Patent Grant
8587513
This application is based on and claims priority from Japanese Patent Application No. 2010-120197, filed on May 26, 2011, the contents of which are incorporated herein by reference.
1. Technical Field
The present invention relates to a technique for compensating for an error in characteristics (particularly, a threshold voltage) of transistors in a pixel circuit.
2. Related Art
JP-A-2009-48202 discloses a technique for compensating for an error in characteristics (a threshold voltage or a mobility) of a driving transistor used to drive an organic EL element.
However, it is difficult to apply the technique disclosed in JP-A-2009-48202 to a configuration in which a high resistance electro-optical element such as an electrophoresis element or a liquid crystal element is connected to the circuit point 96. This is because since a current hardly flows through the electro-optical element, a potential at the circuit point 96 is not determined, and thus the gate-source voltage of the driving transistor 94 does not converge to the target voltage Vrst even if the driving transistor 94 and the switch 95 are controlled to be in a turned-on state during the writing period.
An advantage of some aspects of the invention is to effectively compensate for errors in characteristics of a driving transistor.
According to an aspect of the invention, there is provided an electro-optical device including a pixel circuit, and a driving circuit. The pixel circuit includes a driving transistor that has a first terminal connected to a driving potential line to which a driving potential is supplied, a second terminal connected to a circuit point, and a control terminal controlling a connection state between both the first and second terminals; an electro-optical element that is connected to the circuit point; a first capacitive element that has a first electrode (for example, an electrode E1), and a second electrode (for example, an electrode E2) connected to the control terminal; a first switch (for example, a switch SW1) that controls a connection between the circuit point and the control terminal; and a second switch (for example, a switch SW2) that controls a connection between a signal line and the first electrode. The driving circuit controls the first switch to be turned off, and varies a potential at the control terminal such that the driving transistor is turned on, during a first period (for example, a reset period TRST) when the driving potential is set to a first potential (for example, a higher potential VDR_H); sets the potential at the control terminal to a compensation initial value by controlling the first switch to be turned on, during a second period (for example, a compensation preparing period QA) after the first period has elapsed; controls the first switch to be turned on, and varies the driving potential from the first potential to a second potential (for example, a lower potential VDR_L) such that the driving transistor is turned on, during a third period (for example, a compensation performing period QB) after the second period has elapsed; supplies a grayscale potential corresponding to a designated grayscale to the signal line and controls the second switch to be turned on, during a fourth period (for example, a writing period TWRT) after the third period has elapsed; and varies a voltage between the control terminal and the first terminal with the passage of time, during a fifth period (for example, a driving period TDRV) after the fourth period has elapsed.
Based on the above-described configuration, during the first period, the first potential is supplied to the circuit point from the driving potential line via the first terminal and the second terminal of the driving transistor which is controlled to be turned on depending on the variation in the potential at the control terminal. During the second period, the potential at the control terminal is set to the compensation initial value by controlling the first switch to be turned on. During the third period, since the driving transistor which has the diode connection via the first switch is controlled to be turned on depending on the variation in the driving potential (the potential at the first terminal), charge in the control terminal is moved to the driving signal line via the first switch, the circuit point, the second terminal, and the first terminal. Therefore, a voltage between the control terminal and the first terminal of the driving transistor approaches its threshold voltage (ideally, reaches its threshold voltage). By supplying a grayscale potential to the pixel circuit via the signal line and the second switch during the fourth period and by varying the voltage between the control terminal and the first terminal with the passage of time During the fifth period, the driving transistor is changed from one of the turned-on state and the turned-off state to the other at a time point corresponding to a grayscale potential within the fifth period, and the application and the stopping of the voltage to the electro-optical element are controlled to be changed.
In the above-described configuration, the potential at the circuit point is fixed to the first potential during the first period, and thus if the first potential is appropriately set, it is possible to reliably enable a current to flow through the driving transistor during the third period. Therefore, it is possible to effectively compensate for errors in characteristics of the driving transistor by the compensation operation during the third period even in a state where the high resistance driven element is connected to the circuit point. In addition, the electro-optical element is a driven element which converts one of an electrical operation (an application of an electric field or a supply of a current) and an optical operation (variation in a grayscale or luminance) into the other. For example, a high resistance driven element such as an electrophoresis element or a liquid crystal element is suitably used as the electro-optical element of the invention.
