JP4360128B2 - Electro-optical device and electronic apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electro-optical device and an electronic apparatus, and more particularly, to a protection circuit connected to a data line to which data is supplied at a current level.
[0002]
[Prior art]
Conventionally, there has been proposed an electro-optical device in which a protection circuit for preventing electrostatic breakdown of an internal circuit is connected to a data line (for example, Patent Documents 1 to 6). In particular, Patent Document 4 discloses that the resistance value of the protection circuit is set in consideration of the leakage current flowing through the protection circuit in order to ensure the inspection accuracy during the inspection of the pixel characteristics.
[0003]
[Patent Document 1]
Japanese Patent Publication No. 3-54475 [Patent Document 2]
JP 11-231345 A [Patent Document 3]
Japanese Utility Model Publication No. 64-3827 [Patent Document 4]
Japanese Patent Laid-Open No. 8-22024 [Patent Document 5]
JP-A-10-303431 [Patent Document 6]
Japanese Patent Application Laid-Open No. 7-294952.
[0004]
[Problems to be solved by the invention]
By the way, in the current programming method in which data is supplied to the data line on a current basis, when the leakage current of the protection circuit connected to the data line increases, the display gradation of the pixel shifts from the original gradation. Arise. All of the above-mentioned patent documents are related to a voltage programming method in which data is supplied to a data line on a voltage basis (this method is used for liquid crystal), and the protection circuit which is a problem inherent to the current programming method is described. The gradation shift due to the leak current is not taken into consideration.
[0005]
Therefore, an object of the present invention is to suppress gradation shift caused by a leakage current of a protection circuit in a current programming method.
[0006]
[Means for Solving the Problems]
In order to solve such a problem, the first invention is connected to a plurality of data lines to which a data current defining the gradation of a pixel is supplied and to the data lines in order to prevent electrostatic breakdown of internal circuits. And a protection circuit for discharging static electricity of the data line when a leak current flows. Here, the resistance value in the path through which the leakage current flows in the protection circuit is set to a resistance value that does not cause a shift in the display gradation of the pixel due to the fluctuation of the data current due to the leakage current. This resistance value is preferably set based on the maximum value of the voltage applied to the protection circuit.
[0007]
In the first invention, the protection circuit may be connected to a pair of data lines. In this case, it is preferable that the resistance value in the protection circuit is set to a resistance value such that the leakage current is smaller than ½ of the minimum value of the current change amount between adjacent gradations.
[0008]
In the first invention, the protection circuit may be connected to a data line and a power supply line supplied with a predetermined voltage. In this case, it is preferable that the resistance value in the protection circuit is set to such a resistance value that the leakage current is smaller than the minimum value of the current change amount between adjacent gradations.
[0009]
According to a second aspect of the present invention, there is provided an electronic apparatus comprising the electro-optical device according to the first aspect of the invention.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
FIG. 1 is a block diagram of an electro-optical device according to this embodiment. In the display unit 1, pixels 2 for m dots × n lines are arranged in a matrix (in a two-dimensional plane). The display unit 1 is also provided with horizontal lines Y1 to Yn each extending in the horizontal direction and data lines X1 to Xm each extending in the vertical direction. Pixels 2 are arranged corresponding to the intersections. The scanning line driving circuit 3 and the data line driving circuit 4 cooperate with each other to perform display control of the display unit 1. In other words, the scanning line driving circuit 3 sequentially selects the scanning lines Y1 to Yn corresponding to the pixel row (pixel group existing on one horizontal line) as a data writing target in a predetermined scanning direction. . The data line driving circuit 4 supplies a data current Idata that defines the gray level of the pixel 2 to the pixel row to which data is to be written via the data lines X1 to Xm. Data writing is performed by a current programming method in which data is supplied to the data line X on a current basis.
[0011]
FIG. 2 is a circuit diagram of the pixel 2 as an example in the current programming method. One pixel 2 includes an organic EL element OLED, four transistors T1 to T4, and a capacitor C that holds data. The gate of the switching transistor T1 is connected to the scanning line Y supplied with the scanning signal SEL, and the source thereof is connected to the data line X supplied with the data current Idata. The drain of the first switching transistor T1 is commonly connected to the source of the second switching transistor T2, the drain of the driving transistor T3, and the drain of the control transistor T4. Similarly to the first switching transistor T1, the gate of the second switching transistor T2 is connected to the scanning line Y to which the scanning signal SEL is supplied. The drain of the second switching transistor T2 is commonly connected to one electrode of the capacitor C and the gate of the driving transistor T3. A power supply voltage Vdd is applied to the other electrode of the capacitor C and the source of the driving transistor T3. The control transistor T4 to which the control signal GP is supplied to the gate is provided between the drain of the driving transistor T3 and the anode (anode) of the organic EL element OLED. A voltage Vss lower than the power supply voltage Vdd is applied to the cathode (cathode) of the organic EL element OLED.
