JP2006313189A - Luminescence system, its driving method, and electronic equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To adjust luminescence of images having a simple constitution, without impairing the gradation characteristics. <P>SOLUTION: A pixel circuit P controls OLED elements 15 to a shade corresponding to a voltage of a data line 13. A voltage-adjusting circuit 41 generates a voltage V0 and a voltage V63, and changes the level of the voltage V0 according to the instruction from a control circuit 30. A reference voltage generating circuit 42 generates multiple reference voltages V16, V32, V48 by partial voltages of the voltage V0 and voltage V63 therebetween. A data line drive circuit 23 selects any one of multiple gradation voltages V0 or V63, including multiple reference voltages generated by the reference voltage generating circuit 42 according to image data G, and outputs to a data line 13 as a data voltage XVj. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

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

Conventionally, display devices that adjust the overall brightness of an image (hereinafter referred to as “image brightness”) in accordance with an instruction from a user have been proposed. For example, in a liquid crystal display device equipped with a backlight, the image brightness can be changed by adjusting the amount of light emitted from the backlight. However, since a display device using a light emitting element such as an OLED element does not include a backlight, the luminance cannot be adjusted by this method. In view of this, Patent Document 1 discloses a configuration in which image brightness can be adjusted by controlling a relative ratio of time lengths between a period during which a light emitting element actually emits light and a period during which light emission is stopped.
Japanese Patent Laying-Open No. 2004-325940 (paragraph 0024 and FIG. 2)

  However, under this technique, problems such as flicker may occur due to a change in the interval at which the light emitting element emits light. On the other hand, it is also possible to adjust the image brightness while avoiding the occurrence of flicker by processing the data input to the display device in accordance with the image brightness in order to specify the gradation for each pixel. However, in order to adjust the image brightness while maintaining the relationship between the gradation value specified by the data and the actual gradation of each light emitting element (hereinafter referred to as “gradation characteristics”) to the desired characteristics, it is complicated. There is a problem that an element for executing a proper calculation and control becomes necessary, and the configuration becomes complicated. The present invention has been made in view of such circumstances, and an object of the present invention is to solve the problem of adjusting the brightness of an image with a simple configuration without impairing gradation characteristics.

  In order to solve this problem, a light emitting device according to the present invention is different from a unit circuit (that is, a voltage programming electronic circuit) that controls a light emitting element at a gray level according to a voltage of a data line. A circuit that generates (for example, the voltage V0 in the first embodiment or the voltage V1 in the second embodiment) and a second voltage (for example, the voltage V63 in the first embodiment), and at least one of the first voltage and the second voltage Voltage adjusting circuit that changes the voltage according to an instruction from the outside, and a reference voltage generating circuit that generates a plurality of reference voltages within a range from the first voltage to the second voltage by dividing the first voltage and the second voltage And one of a plurality of gradation voltages including a plurality of reference voltages generated by the reference voltage generation circuit, selected according to a designated gradation value (gradation designated by image data) and output to the data line line Drive circuit. In a configuration in which a plurality of reference voltages are generated by dividing the first voltage and the second voltage as in this configuration, when at least one of the first voltage and the second voltage is changed according to an instruction from the outside Depending on the amount of change, other reference voltages also increase or decrease. That is, the brightness (image brightness) of the unit circuit can be adjusted with a simple configuration of changing the first voltage or the second voltage. In addition, tone characteristics (gamma characteristics) can be maintained when adjusting the image brightness.

  In a preferred aspect of the present invention, a grayscale voltage generation circuit (for example, the resistor string in FIG. 4) that generates a plurality of grayscale voltages including a plurality of reference voltages generated by the reference voltage generation circuit and a voltage between the reference voltages. 252) is further provided. According to this configuration, since the voltage value (level) of the data voltage is diversified as compared with the configuration in which any of the reference voltages generated by the reference voltage generation circuit is output as the data voltage, more light emitting elements are provided. The gradation can be controlled. Note that the position where the gradation voltage generation circuit is arranged is arbitrary. That is, in each embodiment to be described later, a mode in which the gradation voltage generation circuit (resistor string 252) is arranged as a part of the data line driving circuit is illustrated, but the reference voltage generation circuit (reference voltage generation circuit 42 in FIG. 1). It may be arranged as a part. Further, each reference voltage may be used as it is as a gradation voltage if the desired number of gradations is realized by a plurality of reference voltages generated by the reference voltage generation circuit. In this case, it is not necessary to generate the gradation voltage separately from the reference voltage.

  The unit circuit in a specific embodiment of the present invention includes a drive transistor that is inserted in a path of a drive current supplied to the light emitting element and controls the amount of the drive current according to the voltage of the gate. In this aspect, the first voltage and the second voltage are set so that the driving transistor operates in the saturation region even if any voltage within the range from the first voltage to the second voltage is supplied from the data line to the unit circuit. Each voltage value is selected. According to this aspect, since the driving transistor operates in the saturation region regardless of which gradation value is designated, the luminance of the light emitting element for each designated gradation value can be adjusted reliably and with high accuracy. It becomes possible. The current flowing through the drive transistor operating in the saturation region is proportional to the square of the voltage between its gate and source. Therefore, when this mode is grasped from another viewpoint, each voltage of the first voltage and the second voltage is such that the current flowing through the drive transistor is proportional to the square of the voltage between the gate and the source. It can also be said that a value is selected.

