JP2007225653A - Electrooptical device and its driving method, and electronic equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To secure a sufficient time for which an electrooptical element is driven. <P>SOLUTION: A unit circuit U includes a driving transistor Td which has a gate, the electrooptical element 11 which is driven according to a conduction state of the driving transistor Td, a capacity element C1 which has an electrode E1 and an electrode E2, and a transistor for compensation whose threshold voltage Vth nearly matches that of the driving transistor Td. The electrode E2 is connected to the gate of the driving transistor Td. Before the voltage at the electrode E1 converges on a predetermined value (VEL-Vth) corresponding to the threshold voltage Vth through compensating operation wherein a current is supplied to the transistor Tc for compensation, the electrooptical element 11 is driven to a gray scale corresponding to a data voltage Vdata by supplying the data Vdata to the gate of the driving transistor, the gate is placed in a floating state to vary the voltage at the gate according to the voltage at the electrode E1 and the electrooptical element 11 is driven to a gray scale corresponding to the data voltage Vdata and threshold voltage Vth. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to an organic light emitting diode (hereinafter referred to as “OLED (Organic Light Emitting Diode)”).
The present invention relates to a technique for controlling an electro-optical element such as an element.

In an electro-optical device that uses a transistor (hereinafter referred to as “driving transistor”) for driving an electro-optical element, gradation unevenness occurs in each electro-optical element due to an error (variation) in threshold voltage of the driving transistor. There is a problem. In order to solve this problem, for example, Patent Document 1 discloses a configuration for compensating for an error in the threshold voltage of the driving transistor.

In this configuration, the adjustment transistor having the same threshold voltage Vth as that of the drive transistor is
It is connected to the gate of the driving transistor in a diode-connected state. Then, by supplying current to the adjustment transistor, the gate of the driving transistor is converged to a compensation voltage corresponding to the threshold voltage Vth, and the electro-optic element is driven by varying the converged voltage according to a desired gradation. start.
JP 2004-126524 A

However, in the configuration of Patent Document 1, since it takes time to converge the gate voltage of the driving transistor to the compensation voltage, there is a problem that the time that can be actually used for driving the electro-optical element is limited. In view of the above circumstances, an object of the present invention is to solve the problem of ensuring sufficient time available for driving an electro-optical element.

In order to solve the above problems, an electro-optical device according to the present invention includes a drive transistor that is in a conductive state according to a gate voltage, an electro-optical element that is driven according to the conductive state of the drive transistor, and a first A capacitive element (eg, capacitive element C1 in FIG. 2) having an electrode (eg, electrode E1 in FIG. 2) and a second electrode (eg, electrode E2 in FIG. 2) connected to the gate, and corresponding to the threshold voltage of the drive transistor A transistor having a threshold voltage, which is connected to the first electrode in a diode-connected state, and a first switching element that switches a gate between a state in which a data voltage is supplied and an electrically floating state ( For example, the transistor T1) of FIG.

In the above configuration, for example, the first before the voltage of the first electrode converges to a predetermined value corresponding to the threshold voltage of the compensation transistor by the compensation operation for supplying current to the compensation transistor.
In the period, the first switching element controls the control means (for example, the scanning line driving circuit 23 of FIG. 1) so that the data voltage output from the data supply means (for example, the data line driving circuit 25 of FIG. 1) is supplied to the gate. The first switching element is controlled by the control means so that the gate is in an electrically floating state before the voltage of the first electrode converges to a predetermined value after the first period has elapsed. The electro-optic element is driven in a state corresponding to the data voltage supplied to the gate in the first period, and the voltage of the gate which is brought into the floating state by the control means changes according to the voltage of the first electrode during the compensation operation. Thus, it is driven to a state corresponding to the data voltage and the threshold voltage of the compensating transistor.

The gate of the driving transistor is brought into an electrically floating state after the data voltage is supplied, so that the gate voltage changes according to the voltage of the first electrode during the execution of the compensation operation.
When the voltage of the first electrode reaches a predetermined value, the gate voltage of the driving transistor is set to a voltage value corresponding to the data voltage and the threshold voltage of the compensation transistor. Since the threshold voltage of the compensation transistor corresponds to (typically coincides with) the threshold voltage of the driving transistor, according to the above configuration, an error (variation) in the threshold voltage of the driving transistor gives the state of the electro-optic element. Impact is reduced.
In the above configuration, the electro-optic element is driven according to the data voltage by supplying the data voltage to the gate of the driving transistor before the voltage of the first electrode converges to a predetermined value by the compensation operation. Then, the state of the electro-optic element according to the data voltage and the threshold voltage of the compensating transistor is changed by the voltage of the gate which is subsequently in an electrically floating state changing according to the voltage of the first electrode during the compensation operation. Driven by. That is, the compensation operation is executed in parallel with the driving of the electro-optic element according to the data voltage. Therefore, it is possible to use a longer time for driving the electro-optical element as compared with a configuration in which the driving of the electro-optical element and the compensation operation are performed in separate periods.

