JP6540471B2 - Optical writing device and image forming apparatus - Google Patents

Description

  The present invention relates to an optical writing device and an image forming apparatus, and more particularly to a technology for reducing uneven density due to voltage drop in a power supply line supplying current to a light emitting element.

  In recent years, line-optical optical writing devices (OLED-PH: OLED Print Heads) have been arranged in lines using OLEDs (Organic Light Emitting Diodes) as light emission sources for the purpose of downsizing and cost reduction of image forming apparatuses. Development is in progress. Since the OLED-PH can form the light emitting element and its drive circuit on the same substrate by forming the OLED and the thin film transistor (TFT: Thin Film Transistor) on the same substrate, the size and cost can be reduced. can do.

  In the OLED-PH, for example, as shown in FIG. 16, a digital-to-analog converter (DAC: Digital to Analogue Converter) 1601 is used to generate a luminance signal for each OLED 1602, and input to the driving TFT 1603. The amount of light emission is controlled. Note that no current flows in the circuit 1604 from the DAC 1601 to the driving TFT 1603, and thus no voltage drop occurs in the circuit 1604.

On the other hand, the OLED 1602 is a current-driven light emitting element, and in the OLED-PH 1600, a large number of OLEDs 1602 share the power supply line 1605 to receive supply of drive current. Therefore, a current according to the lighting state of the OLED 1602 flows to the circuit from the power supply Vc to the OLED 1602, a voltage drop occurs on the power supply line 1605, and a potential distribution is formed.
The driving TFT 1603 supplies a driving current according to the potential difference between the connection point 1606 with the power supply line 1605 and the luminance signal from the DAC 1601 to the OLED 1602. Therefore, when a voltage drop occurs in the power supply line 1605, the amount of drive current fluctuates according to the position of the connection point 1606 on the power supply line 1605 even if the luminance signal from the DAC 1601 is the same. The amount of light emission of light changes (FIG. 17).

To solve this problem, a dummy load is connected in parallel for each OLED, and for an OLED that is not lit, the current is supplied to the dummy load instead of the OLED, so that the power supply A technique for keeping the amount of voltage drop in a line constant has been proposed (see, for example, Patent Document 1).
If such a technique is used, it is possible to suppress the occurrence of density unevenness in the image by adjusting the luminance signal for each OLED and preventing the fluctuation of the light emission amount caused by the voltage drop in the power supply line. Can.

JP, 2005-144687, A

However, adding a dummy load, a switch for switching between the OLED and the dummy load, a circuit for controlling the switch, and the like for each OLED increases the circuit size and complicates the circuit configuration, resulting in a large OLED-PH. And cost increase.
The present invention has been made in view of the problems as described above, and an optical writing device and an image forming apparatus that suppress the light amount fluctuation due to the voltage drop in the power supply line without complicating the circuit configuration Intended to be provided.

In order to achieve the above object, an optical writing apparatus according to the present invention is an optical writing apparatus which exposes a photosensitive member to form an electrostatic latent image line by line, and a plurality of current driven types arranged in a line and the light emitting element, and extends along a plurality of the light emitting element, a power supply line for supplying power to a plurality of the light emitting element, a constant voltage source connected to one end of the power line, from the reference potential an instruction circuit by the potential difference and outputs an instruction potential for instructing the light emission amount to the light emitting element is interposed between the power supply line and the light emitting element, and the potential at the connection point between the power supply line, the indication circuit A drive circuit group for controlling a drive current input to the light emitting element according to a potential difference between the light emitting element and the indicated potential, a plurality of contacts provided on the power supply line, and lighting of each light emitting element in the one line Depending on the situation, what of the multiple contacts Comprising selecting means for selecting either the reference potential of the indicating circuit is characterized in that given by the potential of the power line at the contact point selected by the selecting unit.

  According to this configuration, there are provided a plurality of contacts provided on the power supply line, and selection means for selecting any of the plurality of contacts in accordance with the lighting state of each light emitting element in the one line, The instruction circuit outputs the instruction potential using the potential of the power supply line at the contact selected by the selection unit as the reference potential. Therefore, unlike the prior art, a dummy circuit, a light emitting element and a dummy circuit are provided for each light emitting element. Since a switch for switching is not necessary, it is possible to suppress the variation in light quantity caused by the voltage drop in the power supply line without complicating the circuit configuration.

