JP2007011214A - Pixel circuit, display device, and driving method of pixel circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pixel circuit and a display device, and a driving method of the pixel circuit that can prevent luminance variance during high-luminance display, prevent deterioration in picture quality without spoiling signal writing response during low-luminance display, and display images of high quality. <P>SOLUTION: The pixel circuit 101 includes a voltage-current converting circuit which converts a voltage signal into a current signal and a current duplicating circuit which refers to a current for high luminance and generates its duplicate current; and the voltage-current converting circuit includes a capacitor C111 for correcting a threshold Vth and the current and a TFT 112 as a reset switch for initialization and a current reference circuit includes a current mirror circuit 120 formed including p-channel TFTs 111 to 113 and has TFTs 115 and 116 as switches for transmitting charges of the current which is referenced to the capacitor C111, and duplicates and transmits an arbitrary reference current during high-luminance display to the voltage-current converting circuit. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a pixel circuit having an electro-optic element whose luminance is controlled by a signal line including an active matrix display device such as an organic EL (Electroluminescence) display device and an LCD (Liquid Crystal Display device), a display device, and a pixel circuit. It is about the method.

In an active matrix display device, an electro-optical element such as a liquid crystal cell or an organic EL element is used as a display element of a pixel.
Among them, the organic EL element has a structure in which a layer made of an organic material, that is, an organic layer is sandwiched between electrodes.
In this organic EL element, by applying a voltage to the element, electrons from the cathode and holes from the anode are injected into the organic layer. As a result, the electrons and holes are recombined to generate light. This organic EL element has the following features.

(1) Since a luminance of several hundred to several tens of thousands of cd / m ² can be obtained by driving at a low voltage of 10 V or less, power consumption can be reduced.
(2) Since it is a self-luminous element, the contrast of the image is high and the response speed is fast, so that the visibility is good and it is also suitable for displaying moving images.
(3) It is an all solid state element having a simple structure, and the element can be made highly reliable and thin.

  An organic EL display device using an organic EL element having these features as a pixel display element (hereinafter referred to as an organic EL display) is considered promising as a next-generation flat panel display.

  By the way, as a driving method of the organic EL display, there are a simple matrix method and an active matrix method. Among these methods, the active matrix method has the following features.

(1) An active matrix system that can hold light emission of an organic EL element in each pixel for one frame period is suitable for high definition and high luminance of an organic EL display.
(2) Since a peripheral circuit using a thin film transistor can be formed over a substrate (panel), the interface with the outside of the panel can be simplified and the function of the panel can be enhanced.

In this active matrix organic EL display, a polysilicon thin film transistor (TFT) using polysilicon as an active layer is generally used as a transistor as an active element.
This is because the polysilicon TFT has a high driving capability and can be designed to have a small pixel size, which is advantageous for high definition.

By the way, it is well known that the polysilicon TFT has the above-mentioned features, but has a large variation in characteristics.
Therefore, in the case of using a polysilicon TFT, it is a big problem in an active matrix type organic EL display using a polysilicon TFT to suppress the characteristic variation and to compensate for the TFT characteristic variation in a circuit. This is due to the following reason.

  That is, in a liquid crystal display using a liquid crystal cell as a pixel display element, the luminance data of each pixel is controlled by a voltage value, whereas in an organic EL display, the luminance data of each pixel is controlled by a current value. It is because the structure to control is taken.

Here, an outline of the active matrix organic EL display will be described.
FIG. 1 is a diagram illustrating an outline of a configuration of a general active matrix organic EL display, and FIG. 2 is a circuit diagram illustrating a configuration example of a pixel circuit of an active matrix organic EL display (for example, Patent Documents). 1 and 2).

  The active matrix organic EL display 1 includes m × n pixel circuits 10 arranged in a matrix, and signal lines for n columns driven by a data driver (DDRV) 2 with respect to the matrix arrangement of the pixel circuits 10. SGL1 to SGLn are wired for each pixel column, and m rows of scanning lines SCNL1 to SCNLm driven by the scan driver (SDRV) 3 are wired for each pixel row.

Further, as shown in FIG. 2, the pixel circuit 10 includes a p-channel TFT 11, an n-channel TFT 12, a capacitor C11, and a light emitting element 13 made of an organic EL element (OLED).
The TFT 11 of each pixel circuit 10 has a source connected to the power supply potential line VCCL and a gate connected to the drain of the TFT 12. The organic EL light emitting element 13 has an anode connected to the drain of the TFT 11 and a cathode connected to a reference potential (for example, ground potential) GND.
The TFT 12 of each pixel circuit 10 is connected to the signal lines SGL1 to SGLn of the column corresponding to the source, and to the scanning lines SCNL1 to SCNLm of the row corresponding to the gate.
The capacitor C11 has one end connected to the power supply potential line VCCL and the other end connected to the drain of the TFT 12.