In the electro-optical device related to the preferred aspect of the invention, the pixel circuit is provided in plurality and the plurality of pixel circuits is connected to the signal line, and the driving circuit performs an operation for setting a potential at the control terminal to the compensation initial value during the second period and a compensation operation for varying the driving potential from the first potential to the second potential during a state where the first switch is controlled to be in the turned-on state, for the plurality of pixel circuits in parallel during the third period. In the above aspect, since the operations during the second and third periods are performed in parallel for a plurality of pixel circuits, there is an advantage in that time required for the compensation operation for the plurality of pixel circuits, for example, as compared with a configuration of setting the second and third periods for each of the plurality of pixel circuits connected to a single signal line (for example, with row units in a configuration where the pixel circuits are arranged in a matrix), is shortened.
During the second period, a method for setting the potential at the control terminal to the compensation initial value is arbitrary. For example, a driving circuit in the aspect of the invention (for example, a first embodiment) may control the first switch to be turned on and then sets the potential at the control terminal to the compensation initial value by varying the potential at the control terminal so as to be reverse to the variation during the first period, during the second period. In the aspect of the invention, since a capacitive component accompanied by the circuit point is disconnected from the control terminal during the first period but is connected to the control terminal during the second period, the varying amount of the potential at the control terminal during the second period is lower than the varying amount of the potential at the control terminal during the first period. By the use of the difference in the varying amount described above, it is possible to set a compensation initial value (for example, if the driving transistor is of an N channel type, the compensation initial value is set to a high potential) such that the driving transistor is easily changed to the turned-on state during the third period.
On the other hand, a driving circuit in the aspect of the invention may set the potential at the control terminal to the compensation initial value by varying the potential at the control terminal so as to be reverse to the variation during the first period before the second period starts and controlling the first switch to be turned on during the second period. In the aspect of the invention, when the potential at the control terminal is varied so as to be reverse to the variation during the first period before the second period starts and the circuit point and the control terminal is connected to each other via the first switch during the second period, charge accumulated in the capacitive component accompanied by the circuit point is moved to the control terminal, and thus the compensation initial value is set. Therefore, it is possible to set a compensation initial value (for example, if the driving transistor is of an N channel type, the compensation initial value is set to a high potential) such that the driving transistor is easily changed to the turned-on state during the third period.
According to the configuration in which a compensation initial value is set such that the driving transistor is easily changed to the turned-on state during the third period like in the aspects of the invention exemplified above, there is an advantage in that an amplitude of the driving potential (a difference between the first potential and the second potential) required to change the driving transistor to the turned-on state during the third period is reduced. In order to sufficiently vary the potential at the control terminal when the first switch is changed to the turned-on state, a configuration is particularly preferable in which an additional capacitive element which is independent from the electro-optical element is connected to the circuit point.
In an appropriate example aiming at a method for varying a voltage between the control terminal and the first terminal with the passage of time, the pixel circuit may further include a second capacitive element that has a third electrode (for example, an electrode E3) connected to a capacitance line to which a capacitance potential is supplied and a fourth electrode (for example, an electrode E4) connected to the control terminal, and wherein the driving circuit varies the potential at the control terminal with the passage of time due to a capacitive coupling with the second capacitive element by varying the capacitance potential during the fifth period. A voltage between the control terminal and the first terminal may be varied with the passage of time by varying a driving potential (a potential at the first terminal of the driving transistor) for a driving potential line during the fifth period.
The electro-optical device related to the above-described respective aspects may employ an aspect in which the driving circuits vary a voltage between the control terminal and the first terminal with the passage of time such that the driving transistor is changed from a turned-off state to a turned-on state at a time point corresponding to a designated grayscale within the fifth period, and an aspect in which the driving circuits vary a voltage between the control terminal and the first terminal with the passage of time such that the driving transistor is changed from a turned-on state to a turned-off state at a time point corresponding to a designated grayscale within the fifth period. However, from the viewpoint of reducing time from the start point of the fifth period till a viewer can recognize the content of a displayed image, the former aspect is particularly preferable in which a viewer can recognize the content of a displayed image from the start point of the fifth period.
In the preferred aspect of the invention, the driving circuit applies a voltage with an opposite polarity to a case where the driving transistor is turned on during the fifth period, to the electro-optical element during the first period. In the above-described aspect, since a voltage (a reverse bias) with an opposite polarity to a voltage (a forward bias) applied in a case where the driving transistor is turned on during the fifth period is applied to the electro-optical element during the first period, an application of a DC component to the electro-optical element is reduced as compared with a configuration where a voltage is not applied to the electro-optical element during the first period. Therefore, it is possible to suppress deterioration in characteristics of the electro-optical element caused by the application of the DC component.
