connected to the drain of the TFT 44. As described above, the TFT portion and the intersection between the data line 42 and the gate line 46 have a thin oxide film (insulating film) of several μm.
And so on.

Further, as shown in FIG.
TAB is connected to the gate line driving circuit board 14
An LCD driver LSI (or IC) 52 is mounted by 50 (tape automated bonding). FIG. 5 shows a sectional view of the TAB. As shown in the figure, the driver LSI 52 encapsulated in the mold resin 52 is connected to the LCD panel by an ACF 56 (anisotropic conductive film) on the output side, and is connected to the drive circuit board by the solder 58 on the input side.

[0007]

In the configuration according to the prior art described above, there is no protection circuit against high voltage such as static electricity, so that high voltage such as static electricity which is slightly applied to the human body during the manufacturing process of the liquid crystal display device or the like is controlled. When applied to the circuit system, there is a problem that the film is broken and causes display failure. In addition, this has been one factor in lowering product productivity.

Further, although a TAB driver LSI has a general protection circuit against static electricity, the protection circuit in the driver LSI cannot completely prevent the above-described film destruction. However, deterioration of display quality such as flickering of the liquid crystal display screen due to static electricity was unavoidable. The present invention has been made in view of the above points, and is directed to a case where a high voltage that causes a film destruction or a deterioration in display quality is applied on an input side or an output side of a driver for driving a liquid crystal display device. , TFT without film destruction
Normal display state can be maintained even during display by converting high voltage to optimal voltage level for driving TFT without damaging elements, gate lines or data lines. It is an object to provide a simple protection circuit and a liquid crystal display device using the protection circuit.

[0009]

To achieve the above object, the present invention is configured as follows. Note that the term “electrode line” includes a data line, a gate line, or both on a liquid crystal display panel. The present invention is a protection circuit that is connected to an electrode line on a liquid crystal display panel in a liquid crystal display device and protects the liquid crystal display panel from a high voltage. The protection circuit receives an input signal, and converts a reference voltage and a voltage of the input signal. A comparator to be compared, connected to the comparator, inputting the input signal to the electrode line when the voltage of the input signal is equal to or lower than the reference voltage, and when the voltage of the input signal is higher than the reference voltage And a conversion circuit for converting the input signal into a voltage lower than the reference voltage and inputting the converted voltage to the electrode line.

According to the present invention, when a high voltage such as static electricity is applied to a liquid crystal display panel, the voltage is converted, so that the pixel is not damaged. Further, by setting the voltage to an optimal level for display, a normal display state can be maintained. To achieve the above object, the present invention is also configured as follows. The present invention is a protection circuit that is connected to an electrode line on a liquid crystal display panel in a liquid crystal display device and protects the liquid crystal display panel from a high voltage. The protection circuit receives an input signal, and converts a reference voltage and a voltage of the input signal. A comparator to be compared, and a first signal line connected to the comparator and the signal line connected to the electrode line when the voltage of the input signal is equal to or lower than the reference voltage. A switching element that selects as a path, and selects a second signal line when the voltage of the input signal is higher than the reference voltage;
A voltage level conversion circuit that is connected to the second signal line and the electrode line, converts a voltage of an input signal to a voltage equal to or lower than the reference voltage, and inputs the voltage to the electrode line.

According to the present invention, when a high voltage such as static electricity is applied to the liquid crystal display panel, the signal path is switched and the voltage is converted, so that the pixel is not damaged. Further, by setting the voltage to an optimal level for display, a normal display state can be maintained. The present invention is also configured such that in the above configuration, the comparator is connected to the comparator,
It may be configured to include a control circuit that controls the signal voltage by feeding back the output from the comparator to the conversion circuit.

According to the present invention, it is possible to perform a control precisely corresponding to a voltage change of an input signal, and it is possible to further improve a display quality. The present invention can also be configured so as to include a control circuit connected to the comparator and controlling a signal voltage by feeding back an output from the comparator to the voltage level conversion circuit.

