JP2009116148A - Light emitting device and electronic equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent reduction in quantity of emitted light in each unit circuit connected to a power supply line due to potential drop in the power supply line. <P>SOLUTION: This light emitting device 10 includes n unit circuits U1-Un. Each of a plurality of unit circuits U1-Un includes a current source transistor Tg for generating driving current Ids by receiving supply of electric power from a high potential side power supply line 16 and a light emitting element 23 for emitting light with the luminance corresponding to the driving current Ids. A gate of the current source transistor Tg in each of a plurality of unit circuits U1-Un is connected between a terminal 22 for supplying reference potential of a reference potential wire 18 and a low potential supply point C. Reference potential VREF is supplied to the terminal 22 for supplying reference potential from power supply circuits 14 and 15, and the electric potential lower than the reference potential VREF is supplied to the low potential supply point C. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a light-emitting device using various light-emitting elements such as an organic light-emitting diode (hereinafter referred to as an “OLED (Organic Light Emitting Diode)) element, and an electronic apparatus including such a light-emitting device.

Conventionally, a light emitting device having a configuration in which a plurality of unit circuits including a light emitting element such as an OLED element and a transistor for supplying current to the substrate are arranged on a substrate has been proposed (for example, Patent Document 1). reference).
In the configuration of Patent Document 1, a power line is further provided on the substrate, and each unit circuit is connected to the power line. The power from the power supply is supplied to the power supply line via the power supply terminal. The transistors of each unit circuit receive a supply of power from the power supply line and generate a drive current. The light emitting element of each unit circuit emits light upon receiving a drive current generated by a transistor. At this time, current flows from the power supply line toward each unit circuit.
JP-A-8-108568

Here, since the power supply line itself is a resistor, a potential drop occurs in the power supply line when a current flows through the power supply line as a driving current is supplied to each light emitting element. The longer the path length of the current from the power supply terminal to the connection point between the transistor of the unit circuit and the power supply line, the greater the resistance value of the current path, and the greater the potential drop at the connection point. As a result, the drive current generated by the transistor is also greatly reduced, and thereby the light emission intensity (luminance) of the light emitting element is greatly reduced.
That is, as the path length of the current from the power supply terminal to the transistor of the unit circuit is increased, the light emission intensity of the light emitting element is greatly reduced, resulting in a variation in the light emission intensity of the light emitting element in each unit circuit.
The present invention has been made in view of such circumstances, and an object of the present invention is to solve the problem of suppressing variations in light emission intensity of light emitting elements in each unit circuit connected to a power supply line.

  In order to solve the above problems, a light emitting device according to the present invention includes a plurality of unit circuits (connected to a high-order power supply line) each connected to a high-order power supply line to which power is supplied via a power supply terminal. All of the plurality of unit circuits (including some of the unit circuits), and each of the plurality of unit circuits is supplied with electric power from the high-level power supply line and generates a drive current. A light emitting element that emits light with a luminance corresponding to a drive current generated by the current source transistor, and the gate of the current source transistor in each of the plurality of unit circuits is connected to the first potential supply point of the reference potential line The resistance value of the current path from the first potential supply point to the gate of the current source transistor increases as the resistance value of the current path connected between the second potential supply point and from the power supply terminal to the current source transistor increases. Listen, the first potential supply point is supplied a reference potential, the second potential supply point, a low potential is supplied than the reference potential.

  According to this invention, even if the power supply potential supplied to the current source transistor of the unit circuit is reduced due to the potential drop that occurs in the high-order side wire, the reference potential supplied to the gate of the current source transistor is used as the power supply. It can be decreased according to the decrease in potential. As a result, a decrease in drive current generated by the current source transistor can be suppressed, and variations in the light emission intensity (luminance) of the light emitting element in each unit circuit can be suppressed.

  As a specific mode of the above-described light emitting device, each of the plurality of unit circuits is disposed between the high potential side power supply line and the low potential side power supply line and connects the low potential side power supply line to the second potential supply point. An aspect may be sufficient. According to the present invention, the potential of the second potential supply point in the reference potential line can be lowered to the potential of the lower power supply line.

  In the light emitting device described above, a resistor is provided between the second potential supply point and the lower power supply line, so that the voltage between the second potential supply point and the lower power supply line is set to a desired value. You can also In the light emitting device described above, the voltage between the second potential supply point and the lower power supply line is changed between the second potential supply point and the lower power supply line according to the input data of a plurality of bits. Voltage changing means can also be provided. According to the present invention, even when the potential drop amount in the higher power line varies, the potential drop amount of the reference potential line at that time can be adjusted. As a result, it is possible to suppress variations in driving current generated by the current source transistors of the unit circuits (variations in light emission intensity of the light emitting elements).

  The voltage changing means described above can be constituted by, for example, a DA converter that outputs an analog signal corresponding to an input signal of digital data. In this case, it is possible to change the voltage value between the second potential supply point, which is the output of the DA converter, and the lower power supply line by controlling the multi-bit digital data input to the DA converter. Further, the voltage changing means described above can be constituted by a transistor. In this case, the voltage between the second potential supply point and the lower power supply line can be changed by adjusting the on-resistance of the transistor by controlling the voltage supplied to the gate of the transistor. Thereby, the amount of potential drop of the reference potential line can be adjusted.

  In the above-described light emitting device, a plurality of connection points to the lower power supply line are provided between the first potential supply point and the second potential supply point of the reference potential supply wiring, and each connection point and the lower power supply As the resistance value between the first potential supply point and the connection point increases, the resistance difference between the first potential supply point and the connection point increases as the resistance value of the current path from the first potential supply point to the connection point increases. It may be an aspect of setting individually. According to the present invention, the potential drop amount of the reference potential supplied to the gate of each current source transistor can be accurately approximated to the potential drop amount of the power supply potential supplied to each current source transistor. Variation in driving current generated by the current source transistor (variation in light emission intensity of the light emitting element) can be further suppressed.