An electro-optical element is an element that converts one of an electrical action and an optical action into the other. Typically, luminance and transmittance are supplied by supplying electric energy (supplying current or applying voltage). This is an element whose optical properties change. For example, various elements such as a current-driven light-emitting element (for example, an OLED element) whose luminance changes with current supply are employed as the electro-optical element of the present invention. That is, one form of the electro-optical device according to the present invention is a light-emitting device using a light-emitting element as an electro-optical element.
In addition, the electrically floating state is a fixedly connected element (for example, the second electrode).
In other words, it means a state in which no electrical conductor is connected (in other words, a state in which no voltage is applied from the outside). Since the gate (second electrode) of the driving transistor and the first electrode are capacitively coupled, the voltage of the gate in the floating state changes according to the voltage of the first electrode.

When the state of the electro-optical element is controlled according to the current (drive current), for example, a configuration in which a drive transistor is arranged on the path of the drive current is employed. In this configuration, the current value of the drive current is controlled according to the gate voltage of the drive transistor. However, the arrangement of the drive transistors is arbitrary. For example, the driving transistor may be arranged in parallel with the electro-optical element on a path branched from the driving current path. In this configuration, since the ratio of the drive current flowing through the electro-optical element and the current flowing through the drive transistor changes according to the gate voltage of the drive transistor, the electro-optical element is driven according to the conduction state of the drive transistor. Is possible. In the above description, a current-driven electro-optical element is assumed. However, the present invention is also applied to a voltage-driven electro-optical element (for example, a liquid crystal element) that is driven by applying a voltage according to the conduction state of the driving transistor. Is done.

In a further preferred aspect, the first electrode is supplied with a reset voltage before the start of the compensation operation. In other words, the first switching element sets the gate to any one of a state where a data voltage is supplied, an electrical floating state, and a state where a reset voltage is supplied. According to this aspect, even when the first electrode is set to an inappropriate voltage (for example, a voltage that hinders subsequent compensation operation) due to noise or the like, the first electrode is set to the reset voltage. It is possible to reliably perform the compensation operation by setting. Although the voltage value of the reset voltage is arbitrary, from the viewpoint of surely executing the compensation operation, the voltage of the first electrode to be set at the end of the compensation operation (for example, the compensation voltage “VEL−Vth” in FIG. 3). ) Is desirable.
More specifically, for example, a second switching element for controlling the electrical connection between the data line and the first electrode is installed, and the control unit is configured to perform the second switching element in the initialization period before the start of the compensation operation. The data supply means supplies a reset voltage to the data line in the initialization period. According to this aspect, since the data line is also used for supplying the data voltage and the reset voltage, there is an advantage that the configuration of the electro-optical device is simplified.
In addition, a second switching element that controls electrical connection between the power supply line to which the reset voltage is supplied and the first electrode is provided, and the control unit sets the second switching element in the initialization period before the start of the compensation operation. A mode in which the switch is turned on is also employed. According to this aspect, since the power supply line for supplying the reset voltage is formed separately from the data line, it is not necessary for the data supply means to switch the output to the data line between the data voltage and the reset voltage. Therefore, there is an advantage that the configuration of the data supply means is simplified.

The threshold voltages of the driving transistor and the compensating transistor are substantially the same. For example, the drive transistor and the compensation transistor have the same size (channel length and channel width) and conductivity type. Further, the drive transistor and the compensation transistor have a common relationship (orthogonal / parallel) between the scanning direction of laser annealing on the semiconductor layer and the channel length direction.

The electro-optical device according to the invention is used in various electronic apparatuses. A typical example of this electronic device is
This is an apparatus using an electro-optical device as a display device. Examples of this type of electronic device include a personal computer and a mobile phone. However, the use of the electro-optical device according to the present invention is not limited to image display. For example, an exposure device (exposure head) for forming a latent image on an image carrier such as a photosensitive drum by irradiation of light, a device (backlight) that is arranged on the back side of the liquid crystal device and illuminates it, or The electro-optical device of the present invention can be applied to various applications such as various illumination devices such as a device that illuminates a document by being mounted on an image reading device such as a scanner.

The present invention is also specified as a method of driving an electro-optical device. The driving method of the present invention includes a driving transistor that is turned on according to the gate voltage, an electro-optic element that is driven according to the conduction state of the driving transistor, a second electrode connected to the first electrode and the gate, And a compensation operation for converging the voltage of the first electrode to a predetermined value corresponding to the threshold voltage of the driving transistor, while the voltage of the first electrode is compensated. A first operation for driving the electro-optical element to a state corresponding to the data voltage by supplying a data voltage to the gate before reaching a predetermined value by the operation, and a gate in an electrically floating state after the first operation. A second operation of driving the electro-optic element to a state corresponding to the data voltage and the threshold voltage by changing the voltage of the gate according to the voltage of the first electrode during the compensation operation; Row. According to this driving method, the same operation and effect as the electro-optical device of the present invention are exhibited.

The electro-optical device includes a compensation transistor having a threshold voltage corresponding to the threshold voltage of the driving transistor and connected to the first electrode in a diode-connected state, and the compensation operation supplies a current to the compensation transistor. This is an operation of changing the voltage of the first electrode to a predetermined value corresponding to the threshold voltage of the compensating transistor by flowing the first electrode. In this aspect, since the voltage of the first electrode changes due to the supply of current to the compensation transistor, the first
In many cases, it takes a long time to converge the voltage of the electrode. Therefore, the present invention that can secure the driving time of the electro-optic element by executing the driving of the electro-optic element and the compensation operation in parallel is particularly preferably employed.