  A graph showing the relationship between each of a plurality of specified gradation values and the data voltage output from the data line driving circuit for the specified gradation value is based on the n (n is a real number) power of the specified gradation value. It may be a polygonal line that approximates a function that changes. According to this configuration, by designing the characteristics of the reference voltage generation circuit, the gradation voltage generation circuit, and the data line driving circuit so that the order n becomes an expected value, the specified gradation value and the luminance of the light emitting element can be reduced. The relationship can be a desired characteristic. For example, when the light-emitting element emits light with a luminance corresponding to the γth power of the specified gradation value, each part of the light-emitting device (particularly the reference voltage generation circuit and the gradation voltage generation) so that the order n is “γ / 2”. Circuit, data line drive circuit) characteristics are selected. The current supplied from the driving transistor to the light emitting element (or the luminance proportional to the current) is a numerical value corresponding to approximately the square of the voltage (more specifically, the data voltage) between the gate and the source of the driving transistor. When the order n is selected as described above, the relationship (total gamma) of the luminance of the light emitting element with respect to the designated gradation number is the desired characteristic (that is, the characteristic where the luminance of the light emitting element is the γth power of the designated gradation number) can do.

  In a desirable mode of the present invention, a black voltage (for example, described later) is a voltage outside the range from the first voltage to the second voltage, and is a voltage value at which the drive transistor is turned off when supplied to the data line. A circuit for generating the voltage V0 in the second embodiment separately from the first voltage and the second voltage (for example, the voltage generation circuit 415 in FIG. 7) is further provided, and the data line driving circuit is designated with a black gradation. When selected, the black voltage is selected and output to the data line. According to this aspect, since the driving transistor can be surely turned off when the black gradation is designated, the gradation of the unit circuit can be brought close to perfect black. A specific example of this aspect will be described later as a second embodiment.

  The light emitting device of each aspect described above is used in various electronic devices. A typical example of this electronic device is a device that uses a light emitting 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 light emitting device according to the present invention is not limited to image display. For example, the light emitting device of the present invention can also be applied as an exposure device (exposure head) for forming a latent image on an image carrier such as a photosensitive drum by irradiation of light.

  In the driving method according to the present invention, the first voltage and the second voltage that are different from each other are divided to generate a plurality of reference voltages within a range from the first voltage to the second voltage, and the plurality of reference voltages are generated. One of a plurality of gradation voltages included is selected according to a designated gradation value and output to the data line. On the other hand, when image brightness is designated, at least one of the first voltage and the second voltage is designated according to the designation. To change. According to this method, the image brightness can be adjusted without impairing the gradation characteristics (gamma characteristics) by a simple procedure of adjusting the first voltage or the second voltage.

<A: First Embodiment>
FIG. 1 is a block diagram showing the overall configuration of the light emitting device according to the first embodiment of the present invention. As shown in the figure, the light emitting device D includes a pixel array unit 10 in which a plurality of pixel circuits P are arranged, and various circuits (scanning line drive circuit 21. A data line driving circuit 23, a control circuit 30, and an adjustment circuit 40). The pixel array section 10 includes M (M is a natural number) scanning lines 11 extending in the X direction and N (N is a natural number) data lines 13 extending in the Y direction intersecting the X direction. It is formed. Each pixel circuit P is disposed at a position corresponding to each intersection of the scanning line 11 and the data line 13. Accordingly, these pixel circuits P are arranged in a matrix of vertical M rows × horizontal N columns. Each pixel circuit P is a circuit for adjusting the gradation (luminance) of the OLED element which is a light emitting element.

  The scanning line driving circuit 21 is a circuit that sequentially selects each of the M scanning lines 11 for each horizontal scanning period. More specifically, the scanning line driving circuit 21 supplies the scanning line 11 to the i-th scanning line 11 in the i-th horizontal scanning period (i is an integer satisfying 1 ≦ i ≦ M) in one vertical scanning period. The scanning signal Yi to be performed is set to the low level, and the scanning signals for the other rows are set to the high level. On the other hand, in the horizontal scanning period in which the scanning line driving circuit 21 selects the scanning line 11 in the i-th row, the data line driving circuit 23 has a voltage corresponding to the gradation specified for each pixel circuit P in the row (hereinafter “ XV1 to XVN (referred to as “data voltage”) are applied to each data line 13. The gradation of each pixel circuit P is specified by the image data G. The image data G in the present embodiment is 6-bit digital data, and designates one of 64 gradation values from “0” to “63”. If the image data G has a gradation value “0”, black, which is the darkest gradation, is designated, and a bright gradation is designated as the gradation value increases, and the image data G having a gradation value “63” is designated. The white color which is the brightest gradation is designated by.