  Since organic EL elements often have rectifying properties, they are sometimes referred to as OLEDs (Organic Light Emitting Diodes). In FIG. 2 and other figures, diode symbols are used as light-emitting elements. However, it does not necessarily require rectification.

In the pixel circuit 10 having such a configuration, in a pixel to which luminance data is written, a pixel row including the pixel is selected by the scan driver 3 via the scanning line SCNL, so that the TFT 12 of the pixel in the row is Turn on.
At this time, the luminance data is supplied as a voltage from the data driver 2 through the signal line SGL, and is written into the capacitor C11 that holds the data voltage through the TFT 12.
The luminance data written in the capacitor C11 is held for one field period. The held data voltage is applied to the gate of the TFT 11.
Thereby, TFT11 drives the organic EL light emitting element 13 with an electric current according to holding | maintenance data. At this time, the gradation expression of the organic EL light emitting element 13 is performed by modulating the gate-source voltage Vdata (<0) of the TFT 11 held by the capacitor C11.

  In general, the luminance Loled of the organic EL element is proportional to the current Ioled flowing through the element. Therefore, the following equation (1) is established between the luminance Loled of the organic EL light emitting element 13 and the current Ioled.

(Equation 1)
Loled∝Ioled = k (Vdata−Vth) ² (1)

In Equation (1), k = 1/2 · μ · Cox · W / L. Here, μ is the carrier mobility of the TFT 11, Cox is the gate capacitance per unit area of the TFT 11, W is the gate width of the TFT 11, and L is the gate length of the TFT 11.
Therefore, it can be seen that the variation in mobility μ and threshold voltage Vth (<0) of the TFT 11 directly affects the luminance variation of the organic EL light emitting element 13.

  In this case, for example, even if the same potential Vdata is written to different pixels, the threshold voltage Vth of the TFT 11 varies from pixel to pixel. As a result, the current Ioled flowing through the light emitting element (OLED) 13 varies greatly from pixel to pixel and is completely different from the desired value. As a result, the display cannot be expected to have high image quality.

  A number of pixel circuits have been proposed in order to improve this problem. A typical example is shown in FIG. 3 (see, for example, Patent Document 3 or Patent Document 4).

The pixel circuit 20 in FIG. 3 includes a p-channel TFT 21, n-channel TFTs 22 to 24, capacitors C21 and C22, and an organic EL light emitting element 25 that is a light emitting element. In FIG. 3, SGL indicates a signal line, SCNL indicates a scanning line, AZL indicates an auto-zero line, and DRVL indicates a drive line.
The operation of the pixel circuit 20 will be described below with reference to the timing chart shown in FIG.

  As shown in FIGS. 4A and 4B, the drive line DRVL and the auto-zero line AZL are set to high level, and the TFTs 22 and 23 are turned on. At this time, since the TFT 21 is connected to the light emitting element (OLED) 25 in a diode-connected state, a current flows through the TFT 21.

  Next, as shown in FIG. 4A, the drive line DRVL is set to low level, and the TFT 22 is turned off. At this time, the scanning line SCNL is at a high level, as shown in FIG. 4C, and the TFT 24 is in a conductive state, and the reference potential Vref is applied to the signal line SGL as shown in FIG. 4D. Since the current flowing through the TFT 21 is cut off, the gate potential Vg of the TFT 21 rises as shown in FIG. 4E. However, when the potential rises to VDD− | Vth | Potential stabilizes. Hereinafter, this operation may be referred to as “auto-zero operation”.

  As shown in FIGS. 4B and 4D, the auto zero line AZL is set to a low level to turn off the TFT 23, and the potential of the signal line SGL is set to a potential that is lower than Vref by ΔVdata. This change in the signal line potential lowers the gate potential of the TFT 21 by ΔVg through the capacitor C21, as shown in FIG.

  As shown in FIGS. 4A and 4C, when the TFT 24 is turned off by setting the scanning line SCNL to the low level and the TFT 22 is turned on by setting the drive line DRVL to the high level, the TFT 21 and the light emitting element (OLED) 25 are turned on. Current flows, and the light emitting element 25 starts to emit light.