As exemplified as one aspect of the electro-optical device according to the present invention, also in a preferred aspect of the driving method according to the present invention, the voltage of the first electrode is set to the reset voltage before the start of the compensation operation. According to this aspect, there is an advantage that malfunction (inhibition of compensation operation) caused by noise or the like can be prevented in advance.
For example, in the configuration in which the electro-optical device includes a first switching element that controls electrical connection between the data line and the gate, and a second switching element that controls electrical connection between the data line and the first electrode. Supplies a reset voltage to the data line and turns on the first switching element and the second switching element before starting the compensation operation, and supplies the data voltage to the data line and the first switching in the first operation. The device is turned on and the second
In operation, the gate is brought into an electrically floating state by turning off the first switching element. A specific example of this embodiment is illustrated in FIG.
The electro-optical device includes a first switching element that controls electrical connection between the data line and the gate, and a second switching element that controls electrical connection between the power supply line and the first electrode. In the configuration including the above, before the compensation operation is started, the reset voltage is supplied to the power supply line and the second switching element is turned on. In the first operation, the data voltage is supplied to the data line and the first switching is performed. The device is turned on, and in the second operation,
The gate is brought into an electrically floating state by turning off the first switching element. A specific example of this aspect is illustrated in FIG.

<A-1: Configuration of electro-optical device>
FIG. 1 is a block diagram showing a configuration of an electro-optical device according to one embodiment of the present invention. The electro-optical device D illustrated in the figure is a device used in various electronic devices as a means for displaying an image, and an element array unit 10 in which a plurality of unit circuits (pixel circuits) U are arranged in a planar shape. And a scanning line driving circuit 23 and a data line driving circuit 25 for driving each unit circuit U.
The scanning line driving circuit 23 and the data line driving circuit 25 may be constituted by thin film transistors formed on the substrate together with the element array unit 10, or may be mounted in the form of an IC chip.

As shown in FIG. 1, the element array unit 10 includes m scanning lines 131 extending in the X direction,
M initialization lines 132 extending in the X direction in pairs with each scanning line 131 and n data lines 15 extending in the Y direction orthogonal to the X direction are formed (m and n). Each is a natural number of 2 or more). Each unit circuit U is arranged at each position corresponding to the intersection of the pair of scanning line 131 and initialization line 132 and the data line 15. Accordingly, these unit circuits U are arranged in a matrix of m rows × n columns. Each unit circuit U is supplied with a power supply potential VEL and a ground potential Gnd from a power supply circuit (not shown).

The scanning line driving circuit 23 includes scanning signals Gwrt [1] to Gwrt [m] and initialization signals Gres [1] to Gres [m].
It is a means to produce | generate. The scanning signal Gwrt [i] is output to the i-th row scanning line 131 (i is an integer satisfying 1 ≦ i ≦ m), and the initialization signal Gres [i] is output to the i-th row initialization line 132. Is done. Note that a circuit that outputs the scanning signal Gwrt [1] to the scanning signal Gwrt [m] and the initialization signal Gres [1] to
It may be installed separately from the circuit that outputs Gres [m]. The data line driving circuit 25 is means for generating data signals S [1] to S [n] based on the gradation data of each unit circuit U. The data signal S [j] is output to the data line 15 in the j-th column (j is an integer satisfying 1 ≦ j ≦ n). The gradation data is supplied for each unit circuit U from various external devices such as a CPU of an electronic device in which the electro-optical device D is mounted. A specific waveform of each signal supplied to the unit circuit U will be described later.

<A-2: Configuration of the unit circuit U>
FIG. 2 is a circuit diagram showing a specific configuration of each unit circuit U. In the figure, only one unit circuit U located in the i-th row and j-th column is shown, but other unit circuits U are shown.
Is the same configuration.

As shown in FIG. 2, the unit circuit U includes a power line (power potential VEL) and a ground line (ground potential Gnd).
The electro-optic element 11 is interposed between the two. The electro-optic element 11 is an element that is driven in a state corresponding to the drive current Idr. The electro-optical element 11 of this embodiment is an organic EL (ElectroLum
an OLED element (light emitting element) in which a light emitting layer made of a material is interposed between an anode and a cathode, and emits light with luminance (gradation) corresponding to the current value of the drive current Idr supplied to the light emitting layer. . The cathode of the electro-optic element 11 is grounded (Gnd).

As shown in FIG. 2, an n-channel driving transistor Td is disposed on a path from the power supply line to the electro-optic element 11. The drain (D) of the driving transistor Td is connected to the power supply line, and the source (S) is connected to the anode of the electro-optic element 11. The driving transistor Td has a gate (G) voltage (hereinafter referred to as “gate voltage”) V in which the conduction state between the source and the drain is V.
It is means for generating a drive current Idr corresponding to the gate voltage Vg by changing according to g. Therefore, the electro-optical element 11 is driven to a gradation corresponding to the conduction state (source-drain resistance) of the drive transistor Td.

As shown in FIG. 2, the unit circuit U of the present embodiment includes two capacitive elements (C1 · C2) and three n-channel transistors (Tc · T1 · T2). As each transistor (Td, Tc, T1, T2) constituting the unit circuit U, for example, a thin film transistor is preferably employed. The capacitive element C1 is an element in which a dielectric is inserted in the gap between the electrode E1 and the electrode E2. The electrode E2 of the capacitive element C1 is connected to the gate of the drive transistor Td. The capacitive element C2 is interposed between the gate of the driving transistor Td and the power supply line (drain of the driving transistor Td).