  The control circuit 30 is means for controlling the overall operation of the light emitting device D. More specifically, the control circuit 30 controls each circuit by outputting various signals such as a clock signal to the scanning line driving circuit 21 and the data line driving circuit 23, and also outputs the image data G of each pixel circuit P. The data is output to the data line driving circuit 23. On the other hand, the adjustment circuit 40 is a means for adjusting the overall brightness (image brightness) of the image displayed by the pixel array unit 10. Note that specific configurations of the data line driving circuit 23 and the adjustment circuit 40 will be described later.

  FIG. 2 is a circuit diagram showing a configuration of the pixel circuit P. In the figure, only one pixel circuit P in the j-th column (j is an integer satisfying 1 ≦ j ≦ N) belonging to the i-th row is shown, but all the pixel circuits P are the same. It is a configuration. As shown in FIG. 2, the pixel circuit P in the present embodiment includes a drive transistor Tdr, a selection transistor Tsl, a capacitive element C, and an OLED element 15. The drive transistor Tdr and the selection transistor Tsl are p-channel TFTs (Thin Film Transistors).

  The drive transistor Tdr and the OLED element 15 are interposed between a power supply line to which a higher voltage (power supply voltage) VH is supplied and a ground line to which a lower voltage (ground voltage) VL is supplied. Among them, the OLED element 15 is an element that emits light with a luminance corresponding to a forward current (hereinafter referred to as “driving current”) Iel, and a light emitting layer made of an organic EL material is interposed in a gap between the anode and the cathode. It has a structure. The OLED element 15 having such a structure is formed, for example, by discharging a droplet of an organic EL material from an ink jet head and drying it. As the material of the light emitting layer of the OLED element 15, an organic light emitting material such as a low molecular weight polymer or a dendrimer is used.

  As shown in FIG. 2, the OLED element 15 has its cathode connected to the ground line and its anode connected to the drain of the drive transistor Tdr. The source of the driving transistor Tdr is connected to the power supply line. The drive transistor Tdr is means for controlling the drive current Iel flowing from the power supply line to the ground line via the OLED element 15 in accordance with the gate voltage. The gate of the driving transistor Tdr is connected to the drain of the selection transistor Tsl. The selection transistor Tsl has a source connected to the data line 13 and a gate connected to the scanning line 11. The capacitive element C is a capacitance interposed between the gate and source of the driving transistor Tdr.

  In the above configuration, when the scanning signal Yi transitions to a low level and the selection transistor Tsl is turned on, the data voltage XVj of the data line 13 at that time is applied to the gate of the driving transistor Tdr via the selection transistor Tsl. As a result, the drive current Iel corresponding to the data voltage XVj is supplied to the OLED element 15. At this time, electric charge corresponding to the data voltage XVj is accumulated in the capacitive element C. Therefore, even if the scanning signal Yi becomes high level and the selection transistor Tsl is turned off, the voltage held in the capacitive element C is continuously applied to the gate of the driving transistor Tdr, so that the data voltage is applied to the OLED element 15. A drive current Iel corresponding to XVj is supplied. On the other hand, the OLED element 15 emits light with a luminance proportional to the drive current Iel. As described above, the pixel circuit P in the present embodiment is an electronic circuit of a method (so-called voltage programming method) in which the luminance of the OLED element 15 is set according to the voltage of the data line 13.

  Next, specific configurations of the adjustment circuit 40 and the data line driving circuit 23 will be described. FIG. 3 is a block diagram showing a configuration of the adjustment circuit 40. As shown in FIGS. 1 and 3, the adjustment circuit 40 includes a voltage adjustment circuit 41 and a reference voltage generation circuit 42. Among them, the voltage adjustment circuit 41 is a circuit for generating a voltage V0 and a voltage V63 which are references for the data voltage XVj. As shown in FIG. 3, a voltage V0 is generated and output to the wiring 451. A generation circuit 411 and a voltage generation circuit 412 that generates a voltage V63 and outputs the voltage V63 to the wiring 452 are included. The voltage V0 is a voltage corresponding to black (tone value “0”) which is the darkest gradation, and the voltage V63 is a voltage corresponding to white (tone value “63”) which is the brightest gradation.

  The reference voltage generation circuit 42 divides the voltage V0 and the voltage V63 generated by the voltage adjustment circuit 41 and includes five types of reference voltages (V0, V16, V32, V48, V63). As shown in FIG. 3, the reference voltage generation circuit 42 includes four resistance elements 421 interposed between the wiring 451 and the wiring 452 while being connected in series with each other. The reference voltage generation circuit 42 outputs the voltage (V16, V32, V48) between the adjacent resistance elements 521 as a reference voltage together with the voltage V0 and the voltage V63. The voltage V16 corresponds to the gradation value “16”, the voltage V32 corresponds to the gradation value “32”, and the voltage V48 corresponds to the gradation value “48”.

  On the other hand, as shown in FIG. 1, the data line driving circuit 23 includes N D / A converters 233 each connected to a separate data line 13 and image data supplied serially from the control circuit 30. A latch circuit 231 that holds G (G1 to GN) and distributes it to each D / A converter 233. Among these, the D / A converter 233 has a gradation value (hereinafter referred to as “the gradation voltage”) designated by the image data Gj among 64 kinds of voltages (hereinafter referred to as “gradation voltages”) V0 to V63 corresponding to different gradations. A voltage corresponding to “designated gradation value” is selected and output to the data line 13 as the data voltage XVj.