  Next, the electronic apparatus according to the present invention preferably includes the above-described light emitting device. Examples of such an electronic device include a printer, a copier, a facsimile, a display device that displays an image, a personal computer, a mobile phone, and the like.

<A: First Embodiment>
FIG. 1 is a perspective view showing a partial configuration of an image forming apparatus using the light emitting apparatus according to the first embodiment of the present invention as an exposure apparatus (optical head). As shown in the figure, the image forming apparatus includes a light emitting device 10, a condensing lens array 11, and a photosensitive drum 12 (image carrier). The light emitting device 10 includes a large number of light emitting elements (not shown in FIG. 1) arranged linearly on the surface of the substrate 13. These light emitting elements selectively emit light according to the form of an image to be printed on a recording material such as paper. The photosensitive drum 12 is supported by a rotating shaft extending in the main scanning direction, and rotates in the sub-scanning direction (direction in which the recording material is conveyed) with the outer peripheral surface facing the light emitting device 10.

  The condensing lens array 11 is disposed in the gap between the light emitting device 10 and the photosensitive drum 12. The condensing lens array 11 includes a large number of gradient index lenses arranged in an array with each optical axis directed to the light emitting device 10. An example of such a condensing lens array 11 is SLA (Selfoc Lens Array) available from Nippon Sheet Glass Co., Ltd. (Selfoc / SELFOC is a registered trademark of Nippon Sheet Glass Co., Ltd.).

  Light emitted from each light emitting element of the light emitting device 10 passes through each gradient index lens of the condensing lens array 11 and reaches the surface of the photosensitive drum 12. By this exposure, a latent image (electrostatic latent image) corresponding to a desired image is formed on the surface of the photosensitive drum 12.

FIG. 2 is a block diagram illustrating an electrical configuration of the light emitting device 10. As shown in FIG. 2, the light emitting device 10 includes power supply circuits 14 and 15, a high power supply line 16, a low power supply line (ground line) 17, a reference potential line 18, and n unit circuits U ( U _{1 to} U _n ) and the drive circuit 19 are arranged on the surface of the substrate 13. The power supply circuits 14 and 15 are arranged in the vicinity of both ends in the longitudinal direction of the long substrate 13. The high-side power line 16 and the low-side power line 17 extend along the main scanning direction. Power supply terminals 20 are provided at both ends of the higher power supply line 16. In addition, power terminals 21 are provided at both ends of the lower power line 17. The power supply potential VEL is supplied from the power supply circuits 14 and 15 to the higher power supply line 16 through the power supply terminal 20. A ground potential VCT (VEL> VCT) is supplied to the lower power supply line 17 from the power supply circuits 14 and 15 through the power supply terminal 21. The reference potential line 18 extends along the main scanning direction, and reference potential supply terminals 22 are provided at both ends thereof. A reference potential VREF (VEL>VREF> VCT) is supplied to the reference potential line 18 from the power supply circuits 14 and 15 through a reference potential supply terminal 22. The unit circuit U ₁ ~U _n, are arranged along the main scanning direction, each of which is disposed between the high-potential power supply line 16 and the low-potential power supply line 17.

As shown in FIG. 2, the unit circuit U ₁ includes a current source transistor Tg, a transistor Tr, and a light emitting element 23. Note that the same configuration as the other unit circuits _U 2 ~U _n be the unit circuit _{U 1.} As shown in FIG. 2, the light emitting element 23 is disposed between the high-potential power supply line 16 and the low-potential power supply line 17. The light emitting element 23 is an element having a gradation corresponding to the drive current Ids. The light emitting element 23 in the present embodiment is an OLED element in which a light emitting layer formed of an organic EL element (ElectroLuminescent) material is interposed in a gap between an anode and a cathode, and has a current value of a drive current Ids supplied to the light emitting layer. Emits light with a corresponding brightness.

As shown in FIG. 2, the current source transistor Tg is a P-channel transistor (typically a thin film transistor) disposed between the high-potential power line 16 and the light emitting element 23. The source of the current source transistor Tg is connected to the high potential side power supply line 16, and the power supply potential VEL is supplied from the high potential side power supply line 16. As shown in FIG. 2, and each _S 1 to S _n to the connection point between the source and the high-potential power supply line 16 of the current source transistor Tg of each unit circuit _U 1 ~U _n. The gate of current source transistor Tg of each unit circuit _U 1 ~U _n is connected to a reference potential line 18. By supplying the power supply potential VEL and the reference potential VREF, each current source transistor Tg functions as a constant current source.

  As illustrated in FIG. 2, the transistor Tr is a transistor (typically a thin film transistor) disposed between the current source transistor Tg and the light emitting element 23. The drive circuit 19 supplies a potential Vd corresponding to gradation data sent from a control device (for example, a CPU or a controller; hereinafter referred to as “higher level device”) of the image forming apparatus to the gate of the transistor Tr. The gradation data in the present embodiment is data that designates either lighting (high gradation) or extinguishing (low gradation) for the light emitting element 23. The transistor Tr is controlled to be in an on state or an off state by the potential Vd corresponding to the gradation data being supplied to the gate. When the transistor Tr changes to the on state, the drive current Ids generated by the current source transistor Tg is supplied to the light emitting element 23, and thereby the light emitting element 23 emits light. On the other hand, when the transistor Tr changes to the OFF state, the current value of the drive current Ids generated by the current source transistor Tg becomes zero and the light emitting element 23 is turned off.

The light emission intensity (luminance) of the light emitting element 23 is determined by the current value of the drive current Ids generated by the current source transistor Tg which is a constant current source. The current source transistor Tg operates in the saturation region, and the drive current Ids generated by the current source transistor Tg is expressed by the following equation (1).
Ids = (μ * Cox / 2) * (W / L) * (Vgs−Vth) ² * (1 + Vds) (1)
In the above formula (1), μ is the electron mobility, Cox is the gate capacitance of the current source transistor Tg, W is the channel width of the current source transistor Tg, L is the channel length of the current source transistor Tg, and Vgs is the current source transistor Tg. , Vth represents the threshold voltage of the current source transistor Tg, and Vds represents the drain-source voltage of the current source transistor Tg.