Similarly, according to the present invention, it is possible to perform control precisely corresponding to a voltage change of an input signal, and it is possible to further improve display quality. The present invention is further configured such that, in the above configuration, the display device is connected to the comparator, monitors an output from the comparator, and stores display data during a period in which the input signal is input at a voltage higher than the reference voltage. It can be configured to include a control circuit for transmitting the stored display data to the electrode lines.

According to the present invention, display data during a period in which a high voltage is applied is stored and can be displayed, so that normal operation can be performed even when a high voltage is applied. To achieve the above object, the present invention is also configured as follows. The present invention is a liquid crystal display device, which has the above protection circuit on a printed circuit board. In the structure, the above protection circuit may be provided on a liquid crystal display panel.

According to these inventions, it is possible to provide a liquid crystal display device having improved reliability and display quality as compared with the prior art.

[0016]

FIG. 6 is a schematic block diagram of an embodiment of the present invention. As shown in FIG.
0, switching element 62, signal voltage level conversion circuit 6
4 and is connected to the gate line 46 of the liquid crystal display device in the example shown here. The comparator 60 is connected to the reference voltage, and has a function of transmitting a signal for switching the signal path of the switching element when the voltage is higher than the reference voltage. The reference voltage is set to a voltage lower than the voltage level at which disconnection occurs when applied to the gate line or the data line,
Alternatively, the voltage is set to a voltage lower than the voltage level at which the TFT is broken down and the pixel is damaged. The switching element 62 has a function of switching the output direction according to an input signal from the comparator 60. When a high voltage signal such as static electricity is input, the switching element 62 selects the path X, and a signal of a normal voltage is output. It has a function of selecting the path Y when input. The signal voltage level conversion circuit 64 has a function of converting a high voltage affecting the LCD panel into an appropriate voltage.

Next, the operation in this configuration will be described. When the signal S is input, the comparator 60 outputs the result of the comparison between the signal S and the reference voltage as a signal S ′, and the switching element 62 receives the signal S and changes the signal path to one of the path X and the path Y. Switch. Normally, when a signal for selecting the path Y is received, the gate signal passes through the path Y and turns on / off the TFT 44 via the gate line 46 to drive the pixel. When a high voltage such as static electricity is applied to the signal S, the comparator 60 compares the reference voltage with S, and sends a signal S ′ for selecting a path X, which is a safe path, to the switching element 62. The signal voltage level conversion circuit 64 instantaneously transmits the signal S ′ without damaging the TFT or the gate line or the data line of the pixel, such as a film breakdown, and at the optimum voltage level for driving the TFT. As a result, it is possible to prevent damage such as film destruction of the pixel and to maintain a normal display state.

FIG. 7 is a circuit diagram more specifically showing an embodiment of the present invention. As shown in the figure, the circuit of the present embodiment includes a comparator 66 using an operational amplifier as a comparator,
A transistor 68 and a transistor 70 and a signal voltage level conversion circuit 72 are provided as switching elements.
The comparator 66 is connected to the reference voltage _Vref ,
The signal S is input, and the comparison result is output as a signal S ′. The transistor 68 is an n-channel MOSFET, and the transistor 70 is a p-channel MOSFET. Since the signal S ′ is input to each gate, when S ′ is at the H level, the transistor 68 is turned on and the transistor 70 is turned on.
Is turned off, the path X is selected, and the high voltage signal is input to the signal voltage level conversion circuit 72. In the signal voltage level conversion circuit 72, the high voltage signal is converted into a signal S "having an optimum voltage level for driving the TFT without damaging the TFT, the gate line, or the data line of the pixel such as film destruction. The TFT 44 is turned on by the signal S ″.
N / OFF to drive the pixel.