  FIG. 4 is a circuit diagram showing a configuration of the D / A converter 233. In FIG. 4, only one D / A converter 233 corresponding to the data line 13 in the j-th column is illustrated, but all the D / A converters 233 have the same configuration. As shown in the figure, each D / A converter 233 includes five input buffers 251 in which reference voltages (V0, V16, V32, V48, and V63) are applied to respective input terminals, and each input. A resistor string 252 in which a large number (63 in total) of resistive elements R are arranged in series between the output ends of the buffer 251; a selection circuit 253 including 64 switches SW; the selection circuit 253 and the j-th column An output buffer 254 interposed between the eye data line 13 and a decoder 255 to which the image data Gj is supplied from the latch circuit 231 are provided. 64 kinds of gradation voltages V0 to V63 are generated by dividing the five kinds of reference voltages inputted through the respective input buffers 251 by the respective resistance elements R of the resistor string 252. That is, the resistor string 252 generates a plurality of reference voltages (V0, V16, V32, V48, and V63) generated by the reference voltage generation circuit 42 and intermediate voltages (for example, V1 to V15, V17 to V31) between the reference voltages. It functions as a means for generating a plurality of gradation voltages including the gradation voltage generation circuit in the present invention.

  The 64 switches SW constituting the selection circuit 253 individually switch between conduction and non-conduction between each of the gradation voltages V0 to V63 and the input terminal of the output buffer 254. The decoder 255 is means for selectively turning on one switch SW of the selection circuit 253 based on the image data Gj supplied from the latch circuit 231. For example, when the gradation value “0” is designated by the image data G, the decoder 255 alternatively turns on the first-stage switch SW shown in FIG. Thus, the voltage corresponding to the image data Gj among the gradation voltages V0 to V63 is output as the data voltage XVj to the data line 13 via the switch SW and the output buffer 254 whose decoder 255 is turned on. 4 illustrates the configuration in which the input buffer 251 and the resistor string 252 are incorporated in each D / A converter 233, the input buffer 251 and the resistor string 252 are shared by a plurality of D / A converters 233. It is good also as a structure to be made.

  Next, FIG. 5 is a graph showing the relationship between the specified gradation value based on the image data G and the data voltage XVj output from the data line driving circuit 23. In the figure, the designated gradation value is shown on the horizontal axis and the data voltage XVj is shown on the vertical axis. As shown by the characteristic F0 in FIG. 5, the relationship between the designated gradation value and the data voltage XVj is as follows when each of the five kinds of reference gradations (V0, V16, V32, V48, V63) is designated. The coordinates corresponding to the data voltage XVj are represented by broken lines that are connected to each other. However, such a relationship is established when the resistance values of the resistor elements R included in the resistor string 252 are equal. When the resistance values of the respective resistance elements R are different from each other, the relationship between the designated gradation value and the data voltage XVj between the reference gradations adjacent to each other is expressed by a finer broken line.

  In the present embodiment, as indicated by an arrow A in FIG. 5, the image brightness is changed by changing the voltage V63 generated by the voltage adjustment circuit 41. That is, when the user of the light emitting device D inputs an instruction to change the image brightness by operating an operator (not shown), the control circuit 30 sets the voltage V63 according to the changed image brightness according to this instruction. The level is designated to the voltage adjustment circuit 41. For example, the control circuit 30 designates a high level voltage V63 when the image brightness designated by the user is low (dark), while it designates a low level voltage when a high (bright) image brightness is designated. Specify V63. Then, the voltage adjustment circuit 41 changes the voltage V63 generated by the voltage generation circuit 412 to a level designated by the control circuit 30.

  The characteristic F1 in FIG. 5 is a broken line showing the relationship between the specified gradation value and the data voltage XVj when the voltage V63 is increased compared to the characteristic F0 indicated by the solid line in FIG. F2 is a broken line showing the relationship between the specified gradation value and the data voltage XVj when the voltage V63 is lowered compared to the characteristic F0. As understood from the characteristics F1 and F2, when the voltage adjustment circuit 41 changes the voltage V63 in response to an instruction from the control circuit 30, three types of reference voltages (V16 · V1) generated by dividing the voltage V63 are used. V32 and V48) and the gradation voltage generated from each change by a level corresponding to the change amount of the voltage V63. When the voltage V63 increases, the data voltage XVj corresponding to each specified gradation value relatively increases, so that the image brightness decreases. When the voltage V63 decreases, each data corresponds to each specified gradation value. Since the data voltage XVj relatively decreases, the image brightness increases. As described above, according to the present embodiment, it is possible to adjust the image brightness with an extremely simple configuration of adjusting the voltage V63 without requiring complicated processing for the image data G. Even when the image brightness is changed by changing the voltage V63, the gradation characteristic (gamma characteristic) does not change before and after the change of the image brightness. This will be described in detail as follows.