In this case, the selecting unit may select a contact further from the constant voltage source on the power supply line as the number of light emitting elements to be lit in the one line increases, or the light emitted in the one line is illuminated As the distribution of elements is farther from the constant voltage source on the power supply line, a contact farther from the constant voltage source on the power supply line may be selected.
Further, the selection unit selects a contact on the power supply line which is further from the constant voltage source as the voltage drop amount from the constant voltage source at the connection point farthest from the constant voltage source on the power supply line is larger. It is also good. In this case, the selection means may select the contact point closest to half of the average value of the voltage drop amounts of at least one line including at least the one line. Furthermore, the one or more lines, and the one line, it is preferable if the two lines of adjacent lines in said one line and the sub scanning direction.

Further, the selection means may select the contact each time the one line is scanned, and the light emitting element may be an OLED.
An image forming apparatus according to the present invention is characterized by including the optical writing device according to the present invention.

FIG. 1 is a diagram showing a main configuration of an image forming apparatus according to a first embodiment of the present invention. FIG. 2 is a diagram showing a main configuration of the optical writing device 100. FIG. 3 is a schematic plan view of the OLED panel 200, together with a cross-sectional view taken along the line B-B ′ and a cross-sectional view taken along the line C-C ′. FIG. 6 is a block diagram showing the main configuration of a TFT substrate 300. FIG. 6 is a circuit diagram showing a pair of selection circuits 401 and a light emission block 402. It is a timing chart explaining an active drive method. FIG. 16 is a circuit diagram showing a configuration for supplying power to a driver IC 302 from a contact 701 provided on a power supply line 531. FIG. 2 is a block diagram showing the main configuration of a control unit 101. 5 is a flowchart showing a procedure of selecting a contact point 701 by the control unit 101. It is a table which illustrates data for power supply selection. It is a graph which compares the driver IC power supply voltage in a main scanning direction, DAC voltage (brightness signal), and gate source voltage Vgs about the case where the number of lighting of OLED201 is large and few. It is a flowchart which shows the procedure which the control part 101 which concerns on the 2nd Embodiment of this invention selects the contact 701. FIG. It is a table which illustrates data for power supply selection concerning a 2nd embodiment of the present invention. It is a flowchart which shows the procedure which the control part 101 which concerns on the 3rd Embodiment of this invention selects the contact point 701. FIG. It is a flowchart which shows the procedure which the control part 101 which concerns on the 4th Embodiment of this invention selects the contact point 701. FIG. It is a circuit diagram which illustrates the composition of a typical OLED-PH. (A) is a graph illustrating the relationship between the lighting state of the OLED 1602 and the voltage drop on the power supply line 1605, and (b) illustrates the change in light emission of the OLED 1602 due to the voltage drop on the power supply line 1605 Is a graph.

Hereinafter, embodiments of an optical writing device and an image forming apparatus according to the present invention will be described with reference to the drawings.
[1] First Embodiment The image forming apparatus according to the first embodiment of the present invention provides a plurality of contacts on a power supply line for supplying a drive current to an OLED, according to the number of OLEDs to be lit. The potential of the luminance signal output from the DAC is adjusted with reference to the potential at the selected contact.

(1) Configuration of Image Forming Apparatus First, the main configuration of the image forming apparatus according to the present embodiment will be described.
As shown in FIG. 1, the image forming apparatus 1 is a so-called tandem type color printer. The image forming stations 110Y, 110M, 110C, and 110K included in the image forming apparatus 1 control toner images of yellow (Y), magenta (M), cyan (C), and black (K) under the control of the control unit 101. Form.

For example, at the image forming station 110Y, the charging device 112 uniformly charges the outer peripheral surface of the photosensitive drum 111. The optical writing device 100 exposes the outer peripheral surface of the photosensitive drum 111 to form an electrostatic latent image.
The developing device 113 supplies toner to the outer peripheral surface of the photosensitive drum 111 and develops (visualizes) the electrostatic latent image to form a Y-color toner image. The primary transfer roller 114 electrostatically transfers (primary transfer) the toner image onto the intermediate transfer belt 102 from the outer peripheral surface of the photosensitive drum 111. The toner remaining on the outer peripheral surface of the photosensitive drum 111 after the primary transfer is removed by the cleaner 115 and discarded.