[0012]
The driving process of the pixel 2 in the current programming method is roughly divided into a data writing process and a subsequent driving process. In the writing process, data is written to the capacitor C. Specifically, in the line selection period in which the scanning signal SEL is at a high level (hereinafter referred to as “H level”), the transistors T1 and T2 are both turned on (conductive). As a result, the data line X and the drain of the transistor T3 are electrically connected, and the transistor T3 has a diode connection in which its gate and its drain are electrically connected. The transistor T3 causes the data current Idata supplied from the data line X to flow through its own channel, and a gate voltage Vg corresponding to this data current Idata is generated at its gate. Charges corresponding to the generated gate voltage Vg are accumulated in the capacitor C connected to the gate of the transistor T3, and data corresponding to the accumulated charge amount is written.
[0013]
In the driving process following the data writing process, the driving current Ioled flows through the organic EL element OLED, and the organic EL element OLED emits light. When the above-described line selection period elapses, the scanning signal SEL becomes L level, and both the transistors T1 and T2 are turned off (non-conducting). As a result, the data line X to which the data current Idata is supplied is electrically isolated from the drain of the transistor T3, and the gate and drain of the transistor T3 are also electrically isolated. However, even after the separation, the gate voltage Vg corresponding to the accumulated charge of the capacitor C is continuously applied to the gate of the transistor T3. When the drive signal GP becomes H level in synchronization with the scanning signal becoming L level, the drive current Ioled through the transistors T3, T4 and the organic EL element OLED is moved from the power supply voltage Vdd toward the reference voltage Vss. Current paths are formed. The drive current Ioled flowing through the organic EL element OLED corresponds to the channel current of the transistor T3, and the current level is controlled by the gate voltage Vg caused by the accumulated charge in the capacitor C. The organic EL element OLED emits light with a luminance corresponding to the drive current Ioled, and thereby the gradation of the pixel 2 is set.
[0014]
The protection circuit 6 is a circuit that prevents electrostatic breakdown of the internal circuit. In the present embodiment, the protection circuit 6 is provided corresponding to the pair of data lines X, and specifically, is connected to an external connection terminal connected to an FPC (Flexible Printed Circuit). FIG. 3 is a circuit diagram showing a configuration of the single protection circuit 6. The protection circuit 6 is generally called a diode ring and has a configuration in which two diode arrays 6a and 6b are connected in parallel in opposite directions. Each of the diode arrays 6a and 6b is formed by connecting a plurality of transistors functioning as nonlinear resistance elements in series by diode connection. The terminals a of the diode rows 6a and 6b are connected to the left data line X1, and the terminals b are connected to the right data line X2 adjacent to the data line X1.
[0015]
The protection circuit 6 discharges the static electricity of the data line X by flowing a leak current Ileak corresponding to the voltage Vab between the two terminals a and b. Specifically, when the voltage of the data line X1 is higher than the voltage of the data line X2, the leakage current Ileak from the terminal a to the terminal b is routed through the diode array 6a. Is formed. On the other hand, when the voltage of the data line X2 is higher than the voltage of the data line X1, the path of the leakage current Ileak from the terminal b to the terminal a is formed through the diode array 6b. The As a result, when the applied voltage Vab of the protection circuit 6 exceeds a predetermined withstand voltage due to static electricity, the leakage current Ileak flows in either direction and acts to reduce the voltage Vab. As a result, static electricity is discharged to prevent electrostatic breakdown of internal circuits including the subsequent circuit system connected to the data lines X1 and X2.
[0016]
By the way, in the current programming method, the display gradation of the pixel 2 is defined by the current value of the data current Idata. Therefore, when the leakage current Ileak flows through the protection circuit 6, the current value of the data current Idata changes. For example, in the configuration of FIG. 3, when a leak current Ileak flows from the data line X1 to the data line X2, the current value I1 ′ of the data line X1 is equal to Ileak from the original value I1 supplied from the data line driving circuit 4. The current value I2 'of the data line X2 increases by Ileak from the original value I2. The amount of leakage current Ileak depends on the applied voltage Vab of the protection circuit 6 and basically increases in proportion to the applied voltage Vab. Therefore, when the leak current Ileak is increased, there is a disadvantage that gradation shift of the pixel 2 or gradation inversion occurs.
[0017]
In the present embodiment, in order to eliminate such inconvenience, the protection circuit 6 regulates the amount of leakage current Ileak by appropriately setting the resistance value R in the path through which the leakage current Ileak flows. FIG. 4 is a schematic characteristic diagram showing the relationship between gradation and data current Idata. Since the gradation changes linearly with respect to the current change, a current change amount ΔI (hereinafter referred to as a step value ΔI) between adjacent gradations is set at equal intervals. In this case, the resistance value R of the protection circuit 6 is set so that the leakage current Ileak is smaller than ½ of the step value ΔI. This resistance value R can be arbitrarily set by setting the channel length, channel width, impurity concentration doped in the channel, or the like of the individual transistors constituting the diode arrays 6a and 6b.