The transistor Tc is a transistor (hereinafter referred to as “compensating transistor”) used to compensate for an error (variation) in the threshold voltage of the driving transistor Td. The compensation transistor Tc is interposed between the electrode E1 of the capacitive element C1 and the power supply line in a diode-connected state. That is, the source of the compensation transistor Tc is connected to the electrode E1 of the capacitive element C1,
Both the drain and the gate are connected to the power supply line (diode connection).

The characteristics of the compensation transistor Tc and the characteristics of the drive transistor Td correspond to each other. More specifically, the threshold voltage Vth of the compensation transistor Tc in this embodiment is substantially equal to the threshold voltage Vth of the drive transistor Td. By forming the compensation transistor Tc and the drive transistor Td of the same size (channel length / channel width) and the same conductivity type (n-channel type) in a common process at positions close to each other, It is possible to make the threshold voltages Vth substantially coincide.

Each semiconductor layer of the compensation transistor Tc and the drive transistor Td is formed by crystallizing an amorphous semiconductor layer by laser annealing, for example. Relationship between laser beam scanning direction and channel length direction (orthogonal / parallel) in the annealing process
Affects the threshold voltage Vth of the transistor. Therefore, laser annealing is performed on the semiconductor layers of the compensation transistor Tc and the drive transistor Td so that the relationship (orthogonal / parallel) between the channel length direction and the laser beam scanning direction coincides.

The transistor T1 is a switching element that is interposed between the gate (electrode E2) of the driving transistor Td and the data line 15 and controls electrical connection (conduction / non-conduction) between the two. The gates of the transistors T1 in each of the n unit circuits U belonging to the i-th row are commonly connected to the i-th scanning line 131. The transistor T2 is a switching element that is interposed between the electrode E1 of the capacitive element C1 and the data line 15 and controls the electrical connection therebetween.
The gates of the transistors T2 in each of the n unit circuits U belonging to the i-th row are commonly connected to the initialization line 132 of the i-th row.

Next, specific waveforms of signals used in the electro-optical device D will be described with reference to FIG. As shown in the figure, the scanning signals Gwrt [1] to Gwrt [m] are signals that sequentially become a high level every predetermined period H in each frame period F. That is, the scanning signal Gwrt [i] maintains a high level in the i-th period H of one frame period F and maintains a low level in the other period (hereinafter referred to as “driving period”) Pdrv. To do.

The period H is a period from the start point until a predetermined time length elapses (hereinafter referred to as “initialization period”) Pres, and a period from the end point of the initialization period Pres to the end point of the period H (hereinafter referred to as “writing period”). And Pwrt). As shown in FIG. 3, the initialization signal Gres [i] is the scanning signal Gwrt.
This is a signal that becomes high level in the initialization period Pres during the period H in which [i] is high level, and maintains the low level in other periods (writing period Pwrt / driving period Pdrv).

In addition, the data signal S [j] is converted into gradation data of the unit circuit U in the j-th column belonging to the i-th row in the writing period Pwrt in the period H in which the scanning signal Gwrt [i] is at a high level. According voltage (
(Hereinafter referred to as “data voltage”) Vdata. Further, the data signal S [j] becomes a predetermined voltage (hereinafter referred to as “reset voltage”) Vres in the initialization period Pres of each period H. The reset voltage Vres of the present embodiment has a voltage value lower than the voltage (VEL−Vth) obtained by subtracting the threshold voltage Vth of the compensation transistor Tc from the power supply potential VEL and is supplied to the gate of the drive transistor Td. Is a voltage value (data voltage Vdat) at which the drive transistor Td is turned off.
a voltage value lower than the minimum value of a).

<A-3: Operation of the electro-optical device>
4 to 6 are circuit diagrams schematically showing the state of the unit circuit U in each of the above periods. Hereinafter, the operation of the unit circuit U in the j-th column belonging to the i-th row will be described with reference to FIGS. 4 to 6 as to the initialization period Pres (FIG. 4), the writing period Pwrt (FIG. 5), and the light emission period Pdrv. (FIG. 6) and it demonstrates.

(a) Initialization period Pres (Fig. 4)
In the initialization period Pres, the data signal S [j] is set to the reset voltage Vres. Since both the scanning signal Gwrt [i] and the initialization signal Gres [i] are at a high level, both the transistor T1 and the transistor T2 are turned on as shown in FIG. Therefore, the reset voltage Vres is supplied to the gate of the drive transistor Td (Vg = Vres).

The electrode E1 of the capacitive element C1 is electrically connected to the data line 15 through the transistor T2. As a result, the reset voltage Vres is also supplied to the electrode E1. Since the reset voltage Vres is lower than the voltage (VEL−Vth) obtained by subtracting the threshold voltage Vth from the power supply potential VEL, the initialization period Pre
In s, a current flows in a path from the power supply line to the data line 15 via the compensation transistor Tc and the transistor T2. Therefore, more strictly speaking, the voltage Vn of the electrode E1 is a voltage value obtained by adding the drain-source voltage of the transistor T2 to the reset voltage Vres (Vn≈Vres).