The intermediate transfer belt 102 is stretched between a secondary transfer roller pair 103 and a driven roller 104, and rotationally travels in the direction of arrow A while carrying a toner image.
Similarly, the toner images of the respective colors MCK formed by the image forming stations 110M, 110C and 110K are primarily transferred onto the intermediate transfer belt 102 at the same timing so as to overlap the Y toner image, Become. As the intermediate transfer belt 102 conveys the color toner image to the secondary transfer roller pair 103, the recording sheet S supplied from the paper feed cassette 120 is also conveyed to the secondary transfer roller pair 103.

The secondary transfer roller pair 103 electrostatically transfers (secondary transfer) the toner image on the intermediate transfer belt 102 onto the recording sheet S. The recording sheet S to which the toner image has been transferred is thermally fixed on the toner image by the fixing device 105, and then discharged onto the paper discharge tray 107 by the paper discharge roller 106.
An operation panel (not shown) is connected to the control unit 101, and presents information to the user of the image forming apparatus 1 or accepts an instruction input from the user.

(2) Configuration of Optical Writing Device 100 Next, the configuration of the optical writing device 100 will be described.
(2-1) Overall Configuration As shown in FIG. 2, the optical writing device 100 is a device in which the OLED panel 200 and the rod lens array 202 are accommodated in the holder 203, and the OLED 201 is mounted on the OLED panel 200. There is. The light beam L emitted by the OLED 201 is condensed on the outer peripheral surface of the photosensitive drum 111 by the rod lens array 202. The rod lens array 202 is an optical element in which a large number of rod lenses are integrated, and may be SLA (SELFOC Lens Array. SELFOC is a registered trademark of Nippon Sheet Glass Co., Ltd.) or MLA (Micro Lens Array). You may use.

(2-2) Schematic Configuration of OLED Panel 200 FIG. 3 is a schematic plan view of the OLED panel 200, together with a cross-sectional view taken along the line B-B 'and a cross-sectional view taken along the line C-C'. Moreover, the schematic plan view part has shown the state which removed the sealing board 301 mentioned later.
As shown in FIG. 3, the OLED panel 200 includes a TFT substrate 300, a sealing plate 301, a driver IC (Integrated Circuit) 302, and the like. On the TFT substrate 300, 15,000 OLEDs 201 are arranged along the main scanning direction. The focusing points of these OLEDs 201 are 21.2 μm pitch (1200 dpi) on the outer peripheral surface of the photosensitive drum 111.

  Further, the substrate surface on which the OLED 201 of the TFT substrate 300 is disposed is a sealing region, and the sealing plate 301 is attached with the spacer frame 303 interposed therebetween. Thus, the OLED 201 is sealed in a state in which dry nitrogen or the like is sealed so as not to be exposed to the outside air. In addition, you may enclose a hygroscopic agent collectively. Further, the sealing plate 301 may be sealing glass or may be made of a material other than glass.

  The driver IC 302 is mounted outside the sealing region of the TFT substrate 300. The control unit 101 inputs image data to the driver IC 302 via a card cable (FFC: Flexible Flat Card) 310. The driver IC 302 converts image data into a luminance signal and inputs it to a drive circuit of each OLED 201. The drive circuit generates drive current for the OLED 201 in response to the luminance signal. The luminance signal may be a current signal or a voltage signal.

As described above, in the OLED-PH, since the OLED and the TFT can be formed on the same substrate, the LED- and the control circuit unit (drive IC etc.) can not but be separated. Cost can be lower than PH.
(2-3) Configuration of TFT Substrate 300 The OLED 201 has a deterioration characteristic that the amount of light emission decreases as the accumulated light emission time becomes longer, while the accumulated light emission time for each OLED 201 varies depending on the image data. The degree of deterioration of the amount of light is different, and the luminance varies. Therefore, in the OLED-PH, it is necessary to adjust the light emission luminance for each of the OLEDs 201 in order to keep the printed image uniform and keep the image quality constant. Therefore, when the driver IC 302 writes the luminance signal generated for each of the OLEDs 201 using the DAC in the drive circuit, the light emission luminance for each of the OLEDs 201 is adjusted.