[0018]
Specifically, the resistance value R is set according to Equation 1. Here, the maximum value Vmax is the applied voltage Vab of the protection circuit 6 generated when the potential difference between the data lines X1 and X2 becomes maximum. For example, in the case of the 64-gradation display shown in FIG. 4, the data current I0 corresponding to the gradation 0 is supplied to the data line X1 (the voltage of the data line X1 becomes the maximum voltage), and the data line X2 is applied. When the data current I63 corresponding to the gradation 63 is supplied (the voltage of the data line X2 becomes the minimum voltage), the applied voltage Vab of the protection circuit 6 becomes the maximum.
[Equation 1]
Ileak <ΔI / 2
Vmax / R <ΔI / 2
R> 2 · Vmax / ΔI
[0019]
Thus, the condition of Ileak <ΔI / 2 is satisfied in the entire voltage region that can occur between the data lines X1 and X2. As a result, even if the leak current Ileak flows through the protection circuit 6, the current amount of the leak current Ileak is effectively regulated, so that the gradation shift of the pixel 2 can be suppressed, and the inversion of gradation between adjacent pixels is also suppressed. it can.
[0020]
In the characteristics shown in FIG. 4, the relationship between the gradation and the data current Idata is linearly shown. However, this is an example, and the nonlinearity is considered in consideration of the characteristics of the organic EL element OLED. It may be set to a simple relationship. In this case, the resistance value R may be set so that the leakage current Ileak is smaller than ½ of the minimum value of the step value ΔI.
[0021]
Furthermore, in the present embodiment, an example in which a diode ring is used as the protection circuit 6 has been described. However, the present invention is not limited to this, and can be widely applied to various circuit configurations that prevent electrostatic breakdown of internal circuits, including the modified examples of FIGS. 5 and 6.
[0022]
FIG. 5 is a circuit diagram of a protection circuit 6 using a resistance element as a modification. The protection circuit 6 is a sheet resistance formed by diffusing impurities such as boron or phosphorus in a polycrystalline silicon layer, for example. The resistance element of the protection circuit 6 is set to a resistance value R that satisfies the above-described conditions.
[0023]
FIG. 6 is a circuit diagram of a protection circuit 6 using a transistor as another modification. The protection circuit 6 includes three transistors 6a, 6b, and 6c. Between the data lines X1 and X2, two transistors 8a and 8b are connected in series, and the gate of one transistor 8a is connected to the data line X1, and the gate of the other transistor 8b is connected to the data line X2. ing. A transistor 8c is also provided between the data lines X1 and X2, and the gate of the transistor 8c is connected to a connection node between the two transistors 8a and 8b. In the protection circuit 6, the resistance in the path through which the leak current Ileak flows corresponds to the channel resistance of the transistor 8c, and this is set to the resistance value R satisfying the above-described conditions.
[0024]
(Second Embodiment)
FIG. 7 is a block diagram of the electro-optical device according to this embodiment. The protection circuit 9 for preventing electrostatic breakdown of the internal circuit is provided for each data line, and is configured by a resistance element that connects the data line X and the power supply line Vss supplied with a predetermined voltage. . The same elements as those shown in FIG. 1 are denoted by the same reference numerals, and description thereof is omitted here.
[0025]
Similar to the first embodiment, also in this embodiment, the protection circuit 9 regulates the amount of leakage current Ileak by appropriately setting the resistance value R in the path through which the leakage current Ileak flows. In this case, the resistance value R of the protection circuit 9 is set so that the leakage current Ileak is smaller than the step value ΔI. Specifically, the resistance value R is set according to Equation 2. Here, the maximum value Vmax is the applied voltage Vab of the protection circuit 9 generated when the voltage of the data line X becomes maximum. For example, in the case of the 64 gradation display shown in FIG. 4, when the data current I 63 corresponding to the gradation 63 is supplied to the data line X (the voltage of the data line X becomes the maximum voltage), the protection circuit 9 The applied voltage Vab is maximized.
[Equation 2]
Ileak <ΔI
Vmax / R <ΔI
R> Vmax / ΔI
[0026]
Thus, the condition of Ileak <ΔI is satisfied in the entire voltage region that can be applied to the data line X. As a result, even if the leakage current Ileak flows through the protection circuit 9, the gradation shift of the pixel 2 can be suppressed, and the inversion of gradation between adjacent pixels can also be suppressed.
