JP2005189388A - Display device, display device drive circuit, and display device drive method - Google Patents

Abstract

The present invention relates to a display device, a display device driving circuit, and a display device driving method. For example, the present invention is applied to a display device using an organic EL element, and a light emitting element is driven by a transistor having a source follower circuit configuration. Prevents surface roughness, black float, and contrast degradation.
The present invention relates to a terminal voltage of a signal level holding capacitor C2 provided between a gate and a source by a signal level Vin of a signal line SIG when the light emitting element 12 is driven by a transistor TR2 having a source follower circuit configuration. And the light emitting element 12 is driven by the voltage between the gate and the source due to the voltage between the terminals of the signal level holding capacitor C2, and the source side end of the transistor TR2 is connected to the reference voltage Vof. The voltage between terminals set in the signal level holding capacitor C2 is offset so that the gradation is lowered by the voltage Vof.
[Selection] Figure 1

Description

  The present invention relates to a display device, a display device drive circuit, and a display device drive method, and can be applied to, for example, a display device using an organic EL (Electro Luminescence) element. The present invention maintains the signal level by setting the terminal voltage of the signal level holding capacitor provided between the gate and the source according to the signal level of the signal line when the light emitting element is driven by a transistor having a source follower circuit configuration. For maintaining the signal level, the light source is driven by the voltage between the gate and the source due to the voltage between the terminals of the capacitor for use, and the source side end of this transistor is connected to the reference voltage and the gradation is lowered by this reference voltage. By offsetting the inter-terminal voltage set in the capacitor, the surface roughness, black floating, and contrast deterioration are prevented in the configuration in which the light emitting element is driven by the transistor having the source follower circuit configuration.

  Conventionally, in organic EL display devices, various applications to display devices have been proposed, for example, in US Pat. No. 5,684,365 and JP-A-8-234683.

  That is, as shown in FIG. 12, in this type of display device 1, the pixel unit 2 has scanning lines SCN provided in the horizontal direction in units of lines with respect to the pixels (PX) 3 arranged in a matrix. A signal line SIG is provided in the vertical direction for each column so as to be orthogonal to the scanning line SCN. For the pixel unit 2 formed in this way, the display device 1 drives the scanning lines SCN by the vertical drive circuit 4 to sequentially drive the pixels 3 of the pixel unit 2 in units of lines. The gray level of each pixel 3 is set by driving the signal line SIG by the horizontal drive circuit 5 so as to correspond to the above driving.

  For this reason, the vertical drive circuit 4 generates a write signal ws for sequentially instructing writing to each pixel 3 in line units by the write scan circuit (WSCN) 4A, and outputs the write signal ws to the scan line SCN. The setting of gradation in the pixel 3 is controlled. The horizontal drive circuit 5 generates a drive signal in accordance with the gradation data D1 indicating the gradation of each pixel 3, distributes this drive signal to each signal line SIG by the horizontal selector (HSEL) 5A, and outputs it. Thus, the display device 1 is configured to set the gradation of each pixel 3 in line units.

  In the organic EL display device, each pixel 3 driven in this manner includes an organic EL element that is a self-luminous element driven by current driving, and a driving circuit (hereinafter referred to as a pixel driving circuit) that drives the organic EL element. (Referred to as a pixel circuit).

  In the display device thus formed, each pixel circuit is formed by an n-channel MOS type TFT (Thin Film Transistor), and the anode of the organic EL element is connected to the transistor. By driving with current, an organic EL element and a pixel circuit can be integrally formed on a glass substrate by applying an amorphous silicon process. As a result, as shown in FIG. It is conceivable to drive the element 12.

  That is, the display device 11 shown in FIG. 14 is formed in each pixel 3 so that the organic EL element 12 is current-driven by the transistor TR2 having a source follower circuit configuration in which the source is connected to the anode of the organic EL element 12. A signal level holding capacitor C1 is provided at the gate of the transistor TR2. The display device 11 outputs the write signal ws from the write scan circuit 4A provided in the vertical drive circuit 4, and this signal level holding capacitor C1 by the switch circuit by the transistor TR1 that is turned on by the write signal ws. Is connected to the signal line SIG, whereby the gate voltage Vg of the transistor TR2 is set according to the signal level of the drive signal output to the signal line SIG in response to the write signal ws. As a result, the display device 11 drives the organic EL element 12 with a current corresponding to the gate voltage Vg set in this way, and emits the organic EL element 12 of each pixel 3 with a gradation corresponding to the gradation data D1. Thus, a desired image can be displayed.

  However, in an organic EL element, as shown in FIG. 14, the current-voltage characteristics change in a direction in which current hardly flows through use. In FIG. 14 and FIG. 15, the symbol L1 indicates the initial characteristics, and the symbol L2 indicates the characteristics due to changes over time. On the other hand, in the driving by the sofa follower circuit described above with reference to FIG. 13, the characteristic curve due to the load intersects the characteristic curve of drain-source voltage Vds−drain source current Ids of the transistor TR2, as shown in FIG. The intersecting point becomes the operating point. Accordingly, in the organic EL element, when the voltage-current characteristic is changed, the current flowing through the organic EL element is reduced correspondingly, thereby causing a problem that the luminance of each pixel is gradually lowered.

As one method for solving this problem, a method of controlling the drive current of the organic EL element 12 by setting the gradation based on the gate-source voltage Vgs can be considered. That is, in the drain current Ids of the TFT, it is expressed by (1/2) × μ × (W / L) Cox (Vgs−Vth) ² (1), thereby setting the gradation by the gate-source voltage Vgs. Thus, it is possible to prevent a change in drive current due to a change with time. Here, μ is the carrier mobility, W is the gate width, L is the gate length, Cox is the gate capacitance per unit area, and Vth is the threshold voltage.

  Accordingly, as shown in FIG. 16 in comparison with FIG. 13, in each pixel 23, the signal level is held between the gate and the source of the transistor TR2 instead of the arrangement of the capacitor C1 for holding the signal level with respect to the gate and drain of the transistor TR2. The capacitor C2 is disposed, and the signal level of the signal line SIG is set in the signal level holding capacitor C2. Further, the transistor TR3 that is turned on by the drive scan signal ds1 is connected to the source of the transistor TR2, and during the period in which the signal level of the signal line SIG is set in the signal level holding capacitor C2, the transistor TR3 causes the source potential of the transistor TR2 to be set. Is set to a constant potential, and the light emission of the organic EL element 12 is stopped. In FIG. 16, this constant potential is the ground potential.

  Corresponding to the configuration of the pixel portion 22 by such pixels 23, in the vertical drive circuit 24, in addition to the write scan circuit 24A, a drive scan signal is synchronized with the output of the write signal ws by the write scan circuit 24A. A drive scan circuit (DSCN) 24B for outputting ds1 is provided, and the horizontal drive circuit 25 generates a drive signal and outputs it from the horizontal selector 25A.

  Accordingly, in practice, as shown in FIGS. 17A to 17D and FIGS. 18A to 18D, the display device 21 thereby operates as an organic EL device by the on / off operation of the transistor TR3 by the drive scan signal ds1. The light emission and non-light emission of the element 12 are controlled. Further, the transistor TR1 is turned on by the write signal ws during the period in which the light emission of the organic EL element 12 is stopped by setting the source potential of the transistor TR2 to a constant potential by the on-control of the transistor TR3, and a signal level holding capacitor The signal level Vin of the signal line SIG is set to C2. Thereafter, the write signal ws is lowered to set the transistor TR1 to the OFF state, and then the drive scan signal ds1 is lowered, whereby the source voltage Vs rises and the signal level Vin set in the signal level holding capacitor C2 is set. The organic EL element 12 is driven by current driving according to the above. 18A to 18D are connection diagrams showing the settings of the transistors TR2 and TR3 corresponding to the periods TA to TD in FIG.

  In this way, when the drive current of the organic EL element 12 is controlled by the gradation setting by the gate-source voltage instead of the gradation setting by the gate-drain voltage Vgd, the characteristics of the organic EL element 12 are changed. Even in such a case, it can be driven by a constant current determined by the gradation data, thereby effectively avoiding deterioration of image quality due to a change with time.

  However, as can be seen from the above-described expression (1), even when the change with time due to the organic EL element 12 is prevented in this way, when the threshold voltage Vth of the transistor TR2 varies, each of the threshold voltages Vth The drive current in the pixel 23 varies, and the image quality deteriorates. As a method for preventing such image quality deterioration due to the variation in the threshold voltage Vth of the transistor TR2, the threshold of the transistor TR2 is set in advance when the signal level of the signal line SIG is set in the signal level holding capacitor C2. A method of setting the value voltage Vth in the capacitor C2 for holding the signal level is conceivable.

  That is, in this case, as shown in FIG. 19 in comparison with FIG. 16, the signal line SIG is connected to the signal level holding capacitor Cs2 provided between the gate and source of the transistor TR2 by the transistor TR1 via the coupling capacitor Cs1. Is set so that the gate side of the transistor TR2 is short-circuited to switch the transistor TR2 to the diode connection, and the transistor TR1 side end of the coupling capacitor Cs1 is set to the reference potential. A switch circuit by the transistor TR6 is provided. Thereby, in this display device 31, the terminal voltage of the signal level holding capacitor Cs2 is set by the signal level Vin of the signal line SIG via the capacitor Cs1 by the transistor TR1. In this case, due to the connection to the signal line SIG via the coupling capacitor Cs1, the voltage between the terminals of the signal level holding capacitor Cs2 is the voltage obtained by dividing the signal level Vin of the signal line SIG by the capacitors Cs1 and Cs2. ΔVin = Vin (Cs1 / (Cs1 + Cs2))... (2). Accordingly, the signal line SIG is driven by the horizontal drive circuit 35 in consideration of this relational expression.

  As a result, as shown in FIGS. 20 and 21, in this case as well, in the pixel 33, the organic EL element 12 is driven by the transistor TR2 by the gate-source voltage Vgs based on the voltage set in the signal level holding capacitor Cs2. Thus, image quality deterioration due to a change with time of the organic EL element 12 is prevented (FIGS. 20D to 20F and FIG. 21A).

  Thus, immediately before the terminal voltage of the capacitor Cs2 for holding the signal level is set by the signal level Vin of the signal line SIG, the display device 31 sets the transistor TR5 to the ON state by the control signal az and makes the transistor TR2 a diode. At the same time, the signal line side end of the coupling capacitor Cs1 is held at a predetermined reference potential (FIG. 20B and FIG. 21B), and then the drive scan signals ds1 and ds2 are switched, whereby the transistor The source of TR2 is set to the reference potential, and the supply of power to the transistor TR2 is stopped (FIGS. 20C, 20D, and 21C).

  As a result, in the transistor TR2, the gate voltage Vg that has risen temporarily decreases gradually, and when the gate-source voltage Vgs reaches the threshold voltage Vth, the decrease in the gate voltage Vg is stopped, thereby maintaining the signal level. The threshold voltage Vth of the transistor TR2 is set in the capacitor Cs2 (FIGS. 20E and 20F).

  As a result, the control signal az is switched to disconnect the signal line side end of the coupling capacitor Cs1 from the reference voltage, and after the diode connection of the transistor TR2 is stopped, the write signal ws is raised and passed through the coupling capacitor Cs1. Thus, the terminal voltage of the signal level holding capacitor Cs2 is set by the signal level Vin of the signal line SIG. As a result, the signal level holding capacitor Cs2 is corrected by the threshold voltage Vth of the transistor TR2, and a voltage corresponding to the signal level of the signal line SIG is set. In the transistor TR2, the capacitor Cs2 is set. The organic EL element 12 is current-driven by the gate-source voltage Vgs due to the voltage (FIG. 21E), and variations due to the threshold voltage Vth of the transistor TR2 can be prevented.

  Corresponding to the configuration of these pixels 33, in the display device 31, a write scan circuit 34A that outputs a write signal ws, a drive scan signal ds1, and a drive scan signal ds2 that output a write signal ws to the vertical drive circuit 34, respectively. 24B and 24C are provided, and an auto-zero circuit (ZERO) 34D that outputs a control signal az of the transistors TR5 and TR6 related to the correction of the threshold voltage Vth is provided. In the horizontal drive circuit 35, the horizontal selector 35A outputs a drive signal to each signal line SIG.

  In this way, even when the threshold voltage Vth of the transistor TR2 that drives the organic EL element 12 varies, it is possible to prevent image quality deterioration due to this variation.

  When the gradation of the organic EL element 12 is set by setting the terminal voltage of the signal level holding capacitor C2 as in the display device 21 described above with reference to FIG. 16, in the black gradation, the signal of the signal line SIG is set. The level Vin is set to 0 [V]. As a result, even if the voltage between the terminals of the signal level holding capacitor C2 is maintained, it is held at 0 [V] which is lower than the threshold voltage of the transistor TR2. Thus, originally, no current is supplied to the organic EL element 12 due to the cutoff of the transistor TR2, so that the organic EL element 12 does not emit any light.

  However, when the characteristic of the transistor TR2 is depletion, the threshold voltage Vth is lower than 0 [V]. Accordingly, in this case, even if the signal level of the signal line SIG is set to 0 [V] and the terminal voltage of the signal level holding capacitor C2 is set, a current flows through the transistor TR2, thereby the organic EL element 12 Emits light with a black gradation. Accordingly, in the display device 21 having the configuration shown in FIG. 16, for example, when the entire surface is set to black gradation, pixels that emit light slightly are generated, and so-called surface roughness that can be seen as if the display screen is rough is generated. There was a problem that occurred. Further, in the display based on the black level as described above, the organic EL element 12 emits light, although slightly, so-called black floating that can be seen as if the gradation based on the black level has floated to the white level side occurs. There was a problem, and there was a problem that the contrast was lowered on the black level side.

  On the other hand, in the display device 31 having the configuration shown in FIG. 19, the threshold voltage Vth of the transistor TR2 is set in advance in the signal level holding capacitor Cs2, thereby obtaining such depletion characteristics. Surface roughness, black float, and contrast reduction can be eliminated.

  However, in setting the threshold voltage Vth to the signal level holding capacitor Cs2, variation cannot be avoided, and the transistor TR2 itself for driving the organic EL element also varies. As a result, when the threshold voltage Vth of the transistor TR2 is corrected in this way, the term of -Vth in the parentheses described above with respect to the expression (1) cannot be completely canceled, and the signal level is maintained slightly. Variation occurs in the voltage set in the capacitor Cs2.

In such a variation, there is almost no problem when the original voltage set to the signal level holding capacitor Cs2 is high, whereas the voltage set to the signal level holding capacitor Cs2 When the voltage is low, the variation voltage becomes relatively large with respect to the original inter-terminal voltage, thereby affecting the display image. Accordingly, the display device 31 having the configuration shown in FIG. 19 also has a problem that surface roughness and black floating occur and the contrast decreases.
USP 5,684,365 JP-A-8-234683

  The present invention has been made in consideration of the above points. In a configuration in which a light emitting element is driven by a transistor having a source follower circuit configuration, a display device and a display device capable of preventing surface roughness, black floating, and contrast deterioration can be prevented. A drive circuit and a display device drive method are proposed.

  In order to solve such a problem, in the invention of claim 1, the pixel is applied to a display device having a pixel portion in which pixels driven by current drive are arranged in a matrix and a drive circuit for driving the pixel portion. A transistor with a source follower circuit that holds a signal level holding capacitor between the light emitting element and the gate source, and drives the light emitting element by a gate-source voltage by a voltage between terminals of the signal level holding capacitor, and a gate of the transistor The signal line switch circuit for connecting to the signal line and the source side switch circuit for connecting the source of the transistor to the reference voltage on the source side, and the drive circuit sets the source side switch circuit to the ON state. With the source potential of the transistor set to the reference voltage on the source side, drive the signal line switch circuit. By setting the terminal voltage of the capacitor for holding the signal level according to the signal level of the signal line, the voltage between the gate and source of the transistor used for driving the light emitting element is set, and the reference voltage on the source side is the signal level of the signal line The voltage between the terminals of the signal level holding capacitor set by the step is set to a voltage that is offset to the side where the gradation is lowered.

  According to a sixth aspect of the present invention, the pixel is applied to a display device in which pixels driven by current are arranged in a matrix, and the pixel holds a signal level holding capacitor between the light emitting element and the gate source, A transistor with a source follower circuit that drives a light emitting element by a gate-source voltage by a voltage between terminals of a capacitor for holding a level, and a terminal voltage of the capacitor for holding a signal level by connecting the gate of the transistor to a signal line for a certain period of time. The signal source switch circuit is set to the reference voltage on the source side during the period in which the gate of the transistor is connected to the signal line by at least the signal line switch circuit. A switch circuit on the source side to be connected, and the reference voltage on the source side is the signal level of the signal line. The terminal voltage of the capacitor for signal level holding set by gradation is to be a voltage that is offset to the side to decrease.

  According to a seventh aspect of the invention, the pixel is applied to a display device in which pixels of organic EL elements are arranged in a matrix, and the pixel holds a signal level holding capacitor between the organic EL element and the gate source. A transistor by a source follower circuit that drives an organic EL element by a gate-source voltage by a voltage between terminals of a signal level holding capacitor, and a signal level holding capacitor by connecting the gate of the transistor to a signal line for a certain period of time. And a signal line switch circuit for setting the terminal voltage of the transistor to the signal line at least during a period in which the gate of the transistor is connected to the signal line by the signal line switch circuit. A switch circuit on the source side connected to the reference voltage, and the reference voltage on the source side is a signal The voltage between the terminals of the capacitor for signal level holding set by the signal level, the gradation is to be a voltage that is offset to the side to decrease.

  According to another aspect of the invention, the pixel is applied to a drive circuit of a display device in which pixels driven by current are arranged in a matrix, and the pixel holds a signal level holding capacitor between the light emitting element and the gate source. A transistor with a source follower circuit that drives a light-emitting element by a gate-source voltage by a voltage between terminals of a capacitor for holding a signal level, and a capacitor for holding a signal level by connecting the gate of the transistor to a signal line for a certain period of time. And a signal line switch circuit for setting the terminal voltage of the transistor to the signal line at least during a period in which the gate of the transistor is connected to the signal line by the signal line switch circuit. A switch circuit on the source side connected to the reference voltage, and the reference voltage on the source side is a signal line The voltage across the terminals of the signal level holding capacitor set according to the signal level is offset to the gradation lowering side, and the drive circuit of the display device has a black level gradation in gradations other than the black level. The signal level drive signal of the signal line is generated by offsetting the signal level of each gradation corresponding to the tone to the side where the gradation is increased by the reference voltage on the capacitor side.

  In the invention of claim 9, the pixel is applied to a driving method of a display device having a pixel portion in which pixels driven by current drive are arranged in a matrix and a driving circuit for driving the pixel portion. And a transistor with a source follower circuit that holds a signal level holding capacitor between the gate and source and drives the light emitting element by a gate-source voltage by a voltage between terminals of the signal level holding capacitor, and a gate of the transistor as a signal line. And a switch circuit for a signal line connected to the source circuit and a source side switch circuit for connecting the source of the transistor to a reference voltage on the source side, and the display device driving method sets the source side switch circuit to an ON state. The switch circuit for the signal line with the source potential of the transistor set to the reference voltage on the source side By driving, the terminal voltage of the capacitor for holding the signal level is set according to the signal level of the signal line, thereby setting the gate-source voltage of the transistor used for driving the light emitting element, and the reference voltage on the source side is The voltage between the terminals of the signal level holding capacitor set by the signal level is set to a voltage that is offset to the side where the gradation is lowered.

  According to the configuration of claim 1, the pixel is applied between a light emitting element and a gate source when applied to a display device having a pixel portion in which pixels driven by current are arranged in a matrix and a driving circuit for driving the pixel portion. A signal follower circuit that holds a signal level holding capacitor and drives a light emitting element by a gate-source voltage by a voltage between terminals of the signal level holding capacitor, and a signal line that connects the gate of the transistor to the signal line And a source-side switch circuit that connects the source of the transistor to a source-side reference voltage, and the drive circuit sets the source-side switch circuit to an on state so that the source potential of the transistor is With the reference voltage set on the source side, drive the signal line switch circuit to determine the signal level of the signal line. By setting the terminal voltage of the capacitor for holding the signal level and setting the gate-source voltage of the transistor used for driving the light emitting element, the terminal voltage of the capacitor for holding the signal level is set according to the signal level of the signal line Thereafter, the light-emitting element can be current-driven by the transistor by the voltage between the gate and the source due to the voltage between the terminals of the signal level holding capacitor, thereby effectively avoiding the influence of the light-emitting element over time and driving the light-emitting element. be able to. At this time, when setting the terminal voltage of the signal level holding capacitor according to the signal level of such a signal line, the reference voltage on the source side connected to the source side is set to the signal level set by the signal level of the signal line. Since the voltage between the terminals of the holding capacitor is offset to the gradation-decreasing side, it is ensured that the light emitting element does not emit light at the black level by simply setting the reference voltage on the capacitor side. Thus, by correcting the threshold voltage of the transistor that drives the light emitting element, surface roughness, black floating, and deterioration of contrast can be prevented.

  Thereby, according to the structure of Claim 6 and Claim 7, it applies to the display apparatus which arrange | positions the pixel by an electric current drive, and the pixel by an organic EL element in a matrix form, and is made surface roughness, a black floating, and contrast degradation. According to the configuration of the eighth aspect, it is possible to provide a display device driving circuit capable of preventing surface roughness, black floating, and contrast deterioration. According to the ninth aspect of the present invention, there is provided a display device that corrects the threshold voltage of the transistor that drives the light emitting element. The display device can prevent surface roughness, black floating, and contrast deterioration. A driving method can be provided.

  According to the present invention, in a configuration in which a light emitting element is driven by a transistor with a source follower circuit configuration, surface roughness, black floating, and contrast deterioration can be prevented.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings as appropriate.

(1) Configuration of Embodiment FIG. 1 is a connection diagram showing a display device according to Embodiment 1 of the present invention in comparison with FIG. In the display device 51, the pixel unit 52 includes pixels 53 that are current-driven and arranged in a matrix. For these pixels 53, scanning lines SCN and SCN 1 are provided in the horizontal direction in units of lines. A signal line SIG is provided in the vertical direction for each column so as to be orthogonal to the scanning lines SCN and SCN1. For the pixel portion 52 formed in this way, the display device 51 drives the scanning lines SCN and SCN1 by the vertical drive circuit 54 to control the operation of the pixel circuit provided in the pixel 53 sequentially in line units. In addition, the gray level of each pixel 53 is set by driving the signal line SIG by the horizontal drive circuit 55 so as to correspond to the control of the pixel circuit.

  Therefore, the vertical drive circuit 54 generates a write signal ws for sequentially instructing writing to each pixel 53 line by line by the write scan circuit (WSCN) 54A, and generates a drive scan signal ds1 in synchronization with the write signal ws. The drive scan circuit 54B generates the write signal ws and the drive scan signal ds1 to the scan lines SCN and SCN1 to control the gradation setting in each pixel 53. In the horizontal drive circuit 55, a drive signal is generated according to the gradation data D1 indicating the gradation of each pixel 53, and this drive signal is distributed to each signal line SIG by a horizontal selector (HSEL) 55A and output. It is made like that.

  In the pixel 53 according to the display device 51, the source of the transistor TR2 is connected to the reference voltage Vof by the transistor TR3. This reference voltage Vof is the signal level of the signal line SIG to the signal level holding capacitor C2. Is set to a power supply that gives a negative offset voltage. Each pixel 53 is configured the same as the pixel 13 of the display device 21 in FIG. 16 except that the setting of the reference voltage Vof related to the transistor TR3 is different.

  Specifically, the reference voltage Vof is set to a positive power supply voltage that rises by a predetermined potential on the power supply Vcc side. In addition, the voltage value is set to a voltage exceeding the threshold voltage Vth even when the threshold voltage Vth of the transistor TR2 varies. The terminal voltage of the signal level holding capacitor C2 is set so that the transistor TR2 can be surely cut off. For this reason, in this embodiment, the power supply circuit for supplying various power supplies to the pixel unit 52 is formed so that the reference voltage Vof can be adjusted. By this adjustment, the reference voltage Vof is changed depending on the gradation of the black level. Each pixel 53 is set to a voltage that does not emit light.

  As a result, in the pixel 53, as shown in FIG. 2, in setting the gradation based on the black level, the transistor TR2 is surely cut off to prevent the organic EL element 12 from being driven. In order to prevent a decrease in contrast. FIG. 2 shows an example in which gradation is displayed by the black level.

  That is, in this case, as shown in FIG. 2, in this pixel 53, the organic EL element 12 is driven by the gate-source voltage Vgs based on the inter-terminal voltage held in the signal level holding capacitor C2, and the drive scan signal Light emission and non-light emission are controlled by ds1 (FIGS. 2B to 2D). In the display device 51, the non-light emission setting by the drive scan signal ds1 causes the source voltage of the transistor TR2 to be held at the reference voltage Vof. In this manner, the source voltage of the transistor TR2 is held to the reference voltage Vof. The write signal ws rises during this non-light emission period (FIG. 2A), whereby the gate of the transistor TR2 is connected to the signal line SIG by the transistor TR1, and the terminal voltage of the signal level holding capacitor C2 Is set to the signal level Vin of the signal line SIG.

  In this case, the other end of the signal level holding capacitor C2 is connected to the source of the transistor TR2 and held at the reference voltage Vof. By setting the terminal voltage based on the signal level Vin of the signal line SIG, the inter-terminal voltage is set to Vin−Vof, whereby the signal level Vin of the signal line SIG is offset by the reference voltage Vof to the side where the gradation is lowered. Is set.

  Thus, in the pixel 53, even when the characteristic of the transistor TR2 is depletion, for the black level gradation, the gate-source voltage Vgs of the transistor TR2 is set to be equal to or lower than the threshold voltage of the transistor TR2. Thus, the organic EL element 12 can be prevented from emitting light, thereby preventing surface roughness, black floating, and contrast deterioration.

(2) Operation of Embodiment In the above configuration, the display device 51 (FIG. 2) is configured such that the signal line SIG driven by the horizontal drive circuit 55 sequentially in units of lines by driving the scanning lines SCN and SCN1 by the vertical drive circuit 54. Is set to each pixel 53. In the display device 51, each pixel 53 emits light according to the signal level set for each pixel 53, and a desired image is displayed.

  In the display device 51, in each pixel 53, a signal level holding capacitor C2 is provided between the gate and the source of the transistor TR2 having a source follower circuit configuration for driving the organic EL element 12, and the transistor TR2 is configured by a switch circuit using the transistor TR1. By connecting the gate to the signal line SIG, the terminal voltage of the signal level holding capacitor C2 is set to the signal level Vin of the signal line SIG. Further, the organic EL element 12 is driven by the transistor TR2 by the gate-source voltage Vgs based on the voltage between the terminals of the signal level holding capacitor C2 set in this way.

  As a result, in the display device 51, it is possible to prevent a change in drive current due to a change with time of the voltage / current characteristics of the organic EL element 12, and to effectively avoid deterioration of image quality due to a change with time of the organic EL element 12. Further, each pixel 53 is formed by an n-channel MOS transistor, and the organic EL element 12 can be driven from the anode side by the transistor TR2. With these, an amorphous silicon process is applied to form the organic EL element and the pixel circuit. It can be integrally formed on a glass substrate.

  In the display device 51, in setting the terminal voltage of the capacitor C2 for holding the signal level based on the signal level Vin of the signal line SIG, the source of the transistor TR2 is set to the reference voltage Vof by the transistor TR3, thereby holding the signal level. In the capacitor C2, the terminal voltage at one end is set by the signal level Vin of the signal line SIG while the other end side is held at the reference voltage Vof, and the inter-terminal voltage is set at Vin-Vof. Thereby, in the signal level holding capacitor C2, the voltage between the terminals is set to be offset to a voltage lower by the reference voltage Vof so as to lower the gradation.

  In this embodiment, even when the characteristic of the transistor TR2 is depletion, for the black level gradation, the reference voltage Vof to be offset to the voltage that can surely cut off the transistor TR2 is set. Therefore, in the display device 51, the black level gradation is set to a voltage that does not cause the organic EL element 12 to emit light by a simple configuration that only sets the reference voltage Vof. Can be prevented.

(3) Effects of the embodiment According to the above configuration, when the light emitting element is driven by the transistor having the source follower circuit configuration, the terminal of the signal level holding capacitor provided between the gate and the source by the signal level of the signal line The voltage is set, and the light emitting element is driven by the voltage between the gate and the source due to the voltage between the terminals of the signal level holding capacitor. The source side end of the transistor is connected to the reference voltage, and the reference voltage By offsetting the inter-terminal voltage set in the signal level holding capacitor so as to lower the tone, the structure in which the light emitting element is driven by the transistor by the source follower circuit configuration, the surface roughness, black floating, Degradation of contrast can be prevented.

  FIG. 3 is a connection diagram showing a display apparatus according to the second embodiment of the present invention in comparison with FIG. In this display device 61, a switch circuit by a transistor TR4 is provided between the drain of the transistor TR2 and the power source Vcc, and the power supply to the transistor TR2 is controlled by this switch circuit. The display device 61 has the same configuration as the display device 51 of the first embodiment except that the configuration related to the transistor TR4 is different. Thus, in the display device 61, the vertical drive circuit 64 is provided with a write scan circuit 64A that outputs a write signal ws and a drive scan circuit 64B that outputs a drive scan signal ds1, and further, a drive scan related to the control of the transistor TR4. A drive scan circuit 64C for outputting the signal ds2 is provided. The horizontal drive circuit 65 generates a drive signal in accordance with the gradation data D1, and outputs this drive signal from the horizontal selector 65A.

  As shown in FIG. 4, in this display device 61, instead of the transistor TR3, the light emission and non-light emission of the organic EL element 12 are controlled by the control of the transistor TR4. The circuit consumes no power and reduces the overall power consumption.

  In addition, by controlling light emission and non-light emission in this way, the write signal ws is raised at a predetermined timing in the non-light emission period, and the terminal voltage of the signal level holding capacitor C2 is set by the signal level Vin of the signal line SIG. To do. In this way, in the period in which the terminal voltage of the signal level holding capacitor C2 is set by the signal level Vin of the signal line SIG, the transistor TR3 is set to the on state, and the voltage at the other end of the signal level holding capacitor C2 is set. Is maintained at the reference voltage Vof.

  As a result, also in this embodiment, the source-side end of the transistor is connected to the reference voltage, and the voltage between the terminals set in the signal level holding capacitor is offset so that the gradation is lowered by this reference voltage. ing.

  According to the configuration of FIG. 3, even if the supply of power to the transistor TR2 is controlled by the switch circuit by the transistor TR4 provided in the power supply Vcc to control light emission and non-light emission, the same effect as in the first embodiment can be obtained. Can be obtained.

  FIG. 5 is a connection diagram showing a display apparatus according to the second embodiment of the present invention in comparison with FIG. In the display device 71, similarly to the display device 31 described above with reference to FIG. 19, the transistor TR4 controls the light emission and non-light emission of the organic EL element 12, and the transistors TR5 and TR6 are turned on during the non-light emission period. Then, the threshold voltage Vth of the transistor TR2 is set in the signal level holding capacitor Cs2, and the terminal voltage of the signal level holding capacitor Cs2 is set by the transistor TR1 by the signal level Vin of the signal line SIG. . As a result, in this embodiment, image quality deterioration due to a change with time of the organic EL element 12 and a variation in the threshold voltage Vth of the transistor TR2 is prevented.

  Therefore, each pixel 73 of the pixel unit 72 is provided with a configuration relating to correction of the threshold voltage Vth by the coupling capacitor Cs1 and the transistors TR5 and TR6. In the vertical drive circuit 74, the write signal ws, A write scan circuit 74A and drive scan circuits 74B and 74D that output drive scan signals ds1 and ds2, respectively, are provided, and an auto-zero circuit 74D that outputs a control signal az related to the control of the transistors TR5 and TR6 is provided. Yes.

  As shown in FIGS. 6 and 7 in comparison with FIGS. 20 and 21, in this display device 71, after setting the threshold voltage Vth of the transistor TR2 to the signal level holding capacitor Cs2 in this way, the transistor When the terminal voltage of the signal level holding capacitor Cs2 is set by the signal level Vin of the signal line SIG by TR1, the other end of the signal level holding capacitor Cs2 is connected to the reference voltage Vof by the transistor TR3. As a result, also in this embodiment, the reference voltage Vof is used to offset the voltage across the terminals of the signal level holding capacitor Cs2 set by the signal level Vin of the signal line SIG to the side where the gradation is lowered. ing. FIG. 20 shows a case where the organic EL element 12 is driven by a black level gradation.

  Specifically, the reference voltage Vof is set to a positive power supply voltage that rises by a predetermined potential on the power supply Vcc side. As for the voltage value, the threshold voltage Vth of the transistor TR2 is set in the signal level holding capacitor Cs2, and even if the threshold voltage Vth is set and the transistor TR2 varies, the black level is reduced. The capacitor Cs2 for holding the signal level is set by adjustment, and the voltage for surely cutting off the transistor TR2 is set to the voltage for setting the capacitor Cs2 for holding the signal level. For this reason, in this embodiment, the power supply circuit for supplying various power supplies to the pixel unit 52 is formed so that the reference voltage Vof can be adjusted.

  Thus, in this pixel 73, in setting the gradation based on the black level, the transistor TR2 is surely cut off to prevent the organic EL element 12 from being driven, thereby preventing surface roughness, black floating, and contrast reduction. It is made like that.

  Corresponding to the supply of the offset voltage by the reference voltage Vof in the pixel 73, in the display device 71, the signal driver 75B corresponds to the gradation indicated by the gradation data D1 in FIGS. A drive signal is generated with the characteristics shown and output to the signal line. Here, FIG. 8 is a characteristic curve diagram showing the gradation based on the voltage Vcs2 between the terminals of the capacitor Cs2 for holding the signal level and the gradation data D1, and FIG. 9 is the signal level Vin of the drive signal output to the signal line SIG. FIG. 6 is a characteristic curve diagram showing the relationship between and gradation data D1. Symbol L3 is a characteristic curve when the source of the transistor TR2 is connected to the ground potential by the transistor TR3, and symbol L4 is a characteristic curve according to this embodiment.

  Usually, the drive signal of this type of display device is set so that the signal level sequentially increases from 0 level as the gray level increases corresponding to the light emission characteristics of the organic EL element 12 (FIG. 8, reference L3). As a result, when the transistor TR3 connects the source of the transistor TR2 to the ground potential to display a black level gradation, the voltage Vcs2 between the terminals of the signal level holding capacitor Cs2 is set to the threshold voltage Vth of the transistor TR2. As a result, the transistor TR2 is originally held in the cut-off state and cannot drive the organic EL element 12, and a black level gradation is expressed.

  However, in this embodiment, by offsetting the voltage between terminals set in the signal level holding capacitor Cs2 by the reference voltage Vof, the transistor TR3 has the same characteristics as when the source of the transistor TR2 is connected to the ground potential. When the drive signal is generated, the gradation sinks on the black side, making it difficult to correctly express the gradation, and in the gradation following the black level, surface roughness in the case of the black level gradation, etc. Will cause problems.

  For this reason, the signal driver 75B generates a drive signal with the same signal level as that when the source of the transistor TR2 is connected to the ground potential by the transistor TR3. For other than the black level, a drive signal is generated with a voltage higher by a level corresponding to the reference voltage Vof than when the source of the transistor TR2 is connected to the ground potential by the transistor TR3. For the high voltage corresponding to the reference voltage Vof, the relationship described above with respect to the expression (2) by setting the terminal voltage of the signal level holding capacitor Cs1 via the coupling capacitor Cs1. The voltage according to the equation. As a result, in this embodiment, in the gradation other than the black level, the signal level of each gradation corresponding to the gradation of the black level is offset to the side where the gradation is increased by the reference voltage Vof on the source side. A drive signal for the line SIG is generated, whereby the voltage between terminals of the signal level holding capacitor Cs2 is offset to prevent surface roughness and the like so that an image can be displayed with a correct gradation. Yes.

  According to this embodiment, after setting the threshold voltage of the transistor TR2 to the signal level holding capacitor Cs2, the terminal voltage of the signal level holding capacitor Cs2 is set according to the signal level of the signal line. By connecting the source side end of this transistor to a reference voltage and offsetting the voltage between terminals set in the signal level holding capacitor so that the gradation is lowered by this reference voltage, surface roughness, black floating, contrast Can be prevented.

  FIG. 10 is a connection diagram illustrating the display apparatus according to the fourth embodiment of the present invention. In the display device 71, the transistor TR3 that grounds the source of the transistor TR2 to the reference voltage Vof is controlled by the write signal ws. As a result, in the display device 71, the drive scan signal ds1, which is a control signal of the transistor TR3, is omitted, and the pixels 73 can be arranged at a higher density, and the configuration of the vertical drive circuit 74 is simplified. It has been made possible.

  The display device 81 is configured in the same manner as the display device 71 according to the third embodiment except that the control related to the write signal ws is different. Thus, the vertical drive circuit 84 is provided with a write scan circuit 84A, a drive scan circuit 84C, and an auto zero circuit 84D that output a write signal ws, a drive scan signal ds2, and a control signal az, respectively. In the horizontal drive circuit 85, a drive signal is generated by the signal driver 85B according to the characteristics described above with reference to FIG. 9, and is distributed to each signal line SIG by the horizontal selector 85A.

  Even when the transistor TR3 that grounds the source of the transistor TR2 to the reference voltage Vof is controlled by the write signal ws as in this embodiment, the same effect as that of the third embodiment can be obtained.

  FIG. 11 is a connection diagram illustrating a display apparatus according to the fifth embodiment of the present invention. In the display device 91, the transistor TR3 that grounds the source of the transistor TR2 to the reference voltage Vof is controlled by a control signal az that instructs correction of the threshold voltage Vth. As a result, in this display device 91, the drive scan signal ds1, which is the control signal for the transistor TR3, is omitted, and the pixels 93 can be arranged at a higher density, and the configuration of the vertical drive circuit 94 is simplified. It has been made possible.

  The display device 91 is configured in the same manner as the display device 71 according to the third embodiment except that the control related to the write signal ws is different. Thus, the vertical drive circuit 94 is provided with a write scan circuit 94A, a drive scan circuit 94C, and an auto zero circuit 94D for outputting a write signal ws, a drive scan signal ds2, and a control signal az, respectively. In the horizontal drive circuit 95, a drive signal is generated by the signal driver 95B according to the characteristics described above with reference to FIG. 9, and is distributed to each signal line SIG by the horizontal selector 95A.

  As in this embodiment, even when the transistor TR3 that grounds the source of the transistor TR2 to the reference voltage Vof is controlled by the control signal az instructing the correction of the threshold voltage Vth, the same effect as in the third embodiment is obtained. be able to.

  In the above-described embodiment, the case where the gradation is corrected by the signal driver which is a digital / analog conversion circuit has been described. However, the present invention is not limited to this, and for example, the gradation is corrected at the front stage of the digital / analog conversion circuit. It may be.

  In the above-described embodiments, the case where the pixel circuit is formed by the n-channel type transistor has been described. However, the present invention is not limited to this, and for example, the present invention can be widely applied to the case where the pixel circuit is formed by the p-channel type transistor. can do.

  Further, in the above-described embodiment, the case where the black level is set in the pixel portion by setting the reference voltage Vof and the gradation is corrected on the horizontal drive circuit side has been described. If sufficient characteristics can be ensured, the correction of gradation on the horizontal drive circuit side may be omitted.

  In the above-described embodiments, the case where the light emitting element by the organic EL element is current-driven has been described. However, the present invention is not limited to this, and can be widely applied to display devices using various light-emitting elements related to current driving. .

  The present invention relates to a display device, a display device drive circuit, and a display device drive method, and can be applied to a display device using an organic EL element, for example.

It is a connection diagram which shows the pixel of the display apparatus which concerns on Example 1 of this invention with a periphery structure. 2 is a time chart for explaining the operation of the pixel in FIG. 1. It is a connection diagram which shows the pixel of the display apparatus which concerns on Example 2 of this invention with a periphery structure. 4 is a time chart for explaining the operation of the pixel in FIG. 3. It is a connection diagram which shows the pixel of the display apparatus which concerns on Example 3 of this invention with a periphery structure. 6 is a time chart for explaining the operation of the pixel in FIG. 5. FIG. 7 is a connection diagram for explaining the time chart of FIG. 6. FIG. 6 is a characteristic curve diagram showing a relationship between a voltage between terminals and a gradation of a signal level holding capacitor according to the pixel of FIG. 5. FIG. 6 is a characteristic curve diagram illustrating a relationship between a driving signal of a signal line and a gray level related to the pixel of FIG. It is a connection diagram which shows the pixel of the display apparatus which concerns on Example 4 of this invention with a periphery structure. It is a connection diagram which shows the pixel of the display apparatus which concerns on Example 5 of this invention with a periphery structure. It is a block diagram which shows the structure of a display apparatus. It is a connection diagram which shows the structure of the display apparatus by an organic EL element. It is a characteristic curve figure which shows the characteristic of an organic EL element. It is a characteristic curve figure with which it uses for description of the change of the operating point of an organic EL element. FIG. 3 is a connection diagram illustrating a pixel circuit having a source follower circuit configuration along with a peripheral configuration. 17 is a time chart for explaining the operation of the pixel circuit of FIG. FIG. 18 is a connection diagram for explaining the time chart of FIG. 17. It is a connection diagram showing a display device relating to threshold voltage correction. 20 is a time chart for explaining the operation of the display device of FIG. 19. FIG. 21 is a connection diagram for explaining the time chart of FIG. 20.

Explanation of symbols

1, 11, 21, 31, 51, 61, 71, 81, 91... Display device, 2, 22, 32, 52, 62, 72, 82, 92... Pixel portion, 3, 23, 33, 53, 63, 73, 83, 93 ... Pixel 4, 24, 34, 54, 64, 74, 84, 94 ... Vertical drive circuit, 4A, 24A, 34A, 54A, 64A, 74A, 84A, 94A ... Light Scan circuit 5, 25, 35, 55, 65, 75, 85, 95 ... Horizontal drive circuit, 12 ... Organic EL element, 24B, 34B, 34C, 54B, 64B, 64C, 74B, 74C, 84C, 94C ... Drive scan circuit, 34D, 74D, 84D, 94D ... Auto-zero circuit, C1, C2, Cs1, Cs2 ... Capacitor, TR1 to TR6 ... Transistor

Claims (9)

In a display device having a pixel portion in which pixels driven by current are arranged in a matrix and a drive circuit for driving the pixel portion,
The pixel is
A light emitting element;
A transistor having a source follower circuit that holds a signal level holding capacitor between a gate and a source and drives the light emitting element by a gate-source voltage based on a voltage between terminals of the signal level holding capacitor;
A switch circuit for a signal line connecting the gate of the transistor to the signal line;
A source-side switch circuit that connects a source of the transistor to a source-side reference voltage;
The drive circuit is
With the source-side switch circuit set to an ON state and the source potential of the transistor set to the source-side reference voltage, the signal line switch level is driven by driving the signal line switch circuit. By setting a terminal voltage of the capacitor for holding the signal level, a gate-source voltage of the transistor used for driving the light emitting element is set,
The display device, wherein the reference voltage on the source side is a voltage for offsetting a voltage between terminals of the signal level holding capacitor set by a signal level of the signal line to a side where gradation is lowered. . The drive circuit is
In the gradation other than the black level, the signal level of each gradation corresponding to the gradation of the black level is offset to the side of increasing the gradation by the reference voltage on the source side, and the drive signal of the signal line is The display device according to claim 1, wherein the display device is generated. The display device according to claim 1, wherein the transistor and each switch circuit are formed by an n-channel MOS transistor. The pixel is
A power supply switch circuit for stopping supply of power to the transistor;
The drive circuit is
The power supply to the transistor is stopped by driving the power switch circuit during a period in which at least the terminal voltage of the signal level holding capacitor is set according to the signal level of the signal line. Item 4. The display device according to Item 1. The pixel is
A coupling capacitor disposed between the switch circuit for the signal line and the gate of the transistor and mediating connection of the signal line to the gate of the transistor by the switch circuit for the signal line;
A switch circuit for short circuit that short-circuits the gate source of the transistor;
A capacitor-side switch circuit that connects the signal line side end of the coupling capacitor to a capacitor-side reference voltage;
The drive circuit is
The capacitor-side switch circuit and the short-circuit switch circuit are set to an on state, and the power supply switch circuit is set to an off state, whereby the voltage between the terminals of the signal level holding capacitor is set to the transistor. After setting the threshold voltage of
The display device according to claim 1, wherein a terminal voltage of the capacitor for holding the signal level is set according to a signal level of the signal line. In a display device in which pixels driven by current are arranged in a matrix,
The pixel is
A light emitting element;
A transistor having a source follower circuit that holds a signal level holding capacitor between a gate and a source and drives the light emitting element by a gate-source voltage based on a voltage between terminals of the signal level holding capacitor;
A signal line switch circuit for connecting a gate of the transistor to a signal line for a certain period, and setting a terminal voltage of the signal level holding capacitor according to a signal level of the signal line;
A source-side switch circuit that connects the source of the transistor to a source-side reference voltage during a period in which the gate of the transistor is connected to the signal line by at least the signal line switch circuit;
The display device, wherein the reference voltage on the source side is a voltage for offsetting a voltage between terminals of the signal level holding capacitor set by a signal level of the signal line to a side where gradation is lowered. . In a display device in which pixels by organic EL elements are arranged in a matrix,
The pixel is
The organic EL element;
A transistor by a source follower circuit that holds a signal level holding capacitor between a gate source and drives the organic EL element by a gate-source voltage by a voltage between terminals of the signal level holding capacitor;
A signal line switch circuit for connecting a gate of the transistor to a signal line for a certain period, and setting a terminal voltage of the signal level holding capacitor according to a signal level of the signal line;
A source-side switch circuit that connects the source of the transistor to a source-side reference voltage during a period in which the gate of the transistor is connected to the signal line by at least the signal line switch circuit;
The display device, wherein the reference voltage on the source side is a voltage for offsetting a voltage between terminals of the signal level holding capacitor set by a signal level of the signal line to a side where gradation is lowered. . In a display device drive circuit in which pixels driven by current are arranged in a matrix,
The pixel is
A light emitting element;
A transistor having a source follower circuit that holds a signal level holding capacitor between a gate and a source and drives the light emitting element by a gate-source voltage based on a voltage between terminals of the signal level holding capacitor;
A signal line switch circuit for connecting a gate of the transistor to a signal line for a certain period, and setting a terminal voltage of the signal level holding capacitor according to a signal level of the signal line;
A source-side switch circuit that connects the source of the transistor to a source-side reference voltage during a period in which the gate of the transistor is connected to the signal line by at least the signal line switch circuit;
The reference voltage on the source side is a voltage for offsetting the voltage between terminals of the signal level holding capacitor set by the signal level of the signal line to the side on which gradation is lowered,
The display device drive circuit comprises:
In the gradation other than the black level, the signal level of each gradation corresponding to the gradation of the black level is offset to the gradation increasing side by the reference voltage on the capacitor side, and the drive signal of the signal line is A display circuit driving circuit. In a driving method of a display device having a pixel portion in which pixels driven by current drive are arranged in a matrix and a driving circuit for driving the pixel portion,
The pixel is
A light emitting element;
A transistor having a source follower circuit that holds a signal level holding capacitor between a gate and a source and drives the light emitting element by a gate-source voltage based on a voltage between terminals of the signal level holding capacitor;
A switch circuit for a signal line connecting the gate of the transistor to the signal line;
A source-side switch circuit that connects a source of the transistor to a source-side reference voltage;
The driving method of the display device is:
With the source-side switch circuit set to an ON state and the source potential of the transistor set to the source-side reference voltage, the signal line switch level is driven by driving the signal line switch circuit. By setting a terminal voltage of the capacitor for holding the signal level, a gate-source voltage of the transistor used for driving the light emitting element is set,
The display device, wherein the reference voltage on the source side is a voltage for offsetting a voltage between terminals of the signal level holding capacitor set by a signal level of the signal line to a side where gradation is lowered. Driving method.

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