(b) Write period Pwrt (Fig. 5)
As shown in FIGS. 3 and 5, in the writing period Pwrt, the initialization signal Gres [i] changes to the low level, and the transistor T2 changes to the off state. Therefore, the voltage V of the electrode E1
As shown in FIG. 3, n gradually increases from the time when the transistor T2 changes to the OFF state (that is, the start point of the writing period Pwrt), and finally the threshold value of the compensation transistor Tc from the power supply potential VEL. It converges to a level obtained by subtracting the voltage Vth (hereinafter referred to as “compensation voltage”). In addition,
In the following, the operation of gradually increasing the voltage Vn of the electrode E1 from the reset voltage Vres by supplying current to the compensation transistor Tc to converge to the compensation voltage is referred to as “compensation operation”, and the voltage Vn increases by the compensation operation. A period from the start time point (start point of the writing period Pwrt) until the compensation voltage is reached is referred to as “compensation period Pcps”.

Further, as shown in FIG. 5, the data line 1 is maintained while the transistor T1 is maintained in the ON state.
5 (data signal S [j]) changes to the data voltage Vdata. Therefore, as shown in FIG. 3, the gate voltage Vg of the drive transistor Td changes from the reset voltage Vres set in the initialization period Pres to the data voltage Vdata. As a result, the drive transistor Td becomes conductive according to the data voltage Vdata, so that the drive current Idr corresponding to the data voltage Vdata is supplied from the power supply line to the electro-optical element 11 via the drive transistor Td. In other words, the electro-optical element 11 has the write voltage Pwrt immediately after the start point (that is, the gate voltage Vg is equal to the data voltage V
The driving is started at the gradation corresponding to the data voltage Vdata from the time when the data is set.

As shown in FIG. 3, the writing period Pwrt ends at a point in the middle of the compensation period Pcps. That is, the scanning signal Gwrt [i] transitions to a low level before the voltage Vn reaches the compensation voltage (VEL−Vth). As shown in FIGS. 3 and 5, the voltage Vn of the electrode E1 at the end point of the writing period Pwrt is
This is an added value “Vres + ΔV1” of the reset voltage Vres set in the initialization period Pres and the increment ΔV1 from the start point to the end point of the write period Pwrt.

(c) Driving period Pdrv (FIG. 6)
As described above, since the voltage Vn is in the middle of the change at the end point of the writing period Pwrt, the voltage Vn continues to increase after the start of the driving period Pdrv, and is in the middle of the driving period Pdrv (compensation period Pcps).
The compensation voltage (VEL−Vth) is reached at the end point of. The voltage ΔV2 shown in FIG.
The change amount of the voltage Vn from the start point of the compensation period to the end point of the compensation period Pcps. That is, the following formula (1)
As expressed by the equation, the sum of the reset voltage Vres, the change amount ΔV1 of the voltage Vn in the writing period Pwrt, and the change amount ΔV2 of the voltage Vn in the driving period Pdrv becomes the compensation voltage (VEL−Vth).
Vres + ΔV1 + ΔV2 = VEL−Vth (1)

Since the scanning signal Gwrt [i] changes to the low level at the start point of the driving period Pdrv, the transistor T1 changes to the off state as shown in FIG. Since the impedance of the gate of the driving transistor Td is sufficiently high, the electrode E2 (gate of the driving transistor Td) is in an electrically floating state during the driving period Pdrv.

Therefore, when the voltage Vn of the electrode E1 fluctuates due to the compensation operation, the voltage of the electrode E2 (gate voltage Vg) is changed to the time when the transistor T1 is turned off by capacitive coupling in the capacitive element C1 (the electrode E2 is in a floating state). It gradually fluctuates (increases) from the point of transition. The gate voltage Vg becomes stable when it changes from the voltage Vdata set in the writing period Pwrt by a change amount corresponding to the change amount ΔV2. The amount of change in the gate voltage Vg at this time is determined according to the relative ratio between the capacitance value of the capacitive element C1 and the capacitance value of the capacitive element C2. More specifically, the amount of change in the gate voltage Vg is expressed as “ΔV 2 · c 1 / (c 1 + c 2)” using the capacitance value c 1 of the capacitive element C 1 and the capacitance value c 2 of the capacitive element C 2. Therefore, in consideration of the voltage ΔV2 derived from the equation (1), the gate voltage Vg of the drive transistor Td is stabilized at the voltage value expressed by the following equation (2) in the drive period Pdrv. The driving transistor T
When considering the gate capacitance of d and the capacitance parasitic to other wiring, the capacitance value c2 is the capacitance element C2.
It is a numerical value combining 2 and these capacities.
Vg = Vdata + k · ΔV2
= Vdata + k ・ (VEL−Vth−Vres−ΔV1) (2)
However, k = c1 / (c1 + c2)