An electro-optical device related to an appropriate example of the aspect of the invention (for example, a third embodiment) may further provide a display portion in which the plurality of pixel circuits is arranged in a planar shape, wherein a first unit period and a second unit period are set each of which includes the first period, the second period, the third period, and the fifth period in a case where a display image on the display portion is changed from a first image including a first grayscale and a second grayscale to a second image, and wherein the driving circuit supplies a grayscale potential according to the first grayscale to first pixel circuits corresponding to pixels of the first grayscale in the first image among the plurality of pixel circuits and supplies a grayscale potential according to the second grayscale to second pixel circuits corresponding to pixels of the second grayscale in the first image during the fourth period in the first unit period, and supplies a grayscale potential corresponding to a grayscale for the second image to the respective pixel circuits during the fourth period in the second unit period. In the above aspect, by applying reverse bias to both of the first pixel circuits and the second pixel circuits during the first period in the first unit period, and by supplying a grayscale potential for the second grayscale to the first pixel circuits and supplying a grayscale potential for the first grayscale to the second pixel circuits during the fourth period in the first unit period, an accumulated charge amount in the first pixel circuits, charge amounts accumulated in the electro-optical element in the second pixel circuits become the same as each other. By applying the reverse bias during the first period in the second unit period, a charge amount in the electro-optical elements of both of the first and second pixel circuits is set to zero. Therefore, it is possible to effectively suppress the application of the DC component to the electro-optical element.
The electro-optical devices related to the above-described respective aspects may be mounted on various kinds of electronic apparatuses, for example, as display devices displaying images. The electro-optical device of the invention is appropriately applicable to various kinds of electronic apparatuses such as a portable information terminal (for example, a portable phone or a wristwatch) and an electronic paper.
The invention is specified as a method for driving the electro-optical device related to the above-described respective aspects. Specifically, according to another aspect of the invention, there is provided a method for driving an electro-optical device which has a pixel circuit including a driving transistor that has a first terminal connected to a driving potential line to which a driving potential is supplied, a second terminal connected to a circuit point, and a control terminal controlling a connection state between both the first and second terminals; an electro-optical element that is connected to the circuit point; a first capacitive element that has a first electrode, and a second electrode connected to the control terminal; a first switch that controls a connection between the circuit point and the control terminal, a second switch that controls a connection between a signal line and the first electrode, the method including controlling the first switch to be turned off, and varying a potential at the control terminal such that the driving transistor is turned on, during a first period when the driving potential is set to a first potential; setting the potential at the control terminal to a compensation initial value by controlling the first switch to be turned on, during a second period after the first period has elapsed; controlling the first switch to be turned on, and varying the driving potential from the first potential to a second potential such that the driving transistor is turned on, during a third period after the second period has elapsed; supplying a grayscale potential corresponding to a designated grayscale to the signal line and controlling the second switch to be turned on, during a fourth period after the third period has elapsed; and varying a voltage between the control terminal and the first terminal with the passage of time, during a fifth period after the fourth period has elapsed. The above-described driving method achieves the operation and the effect which are the same as in the electro-optical device related to the invention.
The invention is also specified as a control circuit (for example, a control circuit 12 in
The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements.
The display portion 20 is provided with M control lines 22, M control lines 28, and N signal lines 24 which intersect the control lines 22 and the control lines 28 (where M and N are natural numbers). The plurality of pixel circuits PIX in the display portion 20 are disposed at the respective intersections of the control lines 22 (control lines 28) and the signal lines 24 and arranged in a matrix of height M rows×width N column. In addition, the display portion 20 includes a driving potential line 26 and a capacitance line 48. The driving potential line 26 and the capacitance line 48 are lines which are commonly connected to all of the pixel circuits PIX in the display portion 20.
The driving circuits 30 drive the respective pixel circuit PIX under the control of the control circuit 12. As shown in
The potential control circuit 36 generates and outputs potentials (a driving potential VDR, a capacitance potential SC, and a common potential VCOM) which are commonly supplied to the respective pixel circuits PIX. The driving potential VDR is respectively set to a higher potential VDR_H or a lower potential VDR_L (VDR_H>VDR_L). The driving potential VDR is supplied to the driving potential line 26, and the capacitance potential SC is supplied to the capacitance line 48. The common potential VCOM is set to a higher potential VCOM_H or a lower potential VCOM_L (VCOM_H>VCOM_L). The higher potential VCOM_H of the common potential VCOM and the higher potential VDR_H of the driving potentials VDR[1] to VDR[m] have the same potential (for example, 15 V), and the lower potential VCOM_L of the common potential VCOM and the lower potential VDR_L and the driving potentials VDR[1] to VDR[m] have the same potential (for example, 0 V).