In order to equalize the light emission intensity of the light emitting element 23 of each unit circuit U ₁ ~U _n is necessary to equalize the current value of the driving current Ids generated by the current source transistor Tg of each unit circuit U ₁ ~U _n is there. Then, in each unit circuit U ₁ ~U _n, it is necessary to set the gate-source voltage Vgs of the current source transistor Tg which greatly affects the current value of the driving current Ids generated by the current source transistor Tg constant.

Here, a configuration (proportional) in which no current flows through the reference potential line 18 is assumed. 3, in the comparative example, the power supply potential VEL is supplied and the position of the connection point _S 1 to S _n in the main scanning direction in the high-potential power source line 16, the current source transistor Tg of each unit circuit _U 1 ~U _n It is a figure which shows the relationship with reference potential VREF. Since the high-order power supply line 16 is a resistor (resistor Rel in FIG. 4), if a current flows through the high-order power supply line 16 as the drive current Ids is supplied to each light emitting element 23, the potential on the high-order power supply line 16 A descent occurs. The greater the path length of the current from the power supply terminal 20 to the connection point S between the current source transistor Tg and the higher power supply line 16, the greater the resistance value in the current path, and the greater the potential drop at the connection point S. That is, as shown in FIG. 3, toward the connection points S ₁ to S _n central position of both the high-potential power supply line 16 from the end portion 20 of the extending direction of the (midpoint between the power supply terminal 20) The power supply potential VEL decreases. In contrast, the unit circuit _{Un / 2} is connected to the center in the extending direction of the high-side power line 16 (the middle point between the power supply terminals 20), and the current source transistor Tg of the unit circuit _{Un / 2} is connected to the unit circuit _{Un / 2.} The amount of decrease in the supplied power supply potential VEL is the largest. As shown in FIG. 3, let the amount of decrease be ΔV. On the other hand, since the impedance at the gate of each current source transistor Tg is very large, almost no current flows through the gate and almost no potential drop occurs in the reference potential line 18. Therefore, as shown in FIG. 3, the gate potential of each current source transistor Tg is substantially the same. Accordingly, no current flows through the reference potential line 18.

As described above, in contrast, a potential drop occurs in the high-order power supply line 16, whereas a potential drop does not occur in the reference potential line 18. Therefore, in each current source transistor Tg as shown in FIG. The gate-source voltage Vgs decreases. As shown in FIG. 3, as the distance from the power supply terminal 20 to the connection point S increases, the gate-source voltage Vgs in the current source transistor Tg decreases greatly, and the current of the drive current Ids generated by the current source transistor Tg. The value is greatly reduced. Thus, variation occurs in the light emission intensity of the light emitting element 23 of each unit circuit U ₁ ~U _n, unevenness in gradation occurs in the image formed by the image forming apparatus.
Since the substrate 13 is a long plate, the distance in the main scanning direction of the high-order power supply line 16 extending from the power supply terminal 20 is large, and the amount of potential drop generated in the high-order power supply line 16 is large. For this reason, in the light emitting device used for the exposure head of the image forming apparatus, the problem that the light emission intensity of the light emitting element 23 varies and the gradation formed in the image formed by the image forming apparatus becomes particularly significant.

  By the way, even if the power supply potential VEL supplied to the source of the current source transistor Tg decreases due to the potential drop in the high-potential power supply line 16, the reference potential VREF supplied to the gate of the current source transistor Tg also decreases the power supply potential VEL. If it decreases accordingly, the decrease in the gate-source voltage Vgs of the current source transistor Tg can be suppressed. The present embodiment focuses on this point. In the present embodiment, the reference potential line 18 is provided with a portion to which a potential lower than the reference potential VREF is provided so that a current flows through the reference potential supply wiring 18, and a potential drop is generated in the reference potential line 18. Yes. Specific embodiments will be described below.

FIG. 4 is a diagram schematically showing an electrical configuration in the vicinity of the center (midpoint between the power supply terminals 20) in the extending direction of the high-order power supply line 16. As shown in FIG. 4, the connection point between the gate of the current source transistor Tg of each unit circuit U _{(n / 2) -1 to} U _{(n / 2) +1} and the reference potential line 18 is the gate connection point G _{(n / 2) -1 to} G _{(n / 2) +1} . Also in the present embodiment, the unit circuit _{Un / 2} is connected to the center in the extending direction of the high-order power supply line 16 (the middle point between the power supply terminals 20), as in the above-described comparison. In the present embodiment, the low potential power supply line 17 is connected to the low potential supply point C of the reference potential line 18 via the resistor Rcon. Since the reference potential VREF is higher than the ground potential VCT, the current Iref flows from the reference potential supply terminal 22 through the low potential supply point C and the resistor Rcon to the low potential side power supply line 17. Since the reference potential line 18 is a resistor (resistor Rref shown in FIG. 4), when the current Iref flows through the reference potential line 18, a potential drop occurs in the reference potential line 18. The larger the resistance value of the current path from the reference potential supply terminal 22 is (the closer to the low potential supply point C), the greater the potential drop in the reference potential line 18.

In the present embodiment, as shown in FIG. 4, a low potential supply point C is provided in the vicinity of the gate connection point G _{n / 2} of the reference potential line 18 (the midpoint between the reference potential supply terminals 22). . Then, as shown in FIG. 5, the reference potential line extends from the reference potential supply terminal 22 to the gate connection point G _{n / 2} so that the potential drop amount at the gate connection point G _{n / 2} of the reference potential line 18 is maximized. 18 potential drops. In this embodiment, the resistance value of the resistor Rcon is selected so that the potential at the gate connection point G _{n / 2} is VREF−ΔV. Therefore, as shown in FIG. 5, the gate-source voltage Vgs of the current source transistor Tg at the end (unit circuit U ₁ , unit circuit U _n ) and the current source transistor Tg at the center (unit circuit U _{n / 2} ). The gate-source voltage Vgs matches.