When S 'is at the L level, the transistor 68 is turned off and the transistor 70 is turned on, and the path Y is selected. Therefore, the signal S drives the pixel without passing through the signal voltage level conversion circuit 72. This is a normal driving state. FIG. 8 shows an embodiment of the signal voltage level conversion circuit 64. As shown in FIG.
4, the anode is connected to the path X,
The cathode is connected at a voltage of a reference voltage level _Vadj . That is, the voltage level _Vadj as a reference is constituted by the diode 74 applied in the reverse direction. In other words, the voltage level _Vadj is clamped by the diode 74. Assuming that the forward voltage of the diode is V _F , when the voltage of the signal S ′ is larger than V _adj + V _F , the current is released in the forward direction of the diode, and the voltage of the high-voltage signal S ′ is changed to a safe voltage level. V _adj +
Converted to V _F or less.

Further, the circuit may be simplified by using a high withstand voltage fixed resistor as the signal voltage level conversion circuit. This method is effective when importance is placed on cost.
FIG. 9 shows an embodiment of a circuit for generating a reference voltage _Vref or _Vadj . This circuit is formed on the LCD control board shown in FIG. 1, and one circuit is provided for each of _Vref and _Vadj . As shown in FIG. 9, this circuit has a voltage follower including a resistor 76, a resistor 78, and an operational amplifier 80. The operation is as follows.

Power is supplied from the power supply section 12 and the resistance 76
The voltage is converted to an appropriate voltage level by the resistor 78, and is output via a voltage follower to obtain a stable output. FIG. 10 shows an embodiment of the present invention, in which an altitude control circuit 82 is added to the configuration shown in FIG. 6 in order to perform fine control of the voltage. As shown in FIG.
By feeding back the 0 signal to the signal voltage level conversion circuit 64 using the altitude control circuit 82, more precise voltage control becomes possible.

The above-described altitude control circuit can have various configurations. For example, FIG. 11 shows an example in which an altitude control circuit 84 having a memory unit 86 and a control unit 88 is added to the embodiment shown in FIG. . In the figure, a video signal is stored in a memory 86
Through the comparator 66 and the transistors 68 and 70. The control unit 88 monitors the output of the comparator 66 described above, and the time when a normal signal does not flow due to application of a high voltage such as static electricity, that is, the comparator 66 is turned on.
Is controlled so that the image data corresponding to the time is transmitted from the memory unit 86 as normal image data. Therefore,
According to the altitude control circuit 84, normal liquid crystal display can be performed even when static electricity is applied.

In the above description, the protection of the pixel on the gate line has been described. However, by connecting this circuit to the data line, high voltage protection on the data line is possible. Actually, since the data line receives various levels of analog signals unlike the gate line, a circuit capable of advanced control as shown in FIGS. 10 and 11 is used on the data line.

Further, the protection circuit according to the present invention uses the L shown in FIG.
It can be formed on any of the CD signal control board 10, the gate line drive circuit board 14, and the data line drive circuit board 16. In any case, the circuit described as the embodiment can be used. By providing the protection circuit of the present invention in a liquid crystal display device as shown in FIG. 1, a liquid crystal display device with improved reliability against high voltages such as static electricity and display quality can be obtained.

Further, the protection circuit can be formed on another printed circuit board, or can be formed on an LCD panel (liquid crystal display panel) by using a thin film manufacturing method. Further, a driver for driving the liquid crystal display device (TAB described above)
By providing the same circuit on both the input side and the output side of section (2), it is possible to further improve the reliability of the liquid crystal display device against high voltages such as static electricity. In that case, the same circuit can be provided on the input side and the output side, and different circuits can be provided on the input side and the output side. For example, it is possible to provide the circuit according to the present embodiment shown in FIG. 7 on one side and the circuit shown in FIG. 11 on the other side. By providing the circuit of the present invention on the input side and the output side in this manner, even if a high voltage such as static electricity is generated on either the input side or the output side, the LCD panel is not affected.

It should be noted that the present invention is not limited to the above-described embodiment, but can be variously modified and applied within the scope of the claims.

[0027]

According to the present invention, even when a high voltage is applied to a liquid crystal display device in a manufacturing process or in a situation where a high voltage such as static electricity is generated, pixels are damaged as in the prior art and display defects occur. Will be lost. Furthermore, since a stable voltage signal is sent to the liquid crystal display device,
The flickering of the liquid crystal display and the like are eliminated, and a liquid crystal display device with significantly improved reliability and display quality can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a configuration of a liquid crystal display device.