The electrophoresis element 40 is a high resistance electro-optical element which represents grayscales using electrophoresis of charged particles, and includes a pixel electrode 42 and an opposite electrode 44 which are opposite to each other, and an electrophoresis layer 46 between both the electrodes. As shown in
The pixel electrode 42 is individually formed for each pixel circuit PIX, and the opposite electrode 44 is formed continuously over the plurality of pixel circuits PIX. As shown in
The driving transistor TDR in
The switch SW1 is constituted by an N channel thin film transistor in the same manner as the driving transistor TDR, and is connected between the gate of the driving transistor TDR and the circuit point p (between the gate and the drain of the driving transistor TDR) so as to control electrical connection between both the terminals (connection and disconnection). The gate of the switch SW1 is connected to the m-th row control line 22. Therefore, if the switch SW1 is changed to be turned on, the gate and the drain of the driving transistor TDR are connected (diode-connected) to each other.
The capacitive element C1 is an electrostatic capacitor including an electrode E1 and an electrode E2. The electrode E2 is connected to the gate of the driving transistor TDR. The switch SW2 is constituted by an N channel thin film transistor in the same manner as the driving transistor TDR or the switch SW1, and is connected between the signal line 24 in the n-th column and the electrode E1 of the capacitive element C1 so as to control electrical connection therebetween (connection and disconnection). The gate of the switch SW2 is connected to the control line 28 in the m-th row.
The capacitive element C2 is an electrostatic capacitor including an electrode E3 and an electrode E4. The electrode E3 is connected to a capacitance line 48, and the electrode E4 is connected to the gate of the driving transistor TDR. The additional capacitive element CP is an electrostatic capacitor including an electrode EP1 and an electrode EP2. The electrode EP1 is connected to the circuit point p, and the electrode EP2 is connected to the ground (GND). In addition, if the electrophoresis element 40 accompanies a sufficient capacitive component, the capacitive component of the electrophoresis element 40 may be used as the additional capacitive element CP.
During the compensation period TCMP, a compensation operation in which the gate-source voltage VGS of the driving transistor TDR in each pixel circuit PIX is set to the threshold voltage VTH of the corresponding driving transistor TDR is performed. The compensation operation is performed in parallel (together) for all of the pixel circuits PIX in the display portion 20. As shown in
During the writing period TWRT, a writing operation is performed in which a grayscale potential VD[m,n] corresponding to a grayscale designated to the pixel circuit PIX is supplied to each pixel circuit PIX. As shown in
During the driving period TDRV, the grayscale for the electrophoresis element 40 is controlled to be varied depending on the grayscale potential VD[m,n] which has been supplied to each pixel circuit PIX during the writing period TWRT. Specifically, during the driving period TDRV, a driving operation (pulse width modulation) is performed in which the driving transistor TDR is turned on at a period of a time length corresponding to the grayscale potential VD[m,n], thereby controlling the grayscale for the electrophoresis element 40. The driving operation is performed in parallel (together) for all (M×N) the pixel circuits PIX in the display portion 20.