As described above, in the driving period Pdrv (more strictly, after the compensation period Pcps has elapsed), the gate voltage Vg expressed by the equation (2) is applied to the gate of the driving transistor Td.
Therefore, the electro-optical element 11 emits light with luminance corresponding to the data voltage Vdata by supplying the driving current Idr corresponding to the data voltage Vdata following the writing period Pwrt. Assuming that the drive transistor Td operates in the saturation region, the drive current Idr is expressed by the following equation (3).
Note that “β” in the following expression is a gain coefficient of the driving transistor Td. In addition, “Vgs
"Is a voltage between the gate and the source of the drive transistor Td. If the source voltage of the drive transistor Td (a voltage determined according to the characteristics of the electro-optical element 11) is" Vs ", then" Vgs = Vs ".
-Vg ".
Idr = (β / 2) (Vgs−Vth) ²
= (Β / 2) (Vs−Vg−Vth) ² (3)
As is clear from the equation (2), the gate voltage Vg in the equation (3) is the threshold voltage Vg of the compensating transistor Tc.
Since it is determined according to th (threshold voltage Vth of the drive transistor Td), according to the present embodiment,
Compared with a configuration in which the voltage Vg is set regardless of the threshold voltage Vth, it is possible to reduce the influence of the error of the threshold voltage Vth on the current value of the drive current Idr. Therefore, according to the present embodiment, each electro-optic element 1 is compensated by compensating for variations in the threshold voltage Vth of each drive transistor Td.
Unevenness of gradation of 1 can be suppressed. In particular, when the capacitance value c2 in the equation (2) is negligibly small (c2≈0), the voltage Vg in the equation (2) is “Vdata + VEL−Vth−Vres−ΔV1”. By substituting this into equation (3), the threshold voltage Vth is canceled out, so that the influence of the error of the threshold voltage Vth on the gradation unevenness is effectively suppressed.

As described above, in the present embodiment, the period (writing period Pwrt / driving period Pdrv) in which the electro-optical element 11 is driven to the luminance according to the gradation data and the compensation period Pcps in which the compensation operation is executed. Therefore, the driving period Pdrv is set to a longer time compared to the configuration in which the operation for compensating for the error of the threshold voltage Vth is performed in a period separate from the period in which the electro-optical element 11 is driven. be able to. In addition, the fact that the drive period Pdrv can be extended means that the electrical energy (particularly the current value of the drive current Idr) to be supplied to the electro-optical element 11 in order to obtain a predetermined amount of light is reduced. Since the deterioration of the electro-optical element 11 proceeds according to the electric energy supplied thereto, the present embodiment has an advantage that the deterioration of the electro-optical element 11 can be prevented and its life can be extended.

By the way, the source of the compensation transistor Tc (the electrode E1 of the capacitive element C1) is connected to the writing period Pw.
Before the start of rt, there is a possibility that it becomes higher than the compensation voltage VEL−Vth due to various causes such as noise. Even if the transistor T2 is changed to the OFF state at the start point of the writing period Pwrt in this state, no current flows through the compensation transistor Tc, so that the potential Vn of the electrode E1 is changed to the compensation voltage (VEL−Vth). I can't let you. In the present embodiment, the writing period Pwr
Since the voltage of the source of the compensation transistor Tc drops to the reset voltage Vres in the initialization period Pres before the start of t, the current flowing through the compensation transistor Tc can be reliably generated in the compensation period Pcps. Therefore, according to this embodiment, there is an advantage that malfunction caused by noise or the like can be prevented in advance. Moreover, in the present embodiment, the reset voltage Vres is supplied to each unit circuit U via the data line 15, so that the reset voltage Vres
It is not necessary to form wiring dedicated for the transmission of the data line 15 independently from the data line 15. Therefore, there is an advantage that the configuration of the electro-optical device D is simplified.

<B: Modification>
Various modifications can be made to each of the above embodiments. An example of a specific modification is as follows. In addition, you may combine each following aspect suitably.

(1) Modification 1
In the above embodiment, the configuration in which the reset voltage Vres is supplied to each unit circuit U through the data line 15 is illustrated. However, as shown in FIG. 7, the wiring 1 to which the reset voltage Vres is applied.
7 is formed separately from the data line 15. In the configuration of FIG. 7, the transistor T2 functions as a switching element that controls the electrical connection between the electrode E1 of the capacitor C1 and the wiring 17. A reset voltage Vres is constantly applied to the wiring 17. The operation of each part is as described in the above embodiment. Also with the configuration of FIG. 7, since the compensation operation is executed in parallel with the operation in the writing period Pwrt and the driving period Pdrv, the same effect as the configuration of FIG. Further, in this modification, it is not necessary to supply the reset voltage Vres to the data line 15, so that the voltage of the data line 15 needs to be switched between the reset voltage Vres and the data voltage Vdata as compared with the configuration of FIG. The configuration of the data line driving circuit 25 is simplified.

(2) Modification 2
The specific configuration of the unit circuit U is not limited to the above examples. For example, in the configuration of FIG. 2, the configuration in which the data voltage Vdata is supplied in a time division manner to each of the m unit circuits U arranged in the Y direction via one data line 15 is illustrated. In this configuration, the data voltage V
In order to sequentially change the unit circuit U that is the output destination of data, the transistor T1 that controls the electrical connection between each unit circuit U and the data line 15 is required. However, for example, in a configuration in which the data line 15 is formed for each unit circuit U (a configuration in which only one data line 15 is supplied with the data voltage Vdata of the unit circuit U corresponding thereto) There is no need to control the electrical connection between and the data line 15. In this configuration, the transistor T1 in FIG.
May be omitted. For example, in an exposure apparatus (optical head) that is used in an image forming apparatus and exposes a photosensitive drum, unit circuits U are arranged in only one direction, and each unit circuit U receives data via an independent data line 15. A configuration connected to the line drive circuit 25 is employed. In this configuration, the transistor T1 in FIG. 2 is omitted, and the gate (electrode E2) of the drive transistor Td is directly connected to the data line 15.