  If the parasitic capacitance can be ignored, ΔVg and the gate potential Vg of the TFT 21 are as follows.

(Equation 2)
ΔVg = ΔVdata × C1 / (C1 + C2) (2)

(Equation 3)
Vg = V _CC − | Vth | −ΔVdata × C1 / (C1 + C2) (3)

  Here, C1 indicates the capacitance value of the capacitor C21, and C2 indicates the capacitance value of the capacitor C22.

  On the other hand, if the current flowing through the light emitting element (OLED) 25 during light emission is Ioled, the current value is controlled by the TFT 21 connected in series with the light emitting element 25. Assuming that the TFT 21 operates in the saturation region, the following relationship is obtained using the well-known MOS transistor equation and the above equation (3).

(Equation 4)
Ioled = μCoxW / L / 2 (V _CC −Vg− | Vth |) ²
= ΜCoxW / L / 2 (ΔVdata × C1 / (C1 + C2)) ²
... (4)

  Here, μ represents carrier mobility, Cox represents gate capacitance per unit area, W represents gate width, and L represents gate length.

  According to the equation (4), Ioled is controlled by ΔVdata given from the outside regardless of the threshold value Vth of the TFT 21. In other words, if the pixel circuit 20 of FIG. 3 is used, it is possible to realize a display device that is relatively unaffected by the threshold value Vth that varies from pixel to pixel and that has a relatively high current uniformity and, consequently, luminance uniformity.

USP 5,684,365 JP-A-8-234683 USP 6,229,506 Fig. 1 of JP-T-2002-514320. 3 JP 2004-133240 A JP 2002-351400 A JP 2002-351402 A

  As described above, when the pixel circuit 10 as shown in FIG. 2 is used, the uniformity of luminance between pixels is impaired due to variations in the threshold voltage Vth of the transistor, and a high-quality display device can be configured. Have difficulty.

  On the other hand, if the pixel circuit of FIG. 3 is used, a display device with relatively high luminance uniformity can be realized, but this has the following problems.

  The first problem is that the gate amplitude ΔVg of the driving transistor decreases according to the equation (2) with respect to the data amplitude ΔVdata driven from the outside. Conversely, in order to obtain the same ΔVg, it is necessary to give a large ΔVdata, which is undesirable from the viewpoint of power consumption and noise.

The second problem is that the above description of the operation relating to the pixel circuit 20 of FIG. 3 is ideal, and in practice, the influence of variations in Vth of the TFT 21 that drives the light emitting element (OLED) 25 is not eliminated. .
This is because the auto zero line AZL and the gate node of the TFT 21 are coupled by the gate capacitance of the TFT 23, and the channel charge of the TFT 23 becomes the gate of the TFT 21 in the process in which the auto zero line AZL transitions to a high level and the TFT 23 becomes nonconductive. This is because it flows into the node. The reason for this will be described next.

That is, the gate potential of the TFT 21 should ideally be VDD− | Vth | after the completion of the auto-zero operation, but actually becomes a slightly higher potential due to the inflow of the charge, and the inflow amount of the charge is It varies depending on the value of Vth. This is because the gate potential of the TFT 21 immediately before the end of the auto-zero operation is approximately VDD− | Vth |. Therefore, this potential is higher as | Vth | is smaller, for example.
On the other hand, when the auto zero operation ends, when the potential of the auto zero line AZL rises and the TFT 23 switches to non-conduction, the higher the source potential, that is, the gate potential of the TFT 21, the more delayed the timing at which the TFT 23 becomes non-conducting. Will flow into the gate of the TFT 21. As a result, the gate potential of the TFT 21 after completion of the auto-zero operation is affected by | Vth |, and thus the above-described equations (3) and (4) are not strictly established and are affected by Vth which varies from pixel to pixel. become.

Therefore, a threshold voltage correction type (offset cancellation type) pixel circuit has been proposed as a pixel circuit capable of compensating for the threshold voltage Vth in which luminance variations are likely to be a problem (see, for example, Patent Document 5).
In this pixel circuit, in order to eliminate characteristic variation, a capacitor element for offset canceling of the threshold value of the transistor is used to eliminate the influence of the threshold value Vth caused by laser annealing during the production of the low-temperature polysilicon TFT. Yes.
However, the mobility variation due to the crystal grain size variation of the laser annealed polysilicon cannot be removed. For this reason, in-plane luminance variation occurs. In particular, in-plane luminance variation occurs when the luminance is higher than when the luminance is low.