1. Reset Period TRST
When the reset period TRST starts, the column driving circuit 34 sets potentials of the instruction signals X[1] to X[N] for the respective signal lines 24 to the reference potential VC as shown in
As shown in
The reset potential VRST of the capacitance potential SC is set such that the driving transistor TDR is maintained to be turned on (VGS=VG1−VDR_H>VTH) in a state where the driving potential VDR is set to the higher potential VDR_H (for example, VRST=30 V). As described above, since the driving transistor TDR is controlled to be turned on during the reset period TRST, as shown in
As shown in
2. Compensation Period TCMP
When the compensation preparing period QA (the time point tb in
If the time point tc comes after the time point tb in the compensation preparing period QA, the potential control circuit 36 decreases the capacitance potential SC from the reset potential VRST to the potential V0 as shown in
When the compensation performing period QB starts (time point td in
On the other hand, the switch SW1 is continuously maintained to be turned on (the diode connection of the driving transistor TDR) during the compensation performing period QB from the compensation preparing period QA. Therefore, if the driving transistor TDR is turned on at the time of starting of the compensation performing period QB, as denoted by the arrow in
When the compensation performing period QB finishes, the row driving circuit 32 controls the switch SW1 and the switch SW2 of each pixel circuit PIX to be turned off by changing both of the control signals GA[1] to GA[m] and the control signals GB[1] to GB[m] to the low level as shown in
3. Writing Period TWRT
As shown in
If the potential at the electrode E1 is varied by the varying amount δ (δ=VD[m,n]−VC) during the selection period H[m], as shown in
4. Driving Period TDRV
When the driving period TDRV starts after the writing period TWRT has elapsed, the potential control circuit 36, as shown in
The potential control circuit 36, as shown in
Since the capacitive element C2 is connected between the capacitance line 48 and the gate of the driving transistor TDR, the potential VG at the gate of the driving transistor TDR of each pixel circuit PIX is capacitively coupled to the capacitive element C2 and thus is varied with the passage of time according to the capacitance potential SC (potential W(t)). First, if the potential W(t) is changed from the potential V0 to the potential VL at the start point of the driving period TDRV, the potential VG at the gate of the driving transistor TDR is changed (decreased) by the varying amount v from the potential VG3 after the setting during the selection period H[m] to the potential VG4 as shown in
The potential VG at the gate of the driving transistor TDR, as shown in
If the driving transistor TDR is turned on at the time point corresponding to the grayscale potential VD[m,n] in the driving period TDRV, since the lower potential VDR_L of the driving potential VDR is supplied to the pixel electrode 42 from the driving potential line 26 via the driving transistor TDR, a voltage with the positive polarity (hereinafter, referred to as a “forward bias”) corresponding to a difference between the lower potential VDR_L of the driving potential VDR and the higher potential VCOM_H of the common potential VCOM is applied to the electrophoresis element 40. Therefore, the black charged particles 462B of the electrophoresis element 40 are moved to the observing side and the white charged particles 462W are moved to the rear surface side, and thus the display grayscale is changed to the black side. When the driving period TDRV finishes, the potential control circuit 36 changes the common potential VCOM to the lower potential VCOM_L (VCOM_L=VDR_L). Therefore, the application of voltages to the electrophoresis element 40 finishes.
As described above, since the forward bias is applied to the electrophoresis element 40 with the variable time length corresponding to the grayscale potential VD[m,n] (pulse width modulation), the grayscale for the electrophoresis element 40 of each pixel circuit PIX is controlled in multiple steps depending on the grayscale potential VD[m,n] for the corresponding pixel circuit PIX. Specifically, as the grayscale potential VD[m,n] is increased (the time length when the driving transistor TDR is turned on during the driving period TDRV is increased), the grayscale for the electrophoresis element 40 is controlled to have a low grayscale (grayscale close to black). Thus, images with multi-grayscales including intermediate grayscales in addition to white and black are displayed on the display portion 20. In addition, the unit period TU is repeated at all times and thus the displayed images are changed.
In the above-described first embodiment, the driving transistor TDR is turned on during the reset period TRST, and thus the potential VP at the circuit point p is reset to the higher potential VDR_H. Thus, it is possible to reliably enable currents to flow between the drain (gate) and the source (that is, the compensation operation is performed) in the case where the driving transistor TDR has the diode connection during the compensation performing period QB. In other words, regardless of the configuration of employing the high resistance electro-optical element (the electrophoresis element 40), it is possible to effectively compensate for the error in the characteristic (the threshold voltage VTH) of the driving transistor TDR (furthermore, unevenness in the grayscales for the displayed image is suppressed). In addition, since the higher potential VDR_H is supplied to the circuit point p by turning on the driving transistor TDR, it is not necessary to mount an element only used to reset the potential VP (supply of the higher potential VDR_H) at the circuit point p on the pixel circuit PIX. Thereby, there is an advantage in that the configuration of the pixel circuit PIX is simplified.
In order to start the compensation operation at the compensation performing period QB, it is necessary to decrease the potential (the driving potential VDR) at the source of the driving transistor TDR as compared with the potential VG at the gate such that the gate-source voltage VGS of the driving transistor TDR becomes greater than the threshold voltage VTH. In the first embodiment, since the potential VG at the gate of the driving transistor TDR is set (increased) to the compensation initial value VINI higher than the initial potential VG0 by the use of the difference between the varying amount δL_H of the potential VG in the state where the additional capacitive element CP is disconnected from the gate and the varying amount δH_L of the potential VG in the state where the additional capacitive element CP is connected to the gate, if compared with a configuration (hereinafter, referred to as a “comparative example”) in which the potential VG is not increased during the compensation preparing period QA, there is an advantage in that necessary conditions for the lower potential VDR_L of the driving potential VDR are mitigated.