Assuming that the drive transistor Tdr operates in the saturation region, the drive current Iel flowing through the drive transistor Tdr is expressed by the following equation (1). However, “Vgs” in the equation (1) is a gate voltage (hereinafter referred to as “gate-source voltage”) with reference to the source of the drive transistor Tdr. In the equation, “β” is the gain coefficient of the drive transistor Tdr, and “Vth” is the threshold voltage of the drive transistor Tdr.
Iel = (β / 2) · (Vgs−Vth) ² …… (1)

Characteristics of the data line driving circuit 23 (more specifically, reference voltage generation) so that the data voltage XVj becomes a level (XVj = k · g ⁿ ) proportional to the nth power of the specified gradation value g based on the image data G. It is assumed that the voltage value of each reference voltage generated by the circuit 42 and the resistance value of the resistance element R in each D / A converter 233 are selected. That is, characteristics F0 shown in FIG. 5 corresponds to fold line approximating a function called "XVj = k · g ^n". At this time, the equation (1) is transformed into the following equation (2). “K” is a proportionality constant. The data voltage XVj is based on the point at which “Vgs−Vth” in equation (1) becomes zero.
Iel = (β / 2) · (k · g ⁿ ) ² (2)

Furthermore, considering that the luminance B of the OLED element 15 is proportional to the drive current Iel, the luminance B of the OLED element 15 is expressed by the following equation (3). However, “α” in Equation (3) is a proportionality constant between the luminance B and the drive current Iel.
B = α ・ Iel
= Α · (β / 2) · (k · g ⁿ ) ²
= Α ・ (β / 2) ・ k ²・ g ²ⁿ ...... (3)
The degree “2n” of the designated gradation value g in the equation (3) is a gamma value γ that defines gradation characteristics (gamma characteristics). The change of the voltage V63 is only equivalent to the change of “k” in the equation (3), and does not affect “g ²ⁿ ”. That is, according to the present embodiment, it is possible to change the image brightness while maintaining the gamma value γ. In other words, the characteristics of the reference voltage generation circuit 42 and the data line driving circuit 23 are selected so that the numerical value n becomes a half value (n = γ / 2) of the desired gamma value γ. When the numerical value n is selected in this way, the gamma value (total gamma) from the input of the image data G to the data line driving circuit 23 to the output by driving the OLED element 15 is obtained as shown in the equation (3). The value γ (= 2n) can be set.

  In the above, it is assumed that the drive transistor Tdr operates in the saturation region. However, depending on the level of the data voltage XVj, the drive transistor Tdr may not operate in the saturation region. Therefore, in the following, the conditions that the data voltage XVj should satisfy in order to operate the drive transistor Tdr in the saturation region will be examined. FIG. 6 is a graph showing the relationship between the gate-source voltage Vgs of the drive transistor Tdr and the drive current Iel flowing from the source to the drain of the drive transistor Tdr (or luminance B proportional to the drive current Iel). Since the drive transistor Tdr is a p-channel type, the drive current Iel corresponds to the absolute value of the gate-source voltage in a region where the gate-source voltage Vgs is lower than the threshold voltage Vth, as shown in FIG. It becomes the amount of current.

The range of the gate-source voltage Vgs for operating the driving transistor Tdr in the saturation region is the range B from the threshold voltage Vth shown in FIG. 6 to the predetermined voltage Va. That is, if the gate-source voltage Vgs is within this range B, the drive current Iel satisfies the formula (1), and therefore the image brightness is changed while the gamma characteristic is reliably maintained regardless of the change of the voltage V63. be able to. In this range B, the gate-source voltage Vgs satisfies the following equation (4). Note that “Vds” in Expression (4) is a drain voltage (hereinafter referred to as “drain-source voltage”) when the source of the driving transistor Tdr is used as a reference.
Vds> Vgs−Vth (4)

  When the current amount of the drive current Iel is small, the voltage drop generated in the OLED element 15 due to the resistance component of the OLED element 15 and the drive current Iel is small. Therefore, since the drain-source voltage Vds of the driving transistor Tdr is maintained at a relatively high level, the equation (4) is established. That is, in this region, the drive current Iel is proportional to the square of the gate-source voltage. On the other hand, if the gate-source voltage Vgs of the drive transistor Tdr is further reduced and falls below a specific level (Va), the expression (1) does not hold for the drive current Iel. When the drive current Iel increases due to the decrease in the gate-source voltage Vgs, the influence of the voltage drop in the OLED element 15 increases, and as a result, the drain-source voltage Vds of the drive transistor Tdr relatively decreases. This is because the condition of formula (4) is not satisfied. Thus, the voltage corresponding to the boundary between the region (range B) where the drive current Iel satisfies the equation (1) and the region where the equation does not satisfy the equation is “Va”. In other words, the voltage Va is defined as a voltage that becomes an inflection point when the drive current Iel changes with respect to the gate-source voltage Vgs of the drive transistor Tdr.