On the other hand, a pixel circuit including a current mirror type current drive circuit has been proposed (see, for example, Patent Documents 6 and 7).
In this pixel circuit, since the current itself is duplicated in a circuit (current mirror circuit) that duplicates the current signal as it is as a current signal, compared to the voltage-current conversion circuit, the variation in luminance can be extremely reduced.
However, since it is a circuit typified by a p-channel current mirror, as shown in FIG. 5, the responsiveness of low current drive during low luminance display is poor. For this reason, when a low luminance (during black display) and a high luminance horizontal line alternate, the black line will appear in the vertical scanning direction. As a result, the image quality is deteriorated.

  The present invention has been made in view of such circumstances, and the purpose thereof is to prevent luminance variation at high luminance, and to prevent deterioration in image quality without impairing signal write response at low luminance, An object of the present invention is to provide a pixel circuit, a display device, and a pixel circuit driving method capable of displaying a high-quality image.

  In order to achieve the above object, a first aspect of the present invention is a pixel circuit that drives an electro-optical element whose luminance changes according to a flowing current, and at least a signal line to which a voltage signal corresponding to luminance information is supplied. A current supply line is formed between at least the first control line, the first and second reference potentials, the first node, the second node, and the first terminal and the second terminal; A drive transistor that controls a current flowing through the current supply line in accordance with a potential of a control terminal connected to the node of the first node, and is connected between the signal line and the second node, and is connected by the first control line. The first switch that is controlled to be conductive, the voltage-current conversion circuit that is connected to the control terminal of the drive transistor and converts the voltage signal into a current signal, and the current having a predetermined luminance is referred to generate a replication current of the current. ,the above And a current replication circuit for transmitting to the piezoelectric current conversion circuit.

  A display device according to a second aspect of the present invention includes a plurality of pixel circuits arranged in a matrix and wiring for each column with respect to the matrix arrangement of the pixel circuits, and a voltage signal corresponding to at least luminance information is supplied. A signal line; at least a first control line wired for each row with respect to the matrix arrangement of the pixel circuit; and first and second reference potentials. Forming a current supply line between the first node, the second node, and the first terminal and the second terminal, the potential of the control terminal connected to the first node And a drive transistor for controlling a current flowing through the current supply line, a first switch connected between the signal line and the second node and controlled to be conductive by the first control line, Driving transistor A voltage-current conversion circuit that converts a voltage signal into a current signal, a current replication circuit that refers to a current of a predetermined luminance, generates a replication current of the current, and transmits the current to the voltage-current conversion circuit ,including.

  Preferably, the voltage-current conversion circuit and the current replication circuit include a first node connected to a control terminal of the driving transistor, a second node, and the first node and the second node. Of the first capacitor connected, the second capacitor connected between the second node and the first reference potential, and at least a first capacitor among the first capacitor and the second capacitor. And a second switch connected to the capacitor and capable of selectively supplying a reference current to the first node.

  Preferably, there is further provided a third switch capable of selectively connecting the first node to a predetermined potential.

  Preferably, the current replication circuit includes a current mirror circuit.

  Preferably, the reference current can be arbitrarily set.

  Preferably, the reference potential is set between a low current and a high current, respectively, of the low brightness and the high brightness of the panel on which the pixel circuit is mounted.

  According to a third aspect of the present invention, there is provided an electro-optical element whose luminance is changed by a flowing current, a signal line to which a voltage signal corresponding to at least luminance information is supplied, at least a first control line, first and second And a first node, a second node, a current supply line between the first terminal and the second terminal, and the control terminal connected to the first node in accordance with the above-described reference potential. A drive transistor for controlling a current flowing in the current supply line; a first switch connected between the signal line and the second node and controlled to be conductive by the first control line; A voltage-current conversion circuit connected to a control terminal and including a voltage-current conversion circuit that converts a voltage signal into a current signal, wherein a current having a predetermined luminance is referred to, a duplicate current of the current is generated, and the voltage Current conversion times Transmitted to, the voltage signal is transmitted to a predetermined time period the second node through a first switch, controlling the light emission of the electro-optical element.

According to the present invention, for example, a reference current is supplied to the first node, and the current is replicated in the current replication circuit.
The replicated current is voltageed to the voltage-current conversion circuit, and the voltage signal is input to the second node through the first switch.
As a result, a signal taking into account the reference current is input to the control terminal of the drive transistor, and the light emission control of the electro-optical element is performed with correction.