According to the configuration of the present embodiment, as shown in FIG. 5, a potential drop also occurs in the reference potential line 18 so as to correspond to the potential drop in the higher power supply line 16. Thus, as compared with the comparative example in which the reference potential VREF of substantially the same potential to the gate of the current source transistor Tg of each unit circuit U ₁ ~U _n is supplied, between the gate and source of each unit circuit U ₁ ~U _n Variations in the voltage Vgs are suppressed. Therefore, variations in the current value of the driving current Ids generated by the current source transistor Tg of each unit circuit U ₁ ~U _n (variation in light emission intensity of the light emitting element 23) is suppressed as compared with the comparative example.

<B: Second Embodiment>
In the light emitting device 10 according to the first embodiment, the resistor Rcon is provided between the low potential supply point C and the low potential side power supply line 17. On the other hand, in the light emitting device 10 of the second embodiment, the low potential supply point C and the low potential side are provided between the low potential supply point C and the low potential power supply line 17 in accordance with the input data of a plurality of bits. It differs from the light emitting device 10 of the first embodiment in that voltage changing means for changing the voltage between the power line 17 and the power source line 17 is provided. Since other configurations are the same as the configurations of the first embodiment, description of overlapping portions will be omitted.

  In this embodiment, the voltage changing means is composed of a DA converter that outputs an analog signal corresponding to input digital data. FIG. 6 is a diagram showing a schematic configuration of the DA converter 24 provided between the low potential supply point C and the lower power supply line 17. As shown in FIG. 6, a plurality of resistors R <b> 1 to R <b> 4 are connected in series between the low potential supply point C and the lower power supply line 17. A switch Sw1 is provided between both ends of the resistor R1, a switch Sw2 is provided between both ends of the resistor R2, and a switch Sw3 is provided between both ends of the resistor R3. Here, the resistance values r1 to r3 of the resistors R1 to R3 are different. Specifically, each resistance value is expressed as a relative ratio of powers of 2 (r1: r2: r3 = 1: 2: 4). In addition, each of each resistor R1-R4 may be comprised by the single resistor, and may be comprised from the some resistor.

  Digital data (hereinafter referred to as “adjustment data”) D of a plurality of bits b (b1 to b3) is input to the DA converter 24 from a control circuit (not shown). Bit b1 is supplied to switch Sw1, bit b2 is supplied to switch Sw2, and bit b3 is supplied to switch Sw3. The switch Sw is switched according to the value of the supplied bit b. In this embodiment, the switch Sw is turned on when the bit b supplied to the switch Sw is “1”, and the switch Sw is turned off when the bit b supplied is “0”. When the switch Sw is turned on, both ends of the resistor R corresponding to the switch Sw are short-circuited.

  The resistance value between the low potential supply point C and the lower power line 17 is selected according to the adjustment data D among the resistors R1 to R3 (the resistance value of the resistor R is not short-circuited between both ends). This is a combined resistance value of r and the resistance value r4 of the resistor R4. For example, when the adjustment data D input to the DA converter 24 is b1 = 1 and b2 = b3 = 0, only the switch Sw1 is turned on and both ends of the resistor R1 are short-circuited, and the low potential supply point The resistance value between C and the lower power line 17 is a resistance value obtained by combining the resistance values r2 to r4 of the resistors R2 to R4 (6r1 + R4).

According to the configuration of the present embodiment, by controlling the adjustment data D input to the DA converter 24, the resistance value between the low potential supply point C and the low potential power line 17 can be adjusted in multiple stages. The voltage value (corresponding to the analog signal output from the DA converter 24) between the low potential supply point C and the lower power supply line 17 can be adjusted. Thereby, the amount of potential drop in the reference potential line 18 can be adjusted. That is, the amount of decrease in the reference potential VREF supplied to the gate of each current source transistor Tg can be adjusted.
For example, even when the potential drop amount in the high power supply line 16 changes, the unit circuit U is adjusted by adjusting the potential drop amount in the reference potential line 18 according to the change in the potential drop amount in the high power supply line 16. the variation of the gate-source voltage Vgs of the current source transistor Tg of ₁ ~U _n can be suppressed. Therefore, variation in current value of the driving current Ids generated by the current source transistor Tg of each unit circuit U ₁ ~U _n (emission intensity of the light emitting element 23) can be suppressed.

  In the present embodiment, the resistance values r1 to r3 of the resistors R1 to R3 are different from each other. However, the present invention is not limited to this. For example, the resistance values r1 to r3 of the resistors R1 to R3 are the same. But you can. Also, the resistance value r4 of the resistor R4 may be different from or the same as the resistance values r1 to r3 of the resistors R1 to R3.

  In the present embodiment, the configuration of the DA converter 24 is exemplified as shown in FIG. 6, but the DA converter 24 is not limited to this, and the DA converter 24 converts the input multi-bit data. Accordingly, the voltage value between the low potential supply point C and the lower power supply line 17 may be changed. For example, the DA converter 24 may have a configuration in which a plurality of resistors R each having a switch Sw provided between both ends thereof are connected in series. However, like the DA converter 24 according to the present embodiment, switches Sw1 to Sw3 are provided between both ends of the plurality of resistors R1 to R3 among the plurality of resistors R1 to R4 connected in series, and the rest According to the configuration in which the switch Sw is not provided between both ends of the resistor R4, the resolution of the resistance value between the low potential supply point C and the low-potential power line 17 is increased with a small number of resistors R (resistance value). This is advantageous in that the step size of the change in (1) can be reduced.

<C: Third Embodiment>
In the light emitting device 10 of the second embodiment, a configuration in which the DA converter 24 is used as an example of the voltage changing unit is illustrated. On the other hand, the light emitting device 10 of the third embodiment is different from the light emitting device 10 of the second embodiment in that a transistor is used as another example of the voltage changing unit. Since other configurations are the same as the configurations of the above-described embodiments, description of overlapping portions will be omitted.