  Further, in the present embodiment, a plurality of OLEDs 201 share a DAC, and an active drive scheme is employed in which a luminance signal is written from the DAC while sequentially switching the OLEDs 201, thereby reducing the circuit size. In the active drive method, the luminance signal written by the DAC is held until the next writing after the main scanning period (1H period) elapses (for example, when light emission data is written, light is emitted for about 1H period). to continue).

As shown in FIG. 4, in the TFT substrate 300, 100 15,000 OLEDs 201 are grouped into 150 light emission blocks 402. Further, 150 DACs 400 are built in the driver IC 302, and correspond to the light emitting blocks 402 one by one, respectively.
When the driver IC 302 receives image data from the control unit 101, the driver IC 302 distributes the input to the DACs 400 for each one scanning period by 100 pixels. A selection circuit 401 is disposed on any circuit going from the DAC 400 to the light emission block. Each DAC 400 converts image data into a luminance signal, and sequentially outputs the luminance signal to the 100 OLEDs 201 under control.

  FIG. 5 is a circuit diagram showing a pair of selection circuits 401 and a light emission block 402. As shown in FIG. As shown in FIG. 5, the light emitting block 402 is composed of 100 light emitting pixel circuits, and each light emitting pixel circuit has one capacitor 521, one driving TFT 522 and one OLED 201. Further, the selection circuit 401 includes a shift register 511 and 100 selection TFTs 512.

  The shift register 511 is connected to the gate terminal of each of the 100 selection TFTs 512, and turns on the selection TFTs 512 in order. The source terminal of the selection TFT 512 is connected to the DAC 400 via the write wiring 530, and the drain terminal is connected to the first terminal of the capacitor 521 and the gate terminal of the driving TFT 522.

With the shift register 511 turning on the selection TFT 512, the luminance signal from the DAC 400 is input to the first terminal of the capacitor 521 (charge) and held until reset (hold).
The first terminal of the capacitor 521 is also connected to the gate terminal of the driving TFT 522, and the second terminal of the capacitor 521 is connected to the source terminal of the driving TFT 522 and the power supply line 531.

The drain terminal of the driving TFT 522 is connected to the anode terminal of the OLED 201, which is a series circuit. The cathode terminal of the OLED 201 is connected to the ground wiring 532. The power supply line 531 is connected to the constant voltage source AVDD received from the power supply unit 612, and the ground wiring 532 is connected to the ground terminal GND.
The constant voltage source AVDD is a supply source of the drive current supplied to the OLED 201, and the drive TFT 522 is a voltage held between the first and second terminals of the capacitor 521, in other words, the drive TFT 522. A drain current corresponding to the source-gate voltage is supplied to the OLED 201 as a drive current. Needless to say, as the source-gate voltage is higher, the driving TFT 522 supplies more driving current, and the light emission amount of the OLED 201 is increased.

  For example, when a luminance signal corresponding to Hi is written to the capacitor 521, the driving TFT 522 is turned on, and the OLED 201 emits light with an amount of light corresponding to the driving current. In addition, when a luminance signal corresponding to Low is written to the capacitor 521, the driving TFT 522 is turned off and the OLED 201 does not emit light. As described above, the light emission amount of the OLED 201 can be controlled by changing the start signal output from the DAC 400.

The reset circuit 540 is connected to the write wiring 530. When the reset circuit 540 is turned on, the wiring from the current DAC 400 to the selection TFT 512 is reset to a predetermined voltage. The reset circuit 540 may be incorporated in the driver IC 302.
By providing such a configuration, the luminance signal is written as follows. As shown in FIG. 6, when the shift register 511 turns on the first selection TFT 512, the luminance signal from the DAC 400 is input to the first capacitor 521, with the on period as a charge period.

Next, when the shift register 511 turns off the first selection TFT 512, a drive current corresponding to the voltage held by the first capacitor 521 is supplied to the first OLED 201, and the OLED 201 is lit (hold period) .
When the first selection TFT 512 is turned off, the second selection TFT 512 is turned on, and a luminance signal is input to the second capacitor 521. When such an operation is performed up to the 100th selection TFT 512, the operation returns to the first selection TFT 512 to repeat the above operation.