  In the present embodiment, the voltage adjustment circuit 41 generates the voltage V0 and the voltage V63 so that the voltage is within the range B in FIG. For example, the voltage V0 is selected to a level substantially equal to the threshold voltage Vth. Therefore, all the reference voltages (V0, V16, V32, V48, V63) generated by dividing the voltage V0 and the voltage V63 and all the grayscale voltages V0 to V63 (data) generated from these reference voltages. Any voltage XVj) falls within the range B in FIG. That is, the drive transistor Tdr can be reliably operated in the saturation region regardless of which gradation value is designated. Therefore, according to the present embodiment, it is possible to appropriately adjust the image brightness while maintaining a desired gamma characteristic. However, when the distortion of the gradation characteristics does not cause a problem in practice (for example, when the difference cannot be perceived by the user who has viewed the image), each voltage is a voltage value outside the range B. There may be.

<B: Second Embodiment>
Next, a second embodiment of the present invention will be described. In the first embodiment, the configuration in which the voltage V0 corresponding to the black gradation is substantially equal to the threshold voltage Vth of the drive transistor Tdr is illustrated. However, when the gate-source voltage Vgs is in the vicinity of the threshold voltage Vth, a slight drive current Iel flows from the drive transistor Tdr into the OLED element 15 and slightly emits light. Even so, it may be difficult to display a complete black color. In view of such circumstances, in the present embodiment, the voltage V0 corresponding to the black gradation is selected independently of the threshold voltage Vth and the gamma value γ. In addition, the same code | symbol is attached | subjected about the element similar to 1st Embodiment among this embodiment, and the description is abbreviate | omitted suitably.

  FIG. 7 is a block diagram showing a configuration of the adjustment circuit 40 in the present embodiment. FIG. 8 is a graph showing the relationship between each specified gradation value based on the image data G and the data voltage XVj output from the data line driving circuit 23. As shown in FIG. 7, the adjustment circuit 40 includes a voltage generation circuit 415 in addition to the two voltage generation circuits (411 and 412) similar to the configuration of FIG. The voltage generation circuit 411 generates a voltage V1 corresponding to a gradation value “1” that is one step higher than the lowest gradation value “0”. On the other hand, the voltage generation circuit 412 variably generates the voltage V63 corresponding to the maximum gradation value “63” in accordance with an instruction from the control circuit 30 as in the first embodiment. Accordingly, the same effects as those of the first embodiment can be obtained in this embodiment.

  On the other hand, the voltage generation circuit 415 is a means for generating the voltage V 0 corresponding to the lowest gradation value “0” and supplying it to the wiring 46. As shown in FIG. 8, the level of the voltage V0 is selected separately from the voltages V1 to V63 corresponding to other gradation values. More specifically, the voltage V0 is selected to a level at which the driving transistor Tdr is surely turned off when it is applied to the data line 13 as the data voltage XVj. Therefore, according to the present embodiment, when the gradation value “0” is designated, the supply of the drive current Iel to the OLED element 15 can be surely stopped to display a complete black color.

<C: 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
The configuration of the adjustment circuit 40 is not limited to the examples in the embodiments. For example, the reference voltage generation circuit 42 can be changed to the configuration of FIG. 9 or FIG. The reference voltage generation circuit 42 in FIG. 9 has a configuration in which a variable resistance element (trimmer) 423 having a variable resistance value is used instead of the resistance element 421 shown in FIG. The resistance value of each variable resistance element 423 is controlled by the control circuit 30 in accordance with an instruction from the user. On the other hand, in the reference voltage generating circuit 42 of FIG. 10, three resistance elements 425 are inserted in parallel between the wiring 451 to which the voltage V0 is applied and the wiring 452 to which the voltage V63 is applied. Then, the control circuit 30 changes the position of contact between the wiring (453, 454, 455) for outputting each reference voltage and each resistance element 425 according to an instruction from the user, whereby the reference voltage (V16 · The level of (V32 / V48) is adjusted. According to these configurations, the gamma characteristic of the light emitting device D (the relationship between the specified gradation value and the luminance B of the OLED element 15) can be adjusted as appropriate.

(2) Modification 2
The configuration of the pixel circuit P is not limited to the example of FIG. For example, the pixel circuit P illustrated in FIG. This pixel circuit P is disclosed as “Fig. 3” in US Pat. No. 6,229,506. The pixel circuit P has a function of compensating for variations in characteristics (particularly threshold voltage Vth) of the drive transistor Tdr. As shown in the figure, the scanning line 11 shown as one wiring in FIG. 1 includes three control lines (111, 112, 113) in this modification. The waveforms of the signals (S1, S2, S3) supplied to the control lines are shown in FIG.

  The source of the drive transistor Tdr shown in FIG. 11B is connected to the power supply line, and the drain is connected to the source of the light emission control transistor Tel. The light emission control transistor Tel is a switching element for defining a period during which the OLED element 15 actually emits light, and has a drain connected to the anode of the OLED element 15 and a gate connected to the control line 113. On the other hand, a first switching element T1 having a gate connected to the control line 112 is interposed between the gate and drain of the driving transistor Tdr. When the signal S2 supplied to the control line 112 becomes low level, the first switching element T1 is turned on and the drive transistor Tdr is diode-connected.