According to the present invention, there is no luminance variation at high luminance, and in particular, the image quality during white display is improved.
Further, since the driving is based on the voltage signal, the response at low luminance is faster than that of the current signal type, and there is no problem, and a good response characteristic can be obtained.
Since an arbitrary luminance reference can be set, it can be set to eliminate a luminance variation deviation when a panel application, for example, the background corresponds to gray display.
Further, since voltage signal driving is used, there is an advantage that the number of connection points to the signal line can be reduced.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 6 is a diagram showing an outline of the configuration of an active matrix organic EL display (display device) according to an embodiment of the present invention.
FIG. 7 is a circuit diagram showing a configuration example of the pixel circuit of the active matrix organic EL display according to the present embodiment.

As shown in FIG. 6, the organic EL display 100 includes a pixel array unit 102 in which pixel circuits 101 are arranged in an m × n matrix, a data driver (DDRV) 103, a scan driver (SDRV) 104, a reset driver ( RSTDRV) 105, reference driver (REFDRV) 106, and reference current supply circuit (IDRV) 107.
Then, signal lines SGL101 to SGL10n for n columns driven by the data driver (DDRV) 103 with respect to the matrix arrangement of the pixel circuit 101, and reference current lines RCL101 to RREF101 to which the reference current supply circuit 107 supplies the reference current IREF. 10n is wired for each pixel column, and m rows of scanning lines SCNL101 to SCNL10m selectively driven by the scan driver (SDRV) 104, reset lines RSL101 to RSL10m selectively driven by the reset driver 105, and Reference lines IRFL101 to IRFL10m selectively driven by the reference driver 106 are wired for each pixel row.

  Next, a specific configuration of each pixel circuit 101 will be described.

As shown in FIG. 7, the pixel circuit 101 includes p-channel TFTs 111 to 113, n-channel TFTs 114 to TFT 116, capacitors C 111 and C 112, a light emitting element 117 including an organic EL element OLED (electro-optical element), and nodes ND 111 to ND 113. Have
In FIG. 7, SGL indicates a signal line, RFL indicates a reference current line, SCNL indicates a scanning line, RSL indicates a reset line, and RFL indicates a reference line.
Among these components, the TFT 111 constitutes a driving transistor, the TFT 114 constitutes a first switch, the TFT 116 constitutes a second switch, the TFT 112 constitutes a third switch, and the capacitor C111 is the first switch. The capacitor C112 constitutes the second capacitor.
The supply line (power supply potential) of the power supply voltage VCC corresponds to the first reference potential, and the potential of the cathode line CSL (for example, the ground potential GND) corresponds to the second reference potential.

As a driving transistor, the source of the TFT 111 is connected to the supply line of the power supply voltage VCC, the drain is connected to the anode side of the organic EL light emitting element 117, and the gate is connected to the node ND 111. The cathode of the light emitting element 117 is connected to a cathode line CSL having a predetermined potential (for example, ground potential).
The source of the TFT 112 is connected to the supply line of the power supply voltage VCC, the drain is connected to the node ND111, and the gate is connected to the reset line RSL.
The source of the TFT 113 is connected to the supply line of the power supply voltage VCC, and the drain and gate are connected to the node ND113.
The source of the TFT 114 is connected to the node ND112, the drain is connected to the signal line SGL, and the gate is connected to the scanning line SCNL.
The source of the TFT 115 is connected to the reference current line RCL, the drain is connected to the node D113, and the gate is connected to the reference line RFL.
The source of the TFT 116 is connected to the node ND113, the drain is connected to the node ND111, and the gate is connected to the reference line IRFL.
A first electrode of the capacitor C111 is connected to the node ND111, and a second electrode is connected to the node ND112.
The first electrode of the capacitor C112 is connected to the supply line of the power supply voltage VCC, and the second electrode is connected to the node ND112.
The node ND111 includes a connection point of the gate of the TFT 111, the drain of the TFT 112, and the first electrode of the capacitor C111. The node ND112 is configured by a connection point between the second electrode of the capacitor C111 and the second electrode of the capacitor C112. The node ND113 includes a connection point between the drain and gate of the TFT 113, the drain of the TFT 115, and the source of the TFT.

  The TFT of this embodiment can be configured by a bottom gate type transistor as shown in FIG. 8 or a top gate type transistor as shown in FIG.

  As shown in FIG. 8, the bottom gate type transistor 130 has a gate 132 made of, for example, Mo formed on a glass substrate 131, and an N + diffusion layer 134 that constitutes a source / drain via a gate insulating film 133 thereon. 135 is formed, and a source electrode 138 and a drain electrode 139 made of, for example, aluminum are formed through contact holes formed in the interlayer insulating films 136 and 137.