Although the case where the driving TFT 522 is a p-channel is described as an example in the present embodiment, it goes without saying that an n-channel driving TFT 522 may be used. The write wiring 530, the power supply line 531 and the ground wiring 532 are all thin film wiring.
(3) Power Supply Circuit Including Power Supply Line 531 Next, the power supply circuit including the power supply line 531 will be described in more detail.

  As shown in FIG. 7, a contact 701 is provided between the light emitting block 402 closest to the constant voltage source AVDD on the power supply line 531 and the constant voltage source AVDD, and between the adjacent light emitting blocks 402. In the present embodiment, since there are 150 light-emitting blocks 402, there will also be 150 contacts 701. In the present embodiment, contact numbers from # 1 to # 150 are assigned in order from the contact closest to the constant voltage source AVDD.

Each contact 701 is connected to the selector 700 via the lead wire 702. The driver IC 302 is also connected to the selector 700 by a circuit unit power supply line 704.
When selector 700 receives switching control from control unit 101 (hereinafter referred to as "power source selection control") and connects any of 150 contacts 701 to driver IC 302, it refers to the potential at selected contact 701. The DAC 400 outputs a luminance signal (DAC voltage) as a potential (hereinafter, referred to as “driver IC power supply voltage”).

(4) Power Supply Selection Control Next, power supply selection control by the control unit 101 will be described.
(4-1) Configuration of Control Unit 101 First, the main configuration of the control unit 101 will be described.
As shown in FIG. 8, the control unit 101 includes a central processing unit (CPU) 800, a read only memory (ROM) 801, and the like. When the image forming apparatus 1 is powered on, the CPU 800 is reset. Ru. Thereafter, the CPU 800 reads a boot program from the ROM 801 and activates the boot program, reads a RAM (Random Access Memory) 802 as a working storage area from an HDD (Hard Disk Drive) 803, and executes the control program.

The CPU 800 inputs image data to the driver IC 302 of the optical writing device 100, and inputs a switching control signal for switching control of the selector 700 to the selector 700. The HDD 803 stores, in addition to the control program, a print job, image data, data for power source selection which the control unit 101 refers to for controlling the selector 700, and the like.
A network interface card (NIC) 804 executes communication processing for receiving a print job from another device via a communication network such as a local area network (LAN). Operation panel 810 presents information to the user of image forming apparatus 1 and accepts instruction input from the user.

(4-2) Power Supply Selection Control Next, power supply selection control will be described.
As shown in FIG. 9, when the control unit 101 receives a print job described in PDL (Page Description Language) (S901: YES), the control unit 101 analyzes the language of the print job, converts it into intermediate data, and rasterizes it. After the image data for each page is generated according to (S902), the processing from step S903 to S906 is executed for each line of the image data.

That is, first, the number of OLEDs 201 to be lighted out of 15,000 OLEDs 201 for the line is counted (S903). Next, the power source selection data stored in the HDD 803 is referred to (S904).
In the power source selection data, as shown in FIG. 10, the contact 701 to be selected is specified for each range of the number of OLEDs 201 to be lit, and the contact farther from the constant voltage source AVDD as the number of lighting of the OLED 201 increases. 701 is to be selected.

After determining the contact 701 to be selected with reference to the power source selection data, the control unit 101 inputs a switching control signal indicating the number of the contact 701 to the selector 700 (S905). After that, the control unit 101 inputs the image data of the one line to the driver IC 302 (S906).
When the above process is performed for all lines of the image data, the control unit 101 proceeds to step S901 and waits for the next print job.

(4-3) Gate-source voltage Vgs
The driving TFT 522 supplies a driving current according to the gate-source voltage Vgs, and the OLED 201 emits light with a light emission amount according to the driving current. Therefore, by looking at the gate-source voltage Vgs, it is possible to evaluate the amount of light quantity variation due to the voltage drop in the power supply line 531.