For example, if the threshold voltage VTH is assumed as 1 V, a case of a comparative example where the compensation operation starts in a state where the potential VG at the gate of the driving transistor TDR is set to the potential VG0 (that is, a configuration where the compensation preparing period QA is omitted) will be described. If the potential VG0 is −3 V, in order to realize the compensation operation based on the comparative example, the lower potential VDR_L of the driving potential VDR is required to be set to −4 V. On the other hand, in the first embodiment, since the potential VG is increased to the compensation initial value VINI of, for example, 3 V, through the connection between the gate of the driving transistor TDR and the additional capacitive element CP during the compensation preparing period QA, the lower potential VDR_L of the driving potential VDR is enough to be set to 2 V or less. That is to say, since conditions necessary for the lower potential VDR_L of the driving potential VDR are mitigated, it is possible to set the respective potentials (VDR_H and VDR_L) of the driving potential VDR to the same potentials as the respective potentials (VCOM_H and VCOM_L) of the common potential VCOM. As described above, the respective potentials are made to be common (the number of kinds of potentials is reduced), and thus there is an advantage in that configurations for generating the respective potentials are simplified. Further, due to the compensation operation during the compensation performing period QB, the operation for diode connecting the driving transistor TDR during the compensation preparing period QA is used to set the compensation initial value VINI. Therefore, for example, as compared with a configuration in which an element only used to increase the potential VG before the compensation operation is performed is specially installed in the pixel circuit PIX, it is possible to simplify a configuration of the pixel circuit PIX.
Since the compensation operation is performed in parallel for all of the pixel circuits PIX in the display portion 20 during the compensation period TCMP, it is possible to reduce the time required for the compensation operation for each pixel circuit PIX as compared with, for example, a configuration in which the compensation operation is performed with row units. Therefore, there is an advantage in that the unit period TU required to update images displayed on the display portion 20 is reduced. In addition, since the switch SW2 is connected between the capacitive element C1 and the signal line 24 for each pixel circuit PIX, a capacitive component accompanied by the signal line 24 is reduced as compared with a configuration in which the capacitive element C1 is directly connected to the signal line 24. Thus, power wasted in the charging and discharging of the signal line 24 is reduced.
Meanwhile, in a configuration in which a voltage with one polarity (DC component) is continuously applied to the electrophoresis element 40, the characteristics of the electrophoresis element 40 may be deteriorated. In the first embodiment, the application and the stopping of the forward bias to the electrophoresis element 40 are selectively performed during the driving period TDRV (that is, a voltage with the negative polarity is not applied to the electrophoresis element 40 during the driving period TDRV), and the reverse bias with a polarity reverse to the polarity of the voltage applied during the driving period TDRV is applied to the electrophoresis element 40 during the reset period TRST. Therefore, it is possible to suppress the deterioration in the electrophoresis element 40 due to the application of the DC component as compared with the configuration in which the reverse bias is not applied. Since the higher potential VDR_H supplied to the circuit point p during the reset period TRST for the realization of the compensation operation is also used to apply the reverse bias to the electrophoresis element 40, there is an advantage in that a configuration of the pixel circuit PIX is simplified as compared with a configuration in which an element only used to apply the reverse bias to the pixel circuit PIX is installed.
Next, a second embodiment of the invention will be described. In addition, the elements having the same operations or functions as in the first embodiment in the respective aspects exemplified below are given the reference numerals described above, and the description thereof will be appropriately omitted.
In the first embodiment, the potential VG is set to the compensation initial value VINI (a potential higher than the potential VG0) by the use of the difference (δL_H>δH_L) between the increasing amount δL_H and the decreasing amount δH_L of the potential VG. The second embodiment is different from the first embodiment in a method for setting (increasing) the potential VG at the gate of the driving transistor TDR to the compensation initial value VINI during the compensation preparing period QA. The configuration of the pixel circuit PIX is the same as in the first embodiment.