Further, the conductivity type of each transistor constituting the unit circuit U is appropriately changed. For example, as shown in FIG. 8, the drive transistor Td may be a p-channel type. In order to make the threshold voltage Vth of the compensation transistor Tc coincide with the threshold voltage Vth of the drive transistor Td,
In the configuration of FIG. 8, the compensation transistor Tc is also a p-channel type. In addition, FIG.
As shown in FIG. 5, the capacitive element C2 may be omitted.

(3) Modification 3
In the above embodiment, the configuration in which the electrode E2 is in a floating state when a predetermined time has elapsed from the start of the compensation operation is illustrated. However, for example, the configuration in which the electrode E2 is in the floating state before the compensation operation is started. Is also adopted. In this configuration, the gate voltage Vg starts to increase from the data voltage Vdata when the compensation operation is started. In the above embodiment, the configuration in which the data voltage Vdata is supplied to the gate of the drive transistor Td at the same time as the start of the compensation operation is illustrated, but the timing when the data voltage Vdata starts to be supplied to the gate is after the start of the compensation operation. There may be. As described above, the data voltage Vdat is applied to the gate of the drive transistor Td.
The operation of supplying a and the operation of bringing the electrode E2 (the gate of the drive transistor Td) into an electrically floating state after this operation are the compensation voltage (VEL) by the compensation operation of the voltage of the electrode E1.
-Vth) (before the end of the compensation period Pcps arrives) is sufficient, and the relationship between the timing at which these operations are performed and the timing at which the compensation operation is started is appropriately changed. .

(4) Modification 4
The electro-optical element in the present invention is an element that converts one of an electrical action and an optical action into the other. An element in which optical characteristics such as light intensity and transmittance are controlled (driven) by applying electric energy is particularly preferably employed as the electro-optical element of the present invention. For this type of electro-optic element, the self-luminous element that emits light and the non-luminous element that modulates external light according to the transmittance, or current drive driven by supplying current The present invention is applied to the present invention regardless of the distinction between a type element and a voltage driven type element driven by application of a voltage. For example, instead of the OLED elements exemplified in the above embodiments, inorganic EL elements, field emission (FE) elements, and surface-conduction emission (SE) are used.
-emitter) elements, ballistic electron surface emitting (BS) elements, LED (Light Emitting Diode) elements, liquid crystal elements, electrophoretic elements, electrochromic elements, and the like can be used in the present invention. it can.

<C: Application example>
Next, electronic equipment using the electro-optical device according to the invention will be described. FIGS. 9 to 11 show forms of electronic apparatuses that employ the electro-optical device D according to any one of the forms described above as a display device.

FIG. 9 is a perspective view showing the configuration of a mobile personal computer employing the electro-optical device D. The personal computer 2000 includes an electro-optical device D that displays various images, and a main body 2010 on which a power switch 2001 and a keyboard 2002 are installed. Since the electro-optical device D uses an OLED element as the electro-optical element 11, it is possible to display an easy-to-see screen with a wide viewing angle.

FIG. 10 is a perspective view illustrating a configuration of a mobile phone to which the electro-optical device D is applied. The cellular phone 3000 includes a plurality of operation buttons 3001, scroll buttons 3002, and an electro-optical device D that displays various images. By operating the scroll button 3002, the screen displayed on the electro-optical device D is scrolled.

FIG. 11 shows a portable information terminal (PDA: Personal Digital Ass) to which the electro-optical device D is applied.
It is a perspective view which shows the structure of istants). The information portable terminal 4000 includes a plurality of operation buttons 40.
01, a power switch 4002, and an electro-optical device D that displays various images.
When the power switch 4002 is operated, various information such as an address book and a schedule book are displayed on the electro-optical device D.

The electronic apparatus to which the electro-optical device according to the present invention is applied includes the digital still camera, the television, the video camera, the car navigation device, the pager, the electronic notebook, and the electronic paper in addition to the apparatuses shown in FIGS. Calculators, word processors, workstations, videophones, POS terminals, printers, scanners, copiers, video players, devices with touch panels, and the like. The use of the electro-optical device according to the invention is not limited to image display. For example, in an image forming apparatus such as an optical writing type printer or an electronic copying machine, an optical head that exposes a photosensitive member in accordance with an image to be formed on a recording material such as paper (
The electro-optical device of the present invention is also used as this type of optical head.

1 is a block diagram illustrating a configuration of an electro-optical device according to an embodiment of the invention. FIG. It is a circuit diagram which shows the structure of one unit circuit. It is a timing chart which shows the waveform of the signal in connection with the drive of a unit circuit. It is a circuit diagram which shows the mode of the unit circuit in an initialization period. It is a circuit diagram which shows the mode of the unit circuit in a writing period. It is a circuit diagram which shows the mode of the unit circuit in a drive period. It is a circuit diagram which shows the structure of the unit circuit which concerns on a modification. It is a circuit diagram which shows the structure of the unit circuit which concerns on a modification. It is a perspective view which shows the form (personal computer) of the electronic device which concerns on this invention. It is a perspective view which shows the form (cellular phone) of the electronic device which concerns on this invention. It is a perspective view which shows the form (mobile information terminal) of the electronic device which concerns on this invention.