FIG. 11 is a graph comparing the driver IC power supply voltage, the DAC voltage (brightness signal), and the gate-source voltage Vgs in the main scanning direction for cases where the number of lighting of the OLED 201 is large and small.
As shown in FIG. 11, the voltage drop amount at the power supply line 531 is larger when the number of lighting of the OLED 201 is smaller (graph 1101) than when the number of lighting is small (graph 1101). It becomes lower as the number of lights increases. Further, the driver IC power supply voltage becomes lower as the contact point 701 is farther from the constant voltage source AVDD.

  On the other hand, as the number of lighting of the OLED 201 increases according to the designation in the power supply selection data, the contact point 701 farther from the constant voltage source AVDD is selected, and the driver IC power supply voltage becomes lower. In the example of FIG. 11, the contact point C0 is selected when the number of lighting of the OLED 201 is small, and the contact point C1 is selected when the number is large. Therefore, when the number of lighting of the OLED 201 is small, the DAC voltage (luminance signal) becomes high (graph 1111), and when the number of lighting of the OLED 201 is large, the DAC voltage (luminance signal) becomes low (graph 1112).

As a result, the gate-source voltage Vgs of the driving TFT 522 corresponding to the near-end OLED 201 becomes lower when the number of lighting of the OLED 201 is small (graph 1121) than when the number of lighting of the OLED 201 is large (graph 1122) . The voltage difference between the gate and source voltages Vgs in this case is Vn.
At the far end, when the number of lighting of the OLED 201 is small (graph 1121), the gate-source voltage Vgs becomes higher than when the number of lighting of the OLED 201 is large (graph 1122). The voltage difference between the gate and source voltages Vgs in this case is Vf.

On the other hand, when the driver IC power supply voltage is taken from the constant voltage source AVDD as in the prior art, when the number of lighting of the OLED 201 is small (graph 1121), the gate when the number of lighting of the OLED 201 is large (graph 1122) The voltage difference Vf 'of the power supply line 531 at the far end of the source voltage Vgs is
| Vf || = | Vn | + | Vf |
Thus, the voltage differences Vn and Vf of the gate-source voltages are smaller in absolute value than the voltage difference Vf 'of the gate-source voltages. Also, the difference between the gate-source voltage Vgs between the graphs 1121 and 1122 is the upper limit of the larger of | Vn | and | Vf | and smaller than | Vf '| The light amount difference can be reduced.

Therefore, regardless of the number of lightings of the OLED 201, the light emission amount of each of the OLEDs 201 is stabilized, so that the density unevenness in the sub-scanning direction can be suppressed.
[2] Second Embodiment Next, a second embodiment of the present invention will be described. The image forming apparatus according to the present embodiment has a configuration substantially common to the image forming apparatus according to the first embodiment, but differs in the operation of selecting the contact 701 according to the position of the OLED 201 to be lit. There is. The following description will focus on the differences. In the present specification, when there is an element common to the embodiments, the same reference numeral is given.

  In the present embodiment, as shown in FIG. 12, the power supply selection data is referred to without counting the number of lights of the OLED 201 (S904). As exemplified in FIG. 13, the power source selection data according to the present embodiment indicates the numbers of the contacts 701 to be selected for each lighting pattern of the 15,000 OLEDs 201. For example, when only the OLED 201 of the number # 1 is turned on and all the other OLEDs 201 are turned off, the contact 701 of the number # 1 is selected.

When the number of the contact 701 to be selected is determined with reference to such data for power source selection, the control unit 101 inputs a switching control signal for instructing the number of the selected contact 701 to the selector 700 (S905). The image data for one line is input to the IC 302 (S906).
As the power source selection data, the number of lighting of the OLED 201 is sequentially added from the light emitting block 402 on the near end side of the constant voltage source AVDD for each lighting state of the OLED 201, and it becomes approximately half of the total number N of the number of lighting of the OLED 201 Thus, the contact 701 for dividing the light emitting block 402 may be designated.

As in the first embodiment, in the case where the contact point 701 is disposed between the light emitting blocks 402, the sum of the number of lighting up to the nth light emitting block 402 is N / 2 with respect to the total number N. In this case, the contact point 701 disposed between the n-th light-emitting block 402 and the (n + 1) -th light-emitting block 402 is designated.
When the sum N0 of the number of lighting up to the (n-1) th light emitting block 402 is smaller than N / 2 and the sum N1 of the number of lighting up to the nth light emitting block 402 is larger than N / 2 Is also conceivable.