When the compensation preparing period QA of the compensation period TCMP after the reset period TRST finishes, the row driving circuit 32 controls the switches SW1 of all of the pixel circuits PIX to be turned on by setting the control signals GA[1] to GA[m] to the high level as shown in
VINI=γp·VDR—H+(1−γp)VG2 (1)
During the compensation performing period QB after the compensation preparing period QA has elapsed, in the same manner as the first embodiment, the driving potential VDR is changed from the higher potential VDR_H to the lower potential VDR_L, and thereby the compensation operation is performed. The operations during the writing period TWRT and the driving period TDRV are the same as in the first embodiment. The second embodiment also achieves the same effect as the first embodiment.
In the above-described embodiments, the forward bias (a voltage with the positive polarity) is applied to the electrophoresis element 40 during the driving period TDRV, and the reverse bias (a voltage with the negative polarity) is applied to the electrophoresis element 40 during the reset period TRST. Therefore, upon comparison with the configuration in which the reverse bias is not applied during the unit period TU (for example, a configuration in which the common potential VCOM is maintained to be the higher potential VCOM_H during the reset period TRST), it is possible to suppress the DC component from being applied to the electrophoresis element 40. However, since a time for applying the forward bias and a time for applying the reverse bias (the reset period TRST) are different from each other, it is difficult to completely prevent the DC component from being applied to the electrophoresis element 40. Therefore, in the third embodiment, the application of the DC component is prevented by appropriately selecting the grayscale potential VD[m,n] over a plurality of unit periods TU in a case where displayed images are changed.
During the reset period in the unit period TU1, the reverse bias is applied to the electrophoresis elements 40 of all of the pixel circuits PIX. As shown in
In addition, during the writing operation in the unit period TU1, the control circuit 12 designates the white grayscale for each of the first pixel circuits PIX which have displayed black pixels of the image IMG1 and designates the black grayscale for each of the second pixel circuits PIX which have displayed white pixels of the image IMG1. Therefore, during the driving operation (the driving period TDRV) in the unit period TU1, as shown in
In the reset operation (reset period TRST) during the unit period TU2 as well, in the same manner as the unit period TU1, the reverse bias is applied to the electrophoresis elements 40 of all of the pixel circuits PIX, and thus charge corresponding to Q is removed from both of the first pixel circuits PIX and the second pixel circuits PIX. Thus, as shown in
According to the third embodiment described above, despite the configuration where only the forward bias is applied to the electrophoresis element 40 during the driving period TDRV, and the reverse bias is collectively applied to the electrophoresis elements 40 of all of the pixel circuits PIX during the reset period TRST, it is possible to effectively prevent the DC component from being applied to the electrophoresis element 40. Therefore, there is an advantage in that deterioration in the electrophoresis element 40 caused by the application of the DC component can be effectively prevented.
In the above description, although in the writing operation during the unit period TU1, each of the first pixel circuits PIX which display black pixels of the image IMG1 is designated to represent the white grayscale, and each of the second pixel circuits PIX which display white pixels of the image IMG1 is designated to represent the black grayscale, the image IMG1 is not limited to binary images of white and black. For example, the above-described embodiment is also applied to a case where the image IMG1 includes intermediate grayscales. Assuming a case where the image IMG1 before being changed includes a first grayscale and a second grayscale (regardless of presence or absence of other grayscales), the writing operation during the unit period TU1 is generalized as an operation in which a grayscale potential VD[m,n] according to the first grayscale is supplied to the respective first pixel circuits PIX which display pixels of the first grayscale of the image IMG1 and a grayscale potential VD[m,n] according to the second grayscale is supplied to the respective second pixel circuits PIX which display pixels of the second grayscale of the image IMG1. As the “grayscale according to the first grayscale” in the above expression, a complementary grayscale of the first grayscale is preferable. In the same manner, as the “grayscale according to the second grayscale”, a complementary grayscale of the second grayscale is preferable. The “complementary grayscale” means a grayscale having the same luminance difference from a central value of white and black (that is, an intermediate luminance between the highest luminance and the lowest luminance). For example, assuming four kinds of grayscales, white, light gray, dark gray, and black, the relationship between white and black or the relationship between light gray and dark gray corresponds to the complementary grayscale. According to the above-described configuration, even when the image IMG1 includes the intermediate grayscales, the grayscales for the electrophoresis elements 40 of both of the first pixel circuits PIX and the second pixel circuits PIX can be arranged to have the intermediate grayscale corresponding to the charge amount +1Q.
The above-described embodiments can be variously modified. Detailed aspects of modifications will be exemplified below. Two or more aspects which are arbitrarily selected from the following examples may be appropriately combined.
In the respective embodiments, although the configuration (hereinafter, referred to as “configuration A”) where the driving transistor TDR is changed from the turned-off state to the turned-on state at a time point corresponding to a designated grayscale during the driving period TDRV has been exemplified, a configuration (hereinafter, referred to as “configuration B”) where the driving transistor TDR is changed from the turned-on state to the turned-off state at the time point corresponding to the designated grayscale during the driving period TDRV may be employed. In configuration B, the capacitance potential SC is decreased from the potential VH to the potential VL during the driving period TDRV. However, configuration A employed in the respective embodiments described above is advantageous in that a time from the starting of the driving period TDRV until a user actually recognizes the content of a displayed image can be reduced as compared with configuration B, as described below in detail.
As shown in
On the other hand, as shown in
The conductivity type of each transistor constituting the pixel circuit PIX is arbitrarily changed. For example, a configuration in
In addition, a material, a structure, or a manufacturing method of each transistor (TDR, SW1 and SW2) of the pixel circuit PIX is arbitrary. For example, a material of a semiconductor layer of each transistor may use, for example, an amorphous silicon, an oxide semiconductor, an organic semiconductor, a polycrystalline semiconductor (for example, a high-temperature poly-silicon or a low-temperature poly-silicon).
In the above-described embodiments, although the gate-source voltage VGS of the driving transistor TDR is varied with the passage of time by setting the capacitance potential SC to the potential W(t) during the driving period TDRV, the method for varying the voltage VGS during the driving period TDRV is appropriately changed. For example, a configuration may be employed in which the voltage VGS of the driving transistor TDR is varied with the passage of time by varying (decreasing) the driving potential VDR supplied to the source of the driving transistor TDR during the driving period TDRV.
Although in the respective embodiments, the potential W(t) is controlled to the ramp waveform (in other words, a waveform which monotonically increases or monotonically decreases in a linear manner), a waveform of the potential W(t) is arbitrary. For example, although the potential W(t) is varied linearly in the above-described embodiments, the potential W(t) may be varied in a curved manner. In addition, in the above-described embodiments, although the potential W(t) monotonically increases during the driving period TDRV, the potential W(t) may be increased and decreased during the driving period TDRV. Specifically, a triangular wave in which a potential is linearly increased (decreased) from the start point of the driving period TDRV and is linearly decreased (increased) in the course thereof or a sinusoidal wave in which a potential is varied in a curved manner during the driving period TDRV may be used as the potential W(t).
Relationships between voltages applied to the electrophoresis element 40 and the grayscales are not limited to the above-described embodiments. For example, in a case of the electrophoresis element 40 using the white charged particles 462W with the negative polarity and the black charged particles 462B with the positive polarity in contrast to the embodiment shown in
The configuration of the electrophoresis element 40 is appropriately changed. For example, a configuration in which the white charged particles 462W are dispersed in the black dispersion medium 464 or the black charged particles 462B are dispersed in the white dispersion medium 464 may be employed (one-particle system). In addition, colors of the charged particles 462 or the dispersion medium 464 constituting the electrophoresis element 40 are arbitrarily changed. The electrophoresis element 40 may be employed in which three kinds or more of particles (for example, one kind is not charged) corresponding to different display colors are dispersed.
However, a target to be driven by the pixel circuit PIX in the above-described respective embodiments is not limited to the electrophoresis element 40. For example, the invention is applicable to driving arbitrary electro-optical elements such as a liquid crystal element, a light emitting element (for example, an organic EL element or an LED (Light Emitting Diode)), a field emission (FE) element, a surface conduction electron emitter (SE) element, a ballistic electron emitting (BS) element, and a non-emissive element. That is to say, the electro-optical element is generalized as a driven element which converts one of an electrical operation (an application of a voltage or a supply of a current) and an optical operation (variation in a grayscale or light emission) into the other. However, from the viewpoint of achieving the above-described object of effectively compensating errors in characteristics of the driving transistor TDR, it is particularly preferable that the invention is applied to a case of driving a high resistance electro-optical element such as the electrophoresis element 40 or the liquid crystal element.
An electronic device to which the invention is applied will be exemplified.
An electronic apparatus to which the invention is applied is not limited to the above-described examples. For example, the electro-optical device of the invention is applicable to various kinds of electronic apparatuses such as a portable phone, a timepiece (wristwatch), a portable audio reproduction device, an electronic diary, and a touch panel mounting display device.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-120197 | May 2010 | JP | national |
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| Number | Date | Country | |
|---|---|---|---|
| 20110292093 A1 | Dec 2011 | US |