[0027]
In the present embodiment, an example in which a resistance element is used as the protection circuit 9 has been described. However, the present invention is not limited to this, and can be widely applied to various circuit configurations that prevent electrostatic breakdown of internal circuits, including the modified examples of FIGS. 8 and 9.
[0028]
FIG. 8 is a circuit diagram of a protection circuit 9 as a modification. The protection circuit 9 is composed of two diode arrays 9a and 9b in which diode-connected n-channel transistors are connected in series. One diode row 9a is connected to the data line X and a power supply line VHH to which a high voltage (a voltage higher than the power supply voltage Vdd) is supplied, and the other diode row 9b is connected to the data line and the low power supply line VHH. It is connected to a power supply line Vss to which a voltage is supplied. The resistance in the path through which the leak current Ileak flows corresponds to the resistance of the diode array 6a (or 6b), and this is set to the resistance value R satisfying the above-described conditions.
[0029]
FIG. 9 is a circuit diagram of a protection circuit 9 as another modification. In this configuration example, the diode array 9a formed of the n-channel transistor in FIG. 8 is changed to a p-channel transistor.
[0030]
In each of the above-described embodiments, the example in which the organic EL element OLED is used as the electro-optical element has been described. However, the present invention is not limited to this, and an electro-optical element (inorganic LED display device, field emission display device, etc.) whose luminance is set according to the drive current, or transmittance according to the drive current. -It can be widely applied to electro-optical devices (electrochromic display devices, electrophoretic display devices, etc.) exhibiting reflectance.
[0031]
In addition, the electro-optical device according to each of the above-described embodiments can be mounted on various electronic devices including, for example, a television, a projector, a mobile phone, a mobile terminal, a mobile computer, a personal computer, and the like. When the above-described electro-optical device is mounted on these electronic devices, the commercial value of the electronic devices can be further increased, and the product appeal of electronic devices in the market can be improved.
[0032]
【The invention's effect】
In the present invention, the resistance value in the path through which the leakage current flows in the protection circuit is set to a resistance value that does not cause a shift in the display gradation of the pixel due to the fluctuation of the data current due to the leakage current. As a result, it is possible to suppress gradation shift caused by the leakage current of the protection circuit.
[Brief description of the drawings]
FIG. 1 is a block configuration diagram of an electro-optical device according to a first embodiment.
FIG. 2 is a circuit diagram of a pixel as an example in a current programming method.
FIG. 3 is a circuit diagram of a protection circuit according to the first embodiment.
FIG. 4 is a schematic characteristic diagram showing a relationship between gradation and data current.
FIG. 5 is a circuit diagram of a protection circuit using a resistor as a modified example.
FIG. 6 is a circuit diagram of a protection circuit using a transistor as another modified example.
FIG. 7 is a block configuration diagram of an electro-optical device according to a second embodiment.
FIG. 8 is a circuit diagram of a protection circuit as a modified example.
FIG. 9 is a circuit diagram of a protection circuit as another modified example.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Display part 2 Pixel 3 Scan line drive circuit 4 Data line drive circuit 6, 9 Protection circuit T1-T4 Transistor C Capacitor
OLED organic EL device

Claims (5)

In an electro-optical device,
A plurality of data lines to which a data current for defining the gradation of the pixel is supplied;
A protection circuit connected to a pair of data lines among the plurality of data lines in order to prevent electrostatic breakdown of the internal circuit;
When a potential difference between the pair of data lines generated when a different gradation is designated for each of the pair of data lines is an applied voltage of the protection circuit,
Oite to the protection circuit, the resistance value in the path of leakage current flows by the applied voltage,
In the relationship between the gradation of the pixel and the data current, the resistance value is set such that the leakage current is smaller than ½ of the current change amount of the data current corresponding to the minimum gradation change. <br/> An electro-optical device characterized by the above. 2. The electro-optical device according to claim 1, wherein the resistance value is set based on a maximum value of a voltage applied to the protection circuit that is generated when a potential difference between the pair of data lines is maximized. apparatus. In an electro-optical device,
A plurality of data lines to which a data current for defining the gradation of the pixel is supplied;
Each of the data lines to prevent electrostatic breakdown of the internal circuit, and a protection circuit connected to a power supply line supplied with a predetermined voltage;
When the voltage generated in the data line when the gradation is designated is the applied voltage of the protection circuit,
In the protection circuit, the resistance value in the path through which the leakage current flows due to the applied voltage is :
In the relationship between the gradation of the pixel and the data current, the resistance value is set such that the leakage current is smaller than the current change amount of the data current corresponding to the minimum gradation change. An electro-optical device. The electro-optical device according to claim 3 , wherein the resistance value is set based on a maximum value of a voltage applied to the protection circuit that is generated when a voltage of the data line becomes maximum . Electronic apparatus, characterized in that mounting the electro-optical device according to any one of claims 1 to 4.

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