  FIG. 7 is a diagram schematically showing an electric circuit near the center in the extending direction of the high-potential power line 16 in the third embodiment. As shown in FIG. 7, a transistor Ts is provided between the low potential supply point C and the low-potential power line 17, and a gate potential Vs is supplied to the gate of the transistor Ts from a control circuit (not shown). . In the present embodiment, the voltage between the low-potential supply point C and the low-potential power line 17 is changed by changing the on-resistance of the transistor Ts by controlling the gate potential Vs supplied to the gate of the transistor Ts. is doing. Also with this configuration, the voltage between the low potential supply point C and the lower power supply line 17 can be adjusted, and the potential drop amount in the reference potential line 18 according to the change in the potential drop amount in the high potential power supply line 16. Can be adjusted.

<D: Modification>
The present invention is not limited to the above-described embodiments, and for example, the following modifications are possible. Also, two or more of the modifications shown below can be combined.
(1) Modification 1
In each of the embodiments described above, an OLED element is taken up as an example of the light emitting element 23, but an inorganic light emitting diode or LED (Light Emitting Diode) may be used. In short, any element may be used as long as it emits light with light emission luminance corresponding to the drive current.

(2) Modification 2
In each of the above-described embodiments, the low-potential power supply line 17 is connected to the reference potential line 18. However, the present invention is not limited to this, and for example, a potential VL lower than the reference potential VREF is supplied as shown in FIG. The low potential supply wiring 25 may be connected to the reference potential line 18. Even in such an aspect, a potential drop can be generated in the reference potential line 18 so as to correspond to the potential drop in the high-potential power supply line 16. In the embodiment of FIG. 8, the amount of potential drop in the reference potential line 18 can be adjusted by adjusting the value of the potential VL supplied to the low potential supply wiring 25. Further, a resistor Rcon may be provided between the low potential supply point C, which is a connection point between the reference potential line 18 and the low potential supply line 25, and the low potential supply line 25, the DA converter 24, A transistor Ts may be provided.

(3) Modification 3
In each of the above-described embodiments, the power supply potential VEL is supplied from both ends of the high-order power supply line 16. However, the present invention is not limited to this. For example, as shown in FIG. The power supply terminal 20 may be provided only at one end, and the power supply potential VEL may be supplied from the power supply circuit 14 via the power supply terminal 20.

  In this configuration, as shown in FIG. 9, a reference potential supply terminal 22 is provided at one end of the reference potential line 18, and the reference potential VREF is supplied from the power supply circuit 14 via the reference potential supply terminal 22. In addition to being supplied, a low potential supply terminal 26 is provided at the other end, and a potential VL lower than the reference potential VREF is supplied from the power supply circuit 15 via the low potential supply terminal 26. You can also. According to this aspect, current flows from the reference potential supply terminal 22 toward the low potential supply terminal 26 so that the amount of potential drop at the reference potential supply terminal 26 is maximized. The potential of the reference potential line 18 drops toward the reference potential supply terminal 26.

As shown in FIG. 9, the gate of the current source transistor Tg of each unit circuit U ₁ ~U _n is the reference potential supply terminal 22 of the reference potential line 18 is connected between the low potential supply terminal 26 As the resistance value of the current path from the power supply terminal 20 to the current source transistor Tg increases, the current path from the reference potential supply terminal 22 to the connection point between the current source transistor Tg and the reference potential line 18 increases. The resistance value is large. As a result, a potential drop occurs in the reference potential line 18 so as to correspond to the potential drop in the higher power supply line 16. Thus, variations in the gate-source voltage Vgs of the current source transistor Tg of each unit circuit U ₁ ~U _n can be suppressed, variations in the driving current Ids generated by the current source transistor Tg (light emitting element 23 Variation in emission intensity). In this aspect, the potential drop amount in the reference potential line 18 may be adjusted by adjusting the potential VL from the power supply circuit 15 supplied to the reference potential line 18 via the low potential supply terminal 26. it can.

(4) Modification 4
In each of the embodiments described above, the current Iref flowing through the reference potential line 18 flows into the low potential supply point C, and flows toward the power supply terminal 21 through the low-potential power supply line 17. Since the lower power supply line 17 is a resistor (for example, the resistor Rct shown in FIG. 4), a potential drop occurs in the lower power supply line 17 when the current Iref flows through the lower power supply line 17. As the resistance value of the current path from the low potential supply point C is larger (closer to the power supply terminal 21), the potential drop amount in the lower power supply line 17 is larger, and as shown in FIG. Is closer to the ground potential VCT as it is closer to the power supply terminal 21. On the other hand, as shown in FIG. 10, the greater the distance from the power supply terminal 20 to the connection point S, the greater the potential drop in the higher power supply line 16. Then, as shown in FIG. 10, the drain-source voltage Vds of the current source transistor Tg of each unit circuit U ₁ ~U _n decreases as the distance from the power supply terminal 20 to the connection point S is large. As understood from the above equation (1), when the drain-source voltage Vds of the current source transistor Tg decreases, the drive current Ids generated by the current source transistor Tg also decreases. As a result, it occurs a variation in emission intensity of the light emitting element 23 of each unit circuit U ₁ ~U _n.

  Here, according to the above equation (1), even if the drain-source voltage Vds of the current source transistor Tg decreases, if the gate-source voltage Vgs of the current source transistor Tg is set large, the current source transistor Tg generates It can be seen that the decrease in the drive current Ids can be suppressed. Therefore, the reference value is set so that the value of the gate-source voltage Vgs of each current source transistor Tg can be compensated for the decrease in the drive current Ids due to the decrease in the drain-source voltage Vds of each current source transistor Tg. The potential drop amount in the potential line 18 may be set larger than the potential drop amount in the higher power line 16. According to this aspect, it is possible to further suppress the variation in the drive current Ids generated by each current source transistor Tg (the variation in the light emission intensity of the light emitting element 23).