  In the top gate type transistor 140, N + diffusion layers 142 and 143 constituting source / drain are formed on a glass substrate 141, a gate 145 made of, for example, Mo is formed through a gate insulating film 144, and an interlayer insulating film 146 is formed. A source electrode 147 and a drain electrode 148 made of, for example, aluminum are formed through the formed contact holes.

The pixel circuit 101 of the present embodiment having such a configuration includes a voltage / current conversion circuit that converts a voltage signal into a current signal, and a current replication circuit that refers to a high-luminance current and generates a replication current thereof.
Specifically, the voltage-current conversion circuit includes a capacitor C111 as an offset capacitance for correcting the threshold value Vth and current, and a TFT 112 as an initialization reset switch, and the current reference circuit is a p-channel TFT 111. The current mirror circuit 120 is formed to include .about.113.
In addition, it has TFTs 115 and 116 as reference switches for transmitting the charge of the referenced current to the capacitor C111 as an offset capacitor.
As a result, an arbitrary fanless (reference) current is replicated at high luminance and transmitted to the voltage-current conversion circuit. The electric charge corresponding to Vth and the reference current is stored in the capacitor C111 as an offset capacitor.

Hereinafter, the operation principle of the pixel circuit according to the present embodiment will be described with reference to FIGS.
10A and 10B are diagrams showing the operating points of the driving transistor according to the related art. FIG. 10A shows the operating point of the driving transistor not subjected to Vth correction, and FIG. 10B shows the operating point of the driving transistor after Vth correction. Each operating point is shown.
11A and 11B are diagrams showing the operating point of the driving transistor of the pixel circuit of this embodiment. FIG. 11A shows the operating point of the driving transistor at high luminance, and FIG. 11B shows the driving at intermediate luminance. The operating points of the transistors are shown respectively.
FIG. 12 is a diagram showing the relationship between visual recognition and luminance deviation depending on the panel.

As shown in FIGS. 10A and 10B, conventionally, the threshold Vth can be corrected by the IDS-VGS (IDS: drain-source current, VGS: gate-source voltage) characteristics of the drive transistor. Therefore, the threshold value Vth is corrected with the offset capacitance. However, due to variations in mobility, it was not possible to correct the deviation when the IDS current was large at high luminance.
IDS = μeff x W / L xCt x (VCC-VTH) ^ 2 = μeff x W / L xCt x (VCC-VSIG) ^ 2 is the current equation of the voltage current circuit (μeff: mobility, W / This is because L: transistor gate width / transistor gate length, VCC: OLED anode voltage, VSIG: image signal voltage) mobility is independent of the threshold value Vth.

On the other hand, in the present embodiment, as shown in FIGS. 11A and 11B, a reference current is used, and a high current at high luminance is first referred to and corrected, and the correction voltage is offset. This is transmitted to the capacitor C111.
As a result, the high luminance can be corrected by this current, so that the luminance variation becomes invisible.
The current variation remains in the low current boundary region of low luminance, but this is not visually perceivable because the luminance deviation variation of the luminance variation on the low luminance side becomes wide as shown in FIG.

  FIG. 13 is a diagram illustrating a timing chart of the pixel circuit according to the present embodiment.

As shown in FIG. 13, in order to drive the pixel circuit 101, first, the reset signal RS is set to a low level for a predetermined period by the reset driver 105. As a result, the TFT 112 as a reset switch is turned on, and the level of the node ND111 and the capacitance of the capacitor C111 are reset to a predetermined level.
Next, after the TFT 111 is turned off, the reference driver 106 sets the referenceless signal RFR to a high level for a predetermined period. Thereby, the TFT 115 as the reference switch and the TFT 116 as the bridge switch are turned on, the reference current RC is supplied to the node ND111 capacitor C112 and the like of the current-voltage conversion circuit, and a voltage corresponding to the reference current value is held in the capacitor. . That is, the current is duplicated in the current mirror circuit 120.
Next, after the TFTs 115 and 116 are turned off, the scan driver 104 sets the write scan signal WS to a high level for a predetermined period. As a result, the TFT 114 is turned on, and a voltage signal is input to the node ND112 through the signal line SGL.
Then, after taking in the data voltage, it becomes the TFT 114, and the light emitting element 117 emits light with luminance according to the voltage signal.