  In such a case, if (N / 2−N0) is smaller than (N / 2−N0) and (N1−N / 2), the (n−1) th light emitting block 402 and n Designate a contact 701 disposed between the second light emission block 402 and the second light emission block 402. Conversely, when (N1−N / 2) is smaller, the contact point 701 disposed between the nth light emitting block 402 and the (n + 1) th light emitting block 402 may be designated.

In this case, since the contact point 701 is selected in consideration of the position of the OLED 201 to be lighted in addition to the number of lights of the OLED 201, uneven density can be eliminated with higher accuracy.
[3] Third Embodiment The image forming apparatus according to the present embodiment has the same configuration as the first and second embodiments in general, but calculates the voltage drop amount for each line of image data. And the point of selecting the contact point 701 is different. In the following, description will be made by focusing on the differences.

As shown in FIG. 14, in the present embodiment, as processing to be executed for each line of image data, first, the position of the OLED 201 to be lit is read (S1401), and the drive current amount of each OLED 201 is specified. (S1402).
Next, among the OLEDs 201 to be lighted, the voltage drop amount at the connection point between the driving TFT 522 corresponding to the OLED 201 closest to the constant voltage power supply AVDD and the power supply line 531, and the OLED 201 to be lighted to the OLED 201 farthest to the constant voltage power supply AVDD. An intermediate value of the voltage drop amount at the connection point between the corresponding driving TFT 522 and the power supply line 531 is calculated (S1403).

Furthermore, the amount of voltage drop is calculated for each contact 701 (S1404), and the contact 701 closest to the intermediate value is selected (S1405).
The voltage drop amount ΔVdown at the connection point between the driving TFT 522 corresponding to the mth OLED 201 and the power supply line 531 is calculated using the following equation (1).

ここで、Ｉｉはｉ番目のＯＬＥＤ２０１に供給される駆動電流量であり、Ｒｉは（ｉ−１）番目のＯＬＥＤ２０１に対応する駆動用ＴＦＴと電源線５３１との接続と、ｉ番目のＯＬＥＤ２０１に対応する駆動用ＴＦＴと電源線５３１との接続との間の配線抵抗である。

Here, Ii is the amount of drive current supplied to the i-th OLED 201, Ri is the connection between the drive TFT corresponding to the (i-1) -th OLED 201 and the power supply line 531, and corresponds to the i-th OLED 201 Wiring resistance between the driving TFT and the power supply line 531.

When the contact point 701 is selected, the control unit 101 inputs a switching control signal indicating the number of the selected contact point 701 to the selector 700 (S905), and inputs the image data of one line to the driver IC 302 (S906).
In this way, it is possible to prevent the voltage at the connection point closest to the constant voltage source AVDD from becoming too high or the voltage at the connection point farthest from the constant voltage source AVDD from becoming too low. Accordingly, since the maximum value of the density unevenness is averaged between the nearest end light emitting element portion and the far end light emitting element portion, the density unevenness in the sub scanning direction is reduced.

[4] Fourth Embodiment The image forming apparatus according to the present embodiment has the same configuration as that of the first to third embodiments, while the amount of voltage drop between adjacent lines in the image data. It is different in that the density unevenness among the lines is prevented by finding the average value of. In the following, description will be made by focusing on the differences.

  As shown in FIG. 15, the control unit 101 first calculates the voltage drop amount of the line using equation (1) as the processing for each line (S1501), and further calculates the voltage drop amount of the previous line. It acquires (S1502), and calculates the average value of these voltage drop amounts (S1503). The previous line is a line adjacent to the current line in the sub-scanning direction, and is a line on which optical writing has been performed before the current line.

In step S1502, the voltage drop amount of the previous line may be calculated. Alternatively, the voltage drop amount calculated when performing the optical writing of the previous line may be stored in advance, and the voltage drop amount of the previous line stored may be read in step S1502.
A half value of the calculated average value is obtained as an intermediate value (S1504), and the contact point 701 whose voltage drop amount is closest to the intermediate value is selected (S1505). Thereafter, the control unit 101 inputs a switching control signal indicating the number of the selected contact point 701 into the selector 700 (S905), and inputs the image data of