(5) Modification 5
In each of the embodiments described above, the reference potential line 18 and the lower power supply line 17 are connected at the low potential supply point C. However, the present invention is not limited to this. For example, as shown in FIG. 22 and the low potential supply point C are provided with a plurality of connection points Z between the reference potential line 18 and the lower power supply line 17, and the connection increases as the resistance value of the current path from the power supply terminal 20 to each connection point Z increases. In order to increase the potential difference between the point Z and the lower power supply line 17, the resistance value between each connection point Z and the lower power supply line 17 (for example, the resistance value of the resistor Rcon in FIG. 11) is individually set. It may be an aspect of setting. According to this aspect, by individually adjusting the voltage between each connection point Z and the lower power supply line 17, the amount of decrease in the power supply potential VEL and the decrease in the reference potential VREF supplied to each current source transistor Tg. It is also possible to make the quantities approximately the same value. Therefore, the gate-source voltage Vgs of the current source transistor Tg can be substantially the same value, that variation in the emission intensity of the light emitting element 22 (luminance) of each unit circuit U ₁ ~U _n more Can be suppressed. In the aspect of FIG. 11, the resistor Rcon is provided between each connection point Z and the lower power line 17, but not limited thereto, for example, the above-described DA converter 24, transistor Ts, and the like are provided. Various modifications and combinations are possible.

(6) Modification 6
For example, there is a possibility that the amount of decrease in the power supply potential VEL in the higher power supply line 16 corresponds to the potential difference between the reference potential VREF and the ground potential VCT. Even in such a case, according to the aspect in which the lower power supply line 17 is connected to the reference potential line 18 without using the resistor Rcon or the like, the reference is made so as to correspond to the potential drop in the higher power supply line 16. A potential drop can be generated in the potential line 18.

(7) Modification 7
In order to make the potential drop in the reference potential line 18 correspond to the potential drop in the higher power supply line 16, the resistance value of the current path from the reference potential supply terminal 22 to each gate connection point G in the reference potential line 18 is individually set. It can also be set as the aspect of setting to. The resistance value may be set by inserting a resistor in the reference potential line 18 or by changing the thickness of the reference potential line 18 without providing a separate resistor. May be.

(8) Modification 8
In each of the above-described embodiments, the power supply potential VEL and the reference potential VREF supplied to the higher power supply line 16 and the reference potential line 18 are supplied from the same power supply circuit. For example, the power supply potential VEL and the reference potential VREF may be supplied from separate power supply circuits.

(9) Modification 9
In the embodiments described above, each of the unit circuits U ₁ ~U _n includes a transistor Tr for switching the current source transistor Tg, whether to supply the current generated by current source transistor Tg to the light emitting element 23, light emission However, the present invention is not limited to this. For example, the unit circuit U may include the current source transistor Tg and the light emitting element 23 without including the transistor Tr. The light emitting device 10 having such a configuration can also be employed in an image reading device such as a scanner as a line-type optical head (illumination device) that irradiates light to a reading target such as a document.

(10) Modification 10
In each of the above-described embodiments, the current source transistor Tg and the transistor Tr are disposed between the high-potential power line 16 and the light emitting element 23. However, the present invention is not limited to this. For example, the current source transistor Tg and the transistor Tr Both or any one of them may be disposed between the light emitting element 23 and the lower power line 17. In each of the above-described embodiments, the transistor Tr is disposed between the current source transistor Tg and the light emitting element 23. However, the present invention is not limited to this. For example, the high-potential power supply line 16 and the current source transistor Tg A transistor Tr may be disposed between them. Further, in the aspect in which both the current source transistor Tg and the transistor Tr are disposed between the light emitting element 23 and the lower power supply line 17, the transistor Tr may be disposed between the light emitting element 23 and the current source transistor Tg. Alternatively, it may be arranged between the current source transistor Tg and the lower power supply line 17.

<E: Electronic equipment>
Next, with reference to FIG. 12, an image forming apparatus which is one aspect of the electronic apparatus according to the invention will be described. This image forming apparatus is a tandem type full color image forming apparatus using a belt intermediate transfer body system.

  In this image forming apparatus, four light emitting devices 10K, 10C, 10M, and 10Y each having the same configuration are provided with four photosensitive drums (image carriers) 110K, 110C, and 110M having the same configuration. , 110Y are arranged at positions facing the image forming surface 110. The light emitting devices 10K, 10C, 10M, and 10Y have the same configuration as the light emitting device 10 according to each of the above embodiments.

  As shown in FIG. 12, this image forming apparatus is provided with a driving roller 121 and a driven roller 122. An endless intermediate transfer belt 120 is wound around these rollers 121 and 122, and an arrow indicates. As shown, the periphery of the rollers 121 and 122 is rotated. Although not shown, tension applying means such as a tension roller that applies tension to the intermediate transfer belt 120 may be provided.

  Around the intermediate transfer belt 120, four photosensitive drums 110K, 110C, 110M, and 110Y each having a photosensitive layer on the outer peripheral surface are arranged at a predetermined interval. The subscripts “K”, “C”, “M”, and “Y” mean that they are used to form black, cyan, magenta, and yellow visible images, respectively. The same applies to other members. The photosensitive drums 110K, 110C, 110M, and 110Y are rotationally driven in synchronization with the driving of the intermediate transfer belt 120.

  Around each photosensitive drum 110 (K, C, M, Y), there is a corona charger 111 (K, C, M, Y), a light emitting device 10 (K, C, M, Y), and a developing unit. 114 (K, C, M, Y) are arranged. The corona charger 111 (K, C, M, Y) uniformly charges the image forming surface 110A (outer peripheral surface) of the corresponding photosensitive drum 110 (K, C, M, Y). The light emitting device 10A (K, C, M, Y) writes an electrostatic latent image on the charged image forming surface 110A of each photosensitive drum. In each light emitting device 10A (K, C, M, Y), a plurality of light emitting elements 20 are arranged along the bus (main scanning direction) of the photosensitive drum 110 (K, C, M, Y). The electrostatic latent image is written by irradiating the photosensitive drum 110 (K, C, M, Y) with light by the plurality of light emitting elements 20. The developing device 114 (K, C, M, Y) has a visible image (that is, a visible image) on the photosensitive drum 110 (K, C, M, Y) by attaching toner as a developer to the electrostatic latent image. ).