Explanation of symbols

D: Electro-optical device, U: Unit circuit, 10: Element array unit, 11: Electro-optical element,
131... Scanning line, 132... Initialization line, 15... Data line, 17... Wiring, Td... Driving transistor, Tc ... Compensation transistor, T1, T2.
... Capacitance element, Pres ... Initialization period, Pwrt ... Writing period, Pcps ... Compensation period, Pdrv ...
Drive period, Gwrt [i] (Gwrt [1] to Gwrt [m]) ... Scan signal, Gres [i] (Gres [1] to Gres [
m]) .. Initialization signal, S [j] (S [1] to S [n]) ... Data signal.

Claims (12)

A drive transistor that is turned on according to the voltage of the gate; an electro-optic element that is driven according to the turn-on state of the drive transistor; and a capacitive element having a first electrode and a second electrode connected to the gate; An electro-optical device driving method including:
While performing a compensation operation to converge the voltage of the first electrode to a predetermined value corresponding to the threshold voltage of the driving transistor,
A first operation of driving the electro-optic element to a state corresponding to the data voltage by supplying a data voltage to the gate before the voltage of the first electrode reaches the predetermined value by the compensation operation; By setting the gate in an electrically floating state after operation, the voltage of the gate is changed according to the voltage of the first electrode during the compensation operation, and the electro-optic element is changed according to the data voltage and the threshold voltage. And a second operation for driving the electro-optical device to a state where the electro-optical device is driven. The electro-optical device includes a compensation transistor connected to the first electrode in a diode-connected state, the transistor having a threshold voltage corresponding to the threshold voltage of the driving transistor,
The electro-optical device according to claim 1, wherein the compensation operation is an operation of changing a voltage of the first electrode to a predetermined value corresponding to a threshold voltage of the compensation transistor by passing a current through the compensation transistor. Driving method. The driving method of the electro-optical device according to claim 1, wherein the voltage of the first electrode is set to a predetermined reset voltage before the start of the compensation operation. The electro-optical device includes a first switching element that controls electrical connection between a data line and the gate, and a second switching element that controls electrical connection between the data line and the first electrode. ,
Before starting the compensation operation, a reset voltage is supplied to the data line and the first switching element and the second switching element are turned on,
In the first operation, a data voltage is supplied to the data line and the first switching element is turned on.
4. The driving method of the electro-optical device according to claim 3, wherein in the second operation, the gate is set in an electrically floating state by turning off the first switching element. 5. The electro-optical device includes a first switching element that controls electrical connection between a data line and the gate, and a second switching element that controls electrical connection between a power supply line and the first electrode,
Before starting the compensation operation, a reset voltage is supplied to the feeder line and the second switching element is turned on.
In the first operation, a data voltage is supplied to the data line and the first switching element is turned on.
4. The driving method of the electro-optical device according to claim 3, wherein in the second operation, the gate is set in an electrically floating state by turning off the first switching element. 5. The driving method of the electro-optical device according to claim 1, wherein the compensation operation is started at a timing at which a data voltage starts to be supplied to the gate in the first operation. A driving transistor that becomes conductive according to the voltage of the gate;
An electro-optic element driven in accordance with the conduction state of the driving transistor;
A capacitive element having a first electrode and a second electrode connected to the gate;
A transistor having a threshold voltage corresponding to the threshold voltage of the driving transistor and connected to the first electrode in a diode-connected state;
An electro-optical device comprising: a first switching element that switches the gate between a state in which a data voltage is supplied and an electrically floating state. The electro-optical device according to claim 7, wherein the first switching element sets the gate to any one of a state in which a data voltage is supplied, an electrical floating state, and a state in which a reset voltage is supplied. . Data supply means for supplying a data voltage to the data line;
Control means for controlling the first switching element,
The control means performs the first operation by a compensation operation for supplying a current to the compensation transistor.
The first switching element is controlled so that the data voltage of the data line is supplied to the gate in a first period before the voltage of the electrode converges to a predetermined value corresponding to the threshold voltage of the compensation transistor. Controlling the first switching element so that the gate is in an electrically floating state after the first period has elapsed and before the voltage of the first electrode converges to the predetermined value.
The electro-optic element is driven in a state corresponding to the data voltage supplied to the gate by the data supply means, and the voltage of the gate which is floated by the control means is the voltage of the first electrode during the compensation operation. The electro-optical device according to claim 7, wherein the electro-optical device is driven to a state corresponding to a data voltage and a threshold voltage of the compensation transistor by changing according to the voltage. A second switching element for controlling electrical connection between the data line and the first electrode;
The control means turns on the second switching element in an initialization period before the start of the compensation operation,
The electro-optical device according to claim 9, wherein the data supply unit supplies a reset voltage to the data line in the initialization period. A second switching element for controlling electrical connection between the power supply line to which a reset voltage is supplied and the first electrode;
The electro-optical device according to claim 9, wherein the control unit turns on the second switching element in an initialization period before the start of a compensation operation. An electronic apparatus comprising the electro-optical device according to claim 7.

Images