FIG. 2 is a sectional view of a TFT array substrate.

FIG. 3 is an equivalent circuit diagram of a unit pixel in a conventional liquid crystal display device.

FIG. 4 is a diagram showing TAB.

FIG. 5 is a sectional view of TAB.

FIG. 6 is a schematic configuration diagram of one embodiment of the present invention.

FIG. 7 is a configuration diagram of one embodiment of the present invention.

FIG. 8 is a diagram showing one embodiment of a signal voltage level conversion circuit.

FIG. 9 is a diagram showing an example of a circuit for generating a reference voltage.

FIG. 10 is a schematic configuration diagram of an embodiment to which an altitude control circuit is added.

FIG. 11 is a configuration diagram of an embodiment to which an altitude control circuit is added.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 LCD control board 12 Power supply part 14 Gate line drive circuit board 16 Data line drive circuit board 18 LCD panel 20 Glass substrate 22 TFT part 24 Display electrode 26 Storage capacitor electrode 28 Gate electrode 30 Gate insulating film 32 а-Si (amorphous silicon) 34 n ⁺ а-Si 36 Source electrode 38 Drain electrode 40 Alignment film 42 Data line 44 TFT 46 Gate line 48 Liquid crystal part (capacitor) 50 TAB 52 Driver LSI 54 Mold resin 56 ACF (anisotropic conductive film) 58 Solder 60 Comparison Device 62 Switching element 64, 72 Signal voltage level conversion circuit 66 Comparator 66 68 N-channel MOSFET 68 P-channel MOSFET 74 Diode 76, 78 Resistance 80 Operational amplifier 82, 84 Advanced control circuit 86 Memory unit 88 Control unit

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H093 NA16 NC12 NC21 NC34 NC62 ND10 ND31 ND53 ND60 NE10 5C006 AA11 AC11 AC21 AF45 AF64 BB16 BF14 BF25 FA33 5C080 AA10 BB05 DD18 EE29 FF11 GG09 JJ02 JJ03 JJ06 JJ06

Claims (7)

[Claims] 1. A protection circuit connected to an electrode line on a liquid crystal display panel in a liquid crystal display device, for protecting the liquid crystal display panel from high voltage, receiving an input signal, receiving a reference voltage and a voltage of the input signal. And a comparator connected to the comparator, wherein when the voltage of the input signal is lower than the reference voltage, the input signal is input to the electrode line, and the voltage of the input signal is higher than the reference voltage. A conversion circuit for converting the input signal to a voltage lower than the reference voltage and inputting the converted voltage to the electrode line. 2. A protection circuit connected to an electrode line on a liquid crystal display panel in a liquid crystal display device and protecting the liquid crystal display panel from a high voltage. The protection circuit receives an input signal, and receives a reference voltage and a voltage of the input signal. And a first signal line connected to the electrode line when the voltage of the input signal is equal to or lower than the reference voltage. And a switching element for selecting a second signal line when the voltage of the input signal is higher than the reference voltage; and a switching element connected to the second signal line and the electrode line, And a voltage level conversion circuit for converting the voltage to a voltage lower than the reference voltage and inputting the voltage to the electrode line. 3. The protection circuit according to claim 1, further comprising a control circuit connected to said comparator and controlling a signal voltage by feeding back an output from said comparator to said conversion circuit. 4. The protection circuit according to claim 2, further comprising a control circuit connected to said comparator and controlling a signal voltage by feeding back an output from said comparator to said voltage level conversion circuit. 5. The display device is connected to the comparator, monitors an output from the comparator, stores display data during a period when the input signal is input at a voltage higher than the reference voltage, and stores the display data. 3. The protection circuit according to claim 1, further comprising a control circuit that transmits display data to the electrode line. 6. A liquid crystal display device comprising the protection circuit according to claim 1 on a printed circuit board. 7. A liquid crystal display device comprising the protection circuit according to claim 1 on a liquid crystal display panel.