  The black, cyan, magenta, and yellow developed images formed by the four-color single-color image forming stations are superimposed on the intermediate transfer belt 120 by being sequentially primary-transferred onto the intermediate transfer belt 120. As a result, a full-color visible image is formed. Four primary transfer corotrons (transfer devices) 112 (K, C, M, Y) are arranged inside the intermediate transfer belt 120. The primary transfer corotron 112 (K, C, M, Y) is disposed in the vicinity of the photosensitive drum 110 (K, C, M, Y), and the photosensitive drum 110 (K, C, M, Y). The electrostatic image is electrostatically attracted from the toner image to transfer the visible image to the intermediate transfer belt 120 passing between the photosensitive drum and the primary transfer corotron.

  A sheet 102 as an object (recording material) on which an image is to be finally formed is fed one by one from the sheet feeding cassette 101 by the pickup roller 103 and is subjected to secondary transfer with the intermediate transfer belt 120 in contact with the driving roller 121. Sent to the nip between the rollers 126. The full-color visible image on the intermediate transfer belt 120 is secondarily transferred to one side of the sheet 102 by the secondary transfer roller 126 and fixed on the sheet 102 through the fixing roller pair 127 as a fixing unit. . Thereafter, the sheet 102 is discharged onto a paper discharge cassette formed in the upper part of the apparatus by a paper discharge roller pair 128.

  Next, another embodiment of the image forming apparatus according to the present invention will be described with reference to FIG. This image forming apparatus is a rotary developing type full-color image forming apparatus using a belt intermediate transfer body system. As shown in FIG. 13, a corona charger 168, a rotary development unit 161, the light emitting device 10 according to the above-described embodiment, and an intermediate transfer belt 169 are provided around the photosensitive drum 110. Yes.

  The corona charger 168 uniformly charges the outer peripheral surface of the photosensitive drum 110. The light emitting device 10 writes an electrostatic latent image on the charged image forming surface (outer peripheral surface) of the photosensitive drum 110. In the light emitting device 10, a plurality of light emitting elements 32 are arranged along the bus line (main scanning direction) of the photosensitive drum 110. The electrostatic latent image is written by irradiating the photosensitive drum 110 with light from these light emitting elements 32.

  The developing unit 161 is a drum in which four developing units 163Y, 163C, 163M, and 163K are arranged at an angular interval of 90 °, and can rotate counterclockwise about the shaft 161a. The developing units 163Y, 163C, 163M, and 163K supply yellow, cyan, magenta, and black toner to the photosensitive drum 110, respectively, and attach the toner as a developer to the electrostatic latent image, thereby the photosensitive drum 110. A visible image (ie, a visible image) is formed.

  The endless intermediate transfer belt 169 is wound around a driving roller 170a, a driven roller 170b, a primary transfer roller 166, and a tension roller, and is rotated around these rollers in a direction indicated by an arrow. The primary transfer roller 166 transfers the visible image to the intermediate transfer belt 169 that passes between the photosensitive drum 110 and the primary transfer roller 166 by electrostatically attracting the visible image from the photosensitive drum 110.

  Specifically, in the first rotation of the photosensitive drum 110, an electrostatic latent image for a yellow (Y) image is written by the light emitting device 10, and a developed image of the same color is formed by the developing device 163Y. The image is transferred to the transfer belt 169. Further, in the next rotation, the electrostatic latent image for the cyan (C) image is written by the light emitting device 10A, and a developed image of the same color is formed by the developing device 163C, and the intermediate transfer is performed so as to overlap the yellow developed image. Transferred to the belt 169. Then, while the photosensitive drum 110 rotates four times in this way, the yellow, cyan, magenta, and black visible images are sequentially superimposed on the intermediate transfer belt 169, and as a result, a full-color visible image becomes a transfer belt 169. Formed on top. When images are finally formed on both sides of a sheet as an object on which an image is to be formed, the same color images of the front and back surfaces are transferred to the intermediate transfer belt 169, and then the front and back surfaces are transferred to the intermediate transfer belt 169. A full-color visible image is formed on the intermediate transfer belt 169 in such a manner that the visible image of the next color is transferred.

  The image forming apparatus is provided with a sheet conveyance path 174 through which a sheet passes. The sheets are picked up one by one from the paper feed cassette 178 by the pick-up roller 179, advanced through the sheet transport path 174 by the transport roller, and between the intermediate transfer belt 169 and the secondary transfer roller 171 in contact with the drive roller 170a. Pass through the nip. The secondary transfer roller 171 transfers the developed image to one side of the sheet by electrostatically attracting a full-color developed image from the intermediate transfer belt 169 collectively. The secondary transfer roller 171 can be moved closer to and away from the intermediate transfer belt 169 by a clutch (not shown). The secondary transfer roller 171 is brought into contact with the intermediate transfer belt 169 when a full-color visible image is transferred onto the sheet, and is separated from the secondary transfer roller 171 while the visible image is superimposed on the intermediate transfer belt 169.

  The sheet on which the image has been transferred as described above is conveyed to the fixing device 172 and is passed between the heating roller 172a and the pressure roller 172b of the fixing device 172, whereby the visible image on the sheet is fixed. The sheet after the fixing process is drawn into the discharge roller pair 176 and proceeds in the direction of arrow F. In the case of double-sided printing, after most of the sheet passes through the paper discharge roller pair 176, the paper discharge roller pair 176 is rotated in the reverse direction and introduced into the double-sided printing conveyance path 175 as indicated by an arrow G. The Then, the visible image is transferred to the other surface of the sheet by the secondary transfer roller 171, the fixing process is performed again by the fixing device 172, and then the sheet is discharged by the discharge roller pair 176.