  A capacitive element C1 is interposed between the gate and source of the driving transistor Tdr. Further, one electrode of the capacitive element C2 is connected to the gate of the driving transistor Tdr. The other electrode L of the capacitive element C2 is connected to the drain of the second switching element T2. The second switching element T 2 has a source connected to the data line 13 and a gate connected to the control line 111.

In the above configuration, compensation of the threshold voltage Vth of the drive transistor Tdr is realized by diode-connecting the drive transistor Tdr with the first switching element T1 while fixing the electrode L of the capacitive element C2 to the specific reference voltage Vref. . More specifically, first, as shown in FIG. 11 (b), in the reset period PRES, the light emission control transistor Tel and the first switching element T1 are turned on to sufficiently set the drain and gate potentials of the drive transistor Tdr. Reduce to a lower level. In the subsequent compensation period P ₁ , the first switching element T1 is turned on so that the drain and gate of the drive transistor Tdr are brought into conduction, thereby converging the potential of the gate of the drive transistor Tdr to “VH− | Vth |”. This electrode L of the capacitor C2 in the compensation period P _1, the reference voltage Vref from the data line 13 via the second switching element T2 in the on state is applied. In the writing period P ₂ following the compensation period P ₁ , the first switching element T 1 is turned off to release the diode connection of the driving transistor Tdr, and then the data voltage XVj is supplied to the data line 13. As a result, the gate potential of the drive transistor Tdr is lowered from “VH− | Vth |” according to the data voltage XVj. More specifically, the difference between the reference voltage Vref and the data voltage XVj is divided according to the capacitance ratio between the capacitive element C1 and the capacitive element C2, and supplied to the gate of the drive transistor Tdr. In the light emission period P _EL , the light emission control transistor Tel is turned on and the drive current Iel is supplied to the OLED element 15.

According to the configuration of the present modification, since the threshold voltage Vth of the driving transistor Tdr is compensated by the operation of the compensation period P _1, it is possible to effectively suppress the unevenness of display due to variation in the threshold voltage Vth. In the pixel circuit P of the present modification, the luminance of the OLED element 15 is set by the difference between the reference voltage Vref and the data voltage XVj, but the drive current Iel is 2 of the data voltage XVj as shown in equation (2). The current amount corresponding to the power is the same as in the first embodiment. Therefore, the same effects as those of the first embodiment can be obtained by this embodiment. As described above, the unit circuit according to the present invention only needs to be a circuit having a function of controlling the light emitting element to a gradation corresponding to the voltage of the data line, and the specific configuration thereof is not limited.

(3) Modification 3
In each embodiment, the case where the voltage V63 is changed according to the image brightness instructed by the user is exemplified, but a configuration in which the voltage V0 is changed in addition to this may be adopted. In short, it suffices if the difference value between the voltage V0 and the voltage V63 is changed by changing one of the voltage V0 and the voltage V63 relative to the other.

(4) Modification 4
In each embodiment, the OLED element 15 is exemplified as a light emitting element, but the light emitting element in the present invention is not limited to this. For example, instead of the OLED element 15, an inorganic EL element, a field emission (FE) element, a surface-conduction electron (SE) element, a ballistic electron surface emitting (BS) element Various self-luminous elements such as LED (Light Emitting Diode) elements can also be used. As described above, it is sufficient that the light emitting element in the present invention is an element that emits light by application of electric energy, and its specific structure and material are not limited.

(5) Modification 5
In each embodiment, the configuration in which each reference voltage (for example, V0, V16, V32, V48, and V63 in the first embodiment) is generated by voltage division using a resistance element is illustrated. A configuration generated by partial pressure using an element is also employed. In this configuration, the reference voltage generation circuit 42 consumes no direct current, so that power consumption is reduced. Similarly, instead of the resistor string 252 functioning as the gradation voltage generation circuit of the present invention, a voltage dividing circuit using a capacitive element may be employed.

<D: Application example>
Next, an electronic apparatus using the light emitting device according to the present invention will be described. FIG. 12 is a perspective view showing the configuration of a mobile personal computer that employs the light emitting device D according to any one of the embodiments described above as a display device. The personal computer 2000 includes a light emitting device D as a display device and a main body 2010. The main body 2010 is provided with a power switch 2001 and a keyboard 2002. Since the light emitting device D uses the OLED element 15 as a light emitting element, it is possible to display an easy-to-see screen with a wide viewing angle.

  FIG. 13 shows a configuration of a mobile phone to which the light emitting device D is applied. A cellular phone 3000 includes a plurality of operation buttons 3001, scroll buttons 3002, and a light emitting device D as a display device. By operating the scroll button 3002, the screen displayed on the light emitting device D is scrolled.

  FIG. 14 shows a configuration of a personal digital assistant (PDA) to which the light emitting device D is applied. The information portable terminal 4000 includes a plurality of operation buttons 4001, a power switch 4002, and a light emitting device D as a display device. When the power switch 4002 is operated, various kinds of information such as an address book and a schedule book are displayed on the light emitting device D.