In the above description, the current mirror circuit is used for current replication. However, other current replication circuits may be used.
Further, the reference luminance can be arbitrarily set by changing the reference current (reference current). This reference potential is preferably set between a low current and a high current, respectively, for the low brightness and high brightness of the panel.
As a result, in addition to the panel usage environment and high luminance, it is possible to perform a display in which luminance variations are eliminated in a panel application with many gray displays.

Further, FIG. 14 shows an organic layer composed of a selector switch unit composed of TFT thin film transistors, a vertical drive circuit, and a COG (CHIP ON GLASS) TAB for generating and converting an image signal (including a shift register and a digital / analog conversion circuit). An example of an EL display is shown.
FIG. 15 is a diagram showing a timing chart of the three-division selector switch of the organic EL display of FIG. 14 and its signal output.
In the example of FIG. 14, the analog switches ASW1 to ASW3 are selectively turned on and off by the selection signals S1, XS1, S2, XS2, S3, and XS3, respectively, and the signal potential from the source driver of the COGTABIC is changed to the signal line SGL101, Communicate to ...

  As described above, according to the present embodiment, the pixel circuit 101 includes the voltage / current conversion circuit that converts a voltage signal into a current signal, and the current replication circuit that refers to a high-luminance current and generates a replication current thereof. Specifically, the voltage-current conversion circuit includes a capacitor C111 as an offset capacitance for correcting the threshold value Vth and current, and a TFT 112 as a reset switch for initialization, and the current reference circuit is p It includes a current mirror circuit 120 formed including channel TFTs 111 to 113, and further includes TFTs 115 and 116 as reference switches for transmitting the charge of the referenced current to the capacitor C111 as an offset capacitor. Duplicates any referenceless current at the time of luminance and transmits it to the voltage-current converter circuit From what has been urchin configuration, it is possible to prevent luminance variation in the time of high brightness, without degrading the signal write response at low intensity, can prevent image degradation, can display high-quality images.

In other words, according to the present embodiment, there is no luminance variation at high luminance, and in particular, the image quality during white display is improved.
Further, since the driving is based on the voltage signal, the response at low luminance is faster than that of the current signal type, and there is no problem, and a good response characteristic can be obtained.
In addition, since an arbitrary luminance reference can be set, it can be set to eliminate a luminance variation deviation when a panel application, for example, the background corresponds to gray display.
Further, since voltage signal driving is used, the number of connection points to the signal line can be reduced. In the current driving circuit, it is necessary to pass a current during the horizontal selection period, so that the signal line changeover switch cannot be used. The voltage signal may be accumulated in the capacity of the signal line.

  Note that the pixel circuit 101 in FIG. 7 is an example, and the present invention is not limited to this. For example, as described above, since the TFTs 112 to 116 are merely switches, it is obvious that all or a part of them can be constituted by p-channel TFTs or other switching elements.

It is a block diagram showing a general active matrix type organic EL display (display device). It is a circuit diagram which shows the 1st structural example of the conventional pixel circuit. It is a circuit diagram which shows the 2nd structural example of the conventional pixel circuit. 4 is a timing chart for explaining a method of driving the circuit of FIG. 3. It is a figure which shows the relationship between the response time and current signal value in a current mirror type pixel circuit. It is a figure which shows the outline of a structure of the active matrix type organic electroluminescent display (display apparatus) which concerns on embodiment of this invention. It is a circuit diagram which shows the structural example of the pixel circuit of the active matrix type organic EL display which concerns on this embodiment. It is a figure which shows the example of a cross-section of a bottom gate type transistor. It is a figure which shows the example of a cross-section of a top gate type transistor. 4A and 4B are diagrams showing operating points of a driving transistor according to a related technique, in which FIG. 5A shows an operating point of a driving transistor not subjected to Vth correction, and FIG. 5B shows an operating point of the driving transistor after Vth correction. . 4A and 4B are diagrams showing the operating point of the driving transistor of the pixel circuit of the present embodiment, where FIG. 5A shows the operating point of the driving transistor at high luminance, and FIG. 5B shows the operating point of the driving transistor at intermediate luminance. ing. It is a figure which shows the relationship between the visual recognition depending on a panel, and a brightness | luminance deviation. It is a figure which shows the timing chart of the pixel circuit which concerns on this embodiment. A diagram showing an example of an organic EL display composed of a selector switch unit composed of TFT thin film transistors, a vertical drive circuit, and a COG (CHIP ON GLASS) TAB (including a shift register and a digital / analog conversion circuit) for generating and converting an image signal. It is. It is a figure which shows the timing chart of the 3 division | segmentation selector switch of the organic electroluminescent display of FIG. 14, and its signal output.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 ... Active matrix type organic electroluminescent display (display apparatus), 101 ... Pixel circuit, 102 ... Pixel array part, 103 ... Data driver (DDRV), 104 ... Scan driver, 111 ... TFT as a drive transistor, 112-116 ... Switch TFT as 117, light emitting element, C111, C112, capacitor, ND111 to ND113.