  Since the image forming apparatus illustrated in FIGS. 12 and 13 uses a light source (exposure means) employing an OLED element as the light emitting element 23, the apparatus is made smaller than when a laser scanning optical system is used. . Note that the light-emitting device 10 of the present invention can also be employed in electrophotographic image forming apparatuses other than those exemplified above. For example, the light emitting device according to the present invention is also applied to an image forming apparatus that directly transfers a visible image from a photosensitive drum to a sheet without using an intermediate transfer belt, and an image forming apparatus that forms a monochrome image. 10 can be applied.

  The use of the light emitting device according to the present invention is not limited to exposure of a photoreceptor. For example, the light emitting device of the present invention is employed in an image reading device such as a scanner as a line type optical head (illumination device) that irradiates a reading target such as an original with light. As this type of image reading apparatus, there is a scanner, a copying machine or a reading part of a facsimile, a barcode reader, or a two-dimensional image code reader for reading a two-dimensional image code such as a QR code (registered trademark). In addition, a light-emitting device in which a plurality of light-emitting elements (particularly light-emitting elements) are arranged in a planar shape is also employed as a backlight unit disposed on the back side of a liquid crystal panel. A light emitting device in which a plurality of unit circuits are arranged in a matrix is employed as a display device for various electronic devices.

1 is a perspective view showing a partial configuration of an image forming apparatus according to a first embodiment. It is a block diagram which shows the electrical structure of the light-emitting device 10 which concerns on the embodiment. It is a diagram illustrating the position of the high-potential power supply line 16, and a power supply voltage VEL and gate voltage VEL is supplied to the drive transistor Tsw of each unit circuit _U 1 ~U _n, the relationship. FIG. 6 is a diagram schematically showing an electrical configuration near the center (midpoint between power supply terminals 20) in the extending direction of the high-order power supply line 16. It is a figure which shows the relationship between the power supply potential VEL supplied to each current source transistor, and the reference potential VREF. It is a figure which shows schematic structure of the DA converter 24 which concerns on 2nd Embodiment. In the light-emitting device 10 which concerns on 3rd Embodiment, it is the figure which showed typically the electrical structure of the center (midpoint between the power supply terminals) of the extension direction of the high power supply line 16 vicinity. FIG. 10 is a diagram schematically showing an electrical configuration in the vicinity of the center (midpoint between power supply terminals 20) in the extending direction of a high-order power supply line 16 in a light emitting device 10 according to Modification 2. FIG. 11 is a diagram schematically showing an electrical configuration in the vicinity of the center (midpoint between power supply terminals 20) in the extending direction of a high-order power supply line 16 in a light emitting device 10 according to Modification 3. It is a figure which shows the relationship between the power supply potential VEL supplied to each current source transistor, the reference potential VREF, and the ground potential VCT. FIG. 11 is a diagram schematically showing an electrical configuration in the vicinity of the center (midpoint between power supply terminals 20) in the extending direction of the high-order power supply line 16 in the light emitting device 10 according to Modification Example 5. It is a perspective view which shows the specific example (image forming apparatus) of the electronic device which concerns on this invention. It is a perspective view which shows the specific example (image forming apparatus) of the electronic device which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Light-emitting device, 16 ... High side power line, 17 ... Low side power line, 18 ... Reference potential line, 20 ... Power supply terminal, 21 ... Power supply terminal, 22 ... Reference potential supply terminal, 23... Light emitting element, 24... DA converter U _{1 to} U _n ... Unit circuit, Tg... Current source transistor, Tr ... Transistor (switch), Ts ... Transistor, S _{1 to} S _n. Point, G _{1 to} G _n ... Gate connection point, Ids... Drive current.

Claims (8)

A light-emitting device including a plurality of unit circuits each connected to a high-order power supply line to which power is supplied via a power supply terminal,
Each of the plurality of unit circuits is
A current source transistor that receives a supply of power from the high-level power supply line and generates a drive current;
A light emitting element that emits light with a luminance according to the drive current generated by the current source transistor,
A gate of a current source transistor in each of the plurality of unit circuits is connected between a first potential supply point and a second potential supply point of a reference potential line;
The larger the resistance value of the current path from the power supply terminal to the current source transistor, the larger the resistance value of the current path from the first potential supply point to the gate of the current source transistor,
A light emitting device, wherein a reference potential is supplied to the first potential supply point, and a potential lower than the reference potential is supplied to the second potential supply point. Each of the plurality of unit circuits is disposed between the high-order power supply line and the low-order power supply line,
The light emitting device according to claim 1, wherein the lower power supply line is connected to the second potential supply point.   The light emitting device according to claim 2, wherein a resistor is provided between the second potential supply point and the lower power supply line.   Voltage changing means for changing a voltage between the second potential supply point and the lower power supply line is provided between the second potential supply point and the lower power supply line. The light-emitting device according to claim 2.   5. The light emitting device according to claim 4, wherein the voltage changing unit changes a voltage between the second potential supply point and the lower power supply line according to input data of a plurality of bits. The voltage changing means is composed of a transistor,
5. The light emitting device according to claim 4, wherein a voltage between the second potential supply point and the lower power supply line is changed by controlling a voltage supplied to the gate of the transistor. 6. In the reference potential supply wiring, a plurality of connection points to the lower power supply line are provided between the first potential supply point and the second potential supply point.
The resistance value between each of the connection points and the lower-side power supply line is such that the larger the resistance value of the current path from the first potential supply point to the connection point, the higher the potential of the first potential supply point and the connection. The light emitting device according to any one of claims 3 to 6, wherein the light emitting device is individually set so that a potential difference with respect to a potential of a point is increased. The electronic device which comprises the light-emitting device of any one of Claim 1 to 7.

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