  Note that electronic devices to which the light emitting device according to the present invention is applied include those shown in FIGS. 12 to 14, digital still cameras, televisions, video cameras, car navigation devices, pagers, electronic notebooks, electronic papers, calculators. , Word processors, workstations, videophones, POS terminals, printers, scanners, copiers, video players, devices equipped with touch panels, and the like. The application of the light emitting device D according to the present 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, a writing head that exposes a photosensitive member according to an image to be formed on a recording material such as paper is used. However, the light emitting device of the present invention is used. The unit circuit referred to in the present invention is a concept including not only a pixel circuit that constitutes a pixel of a display device as in each embodiment but also a circuit that is a unit of exposure in the image forming apparatus.

It is a block diagram which shows the structure of the light-emitting device which concerns on 1st Embodiment. It is a circuit diagram which shows the structure of one pixel circuit. It is a block diagram which shows the structure of an adjustment circuit. It is a block diagram which shows the structure of a data line drive circuit. It is a graph which shows the relationship between the gradation value which image data designates, and a data voltage. It is a graph which shows the relationship between the gate-source voltage of a drive transistor, and a drive current. It is a block diagram which shows the structure of the adjustment circuit of the light-emitting device which concerns on 2nd Embodiment. It is a graph which shows the relationship between the gradation value which image data designates, and a data voltage. It is a block diagram which shows the structure of the adjustment circuit which concerns on a modification. It is a block diagram which shows the structure of the adjustment circuit which concerns on a modification. It is a block diagram which shows the structure of the pixel circuit which concerns on a modification. It is a perspective view which shows the structure of the personal computer to which this invention is applied. It is a perspective view which shows the structure of the mobile telephone to which this invention is applied. It is a perspective view which shows the structure of the portable information terminal to which this invention is applied.

Explanation of symbols

D: Light emitting device, P: Pixel circuit, 10: Pixel array unit, 11: Scan line, 13: Data line, 21: Scan line drive circuit, 23 ... Data line drive circuit, 231 ... Latch circuit, 233 ... D / A converter, 251 ... input buffer, 252 ... resistor string, 253 ... selection circuit, 254 ... output buffer, 255 ... decoder, 30 ... control circuit, 40 ... Adjustment circuit 41... Voltage adjustment circuit 411 412 415 Voltage generation circuit 42 Reference voltage generation circuit 421 Resistance element V0, V16, V32, V48, V63 Reference voltage XVj ... data voltage.

Claims (8)

A unit circuit for controlling the light emitting element to a gradation corresponding to the voltage of the data line;
A voltage adjusting circuit that generates a first voltage and a second voltage that are different from each other, and changes at least one of the first voltage and the second voltage according to an instruction from the outside;
A reference voltage generation circuit that generates a plurality of reference voltages within a range from the first voltage to the second voltage by dividing the first voltage and the second voltage;
A data line driving circuit that selects any one of a plurality of gradation voltages including the plurality of reference voltages generated by the reference voltage generation circuit according to a specified gradation value and outputs the selected data as a data voltage to the data line. apparatus. 2. The light emitting device according to claim 1, further comprising: a gradation voltage generation circuit that generates the plurality of gradation voltages including a plurality of reference voltages generated by the reference voltage generation circuit and an intermediate voltage between the reference voltages. The unit circuit includes a drive transistor that is inserted in a path of a drive current supplied to the light emitting element and controls a current amount of the drive current according to a voltage of a gate,
As for the first voltage and the second voltage, the driving transistor operates in the saturation region even if any voltage within the range from the first voltage to the second voltage is supplied from the data line to the unit circuit. The light-emitting device according to claim 1, wherein each voltage value is selected. A graph showing the relationship between each of a plurality of specified gradation values and the data voltage output from the data line driving circuit for the specified gradation value is based on the n (n is a real number) power of the specified gradation value. The light-emitting device according to claim 3, wherein the light-emitting device is a polygonal line that approximates a function that changes. The order n of the specified gradation value in the function is set to γ / 2 in order to cause the light emitting element to emit light with a luminance corresponding to the γth power of the specified gradation value (γ is an arbitrary real number). The light-emitting device of description. The unit circuit includes a drive transistor that is inserted in a path of a drive current supplied to the light emitting element and controls a current amount of the drive current according to a voltage of a gate,
The black voltage that is outside the range from the first voltage to the second voltage and that turns off the driving transistor when supplied to the data line is the first voltage and the second voltage. Comprises a separately generating circuit,
The light emitting device according to any one of claims 1 to 5, wherein the data line driving circuit selects the black voltage and outputs it to the data line when a black gradation is designated.   The electronic device which comprises the light-emitting device of any one of Claims 1-6. A method of driving a light-emitting device in which unit circuits for controlling light-emitting elements to a gradation corresponding to a voltage of a data line are arranged,
The first voltage and the second voltage that are different from each other are divided to generate a plurality of reference voltages within a range from the first voltage to the second voltage, and a plurality of grayscale voltages including the plurality of reference voltages are generated. While either one is selected according to the specified gradation value and output to the data line,
A method of driving a light emitting device, wherein when an image brightness is designated, a voltage value of at least one of the first voltage and the second voltage is changed according to the designated image brightness.

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