Claims (13)

A pixel circuit that drives an electro-optic element whose luminance changes according to a flowing current,
A signal line to which a voltage signal corresponding to at least luminance information is supplied;
At least a first control line;
First and second reference potentials;
A first node;
A second node;
A drive transistor that forms a current supply line between the first terminal and the second terminal and controls a current flowing through the current supply line according to a potential of a control terminal connected to the first node;
A first switch connected between the signal line and the second node, the conduction of which is controlled by the first control line;
A voltage-current conversion circuit that is connected to the control terminal of the drive transistor and converts a voltage signal into a current signal;
A pixel circuit having a current replication circuit that refers to a current having a predetermined luminance, generates a replication current of the current, and transmits the replication current to the voltage-current conversion circuit. The voltage-current conversion circuit and the current replication circuit are:
A first node connected to the control terminal of the drive transistor;
A second node;
A first capacitor connected between the first node and the second node;
A second capacitor connected between the second node and the first reference potential;
2. The pixel according to claim 1, further comprising: a second switch connected to at least the first capacitor of the first capacitor and the second capacitor and capable of selectively supplying a reference current to the first node. circuit. The pixel circuit according to claim 2, further comprising a third switch capable of selectively connecting the first node to a predetermined potential. The pixel circuit according to claim 2, wherein the current replication circuit includes a current mirror circuit. The pixel circuit according to claim 2, wherein the reference current can be arbitrarily set. The pixel circuit according to claim 5, wherein the reference potential is set between a low current and a high current, respectively, of a low luminance and a high luminance of a panel on which the pixel circuit is mounted. A plurality of pixel circuits arranged in a matrix;
A signal line that is wired for each column with respect to the matrix arrangement of the pixel circuit and that is supplied with a voltage signal corresponding to at least luminance information;
At least a first control line wired for each row to the matrix arrangement of the pixel circuit;
First and second reference potentials,
The pixel circuit is
An electro-optic element whose luminance varies depending on the flowing current;
A first node;
A second node;
A drive transistor that forms a current supply line between the first terminal and the second terminal and controls a current flowing through the current supply line according to a potential of a control terminal connected to the first node;
A first switch connected between the signal line and the second node, the conduction of which is controlled by the first control line;
A voltage-current conversion circuit that is connected to the control terminal of the drive transistor and converts a voltage signal into a current signal;
A current replication circuit that refers to a current having a predetermined luminance, generates a replication current of the current, and transmits the replication current to the voltage-current conversion circuit. The voltage-current conversion circuit and the current replication circuit are:
A first node connected to the control terminal of the drive transistor;
A second node;
A first capacitor connected between the first node and the second node;
A second capacitor connected between the second node and the first reference potential;
The display according to claim 7, further comprising: a second switch connected to at least the first capacitor of the first capacitor and the second capacitor and capable of selectively supplying a reference current to the first node. apparatus. The display device according to claim 8, further comprising a third switch capable of selectively connecting the first node to a predetermined potential. The display device according to claim 8, wherein the current replication circuit includes a current mirror circuit. The display device according to claim 8, wherein the reference current can be arbitrarily set. The display device according to claim 11, wherein the reference potential is set between a low current and a high current, respectively, of the low brightness and the high brightness of the panel on which the pixel circuit is mounted. An electro-optic element whose luminance varies depending on the flowing current;
A signal line to which a voltage signal corresponding to at least luminance information is supplied;
At least a first control line;
First and second reference potentials;
A first node;
A second node;
A drive transistor that forms a current supply line between the first terminal and the second terminal and controls a current flowing through the current supply line according to a potential of a control terminal connected to the first node;
A first switch connected between the signal line and the second node, the conduction of which is controlled by the first control line;
A voltage-current conversion circuit that is connected to the control terminal of the drive transistor and converts a voltage signal into a current signal,
A current having a predetermined luminance is referred to, a replication current of the current is generated, and transmitted to the voltage-current conversion circuit.
A method for driving a pixel circuit, wherein a voltage signal is transmitted to the second node through the first switch for a predetermined period to perform light emission control of the electro-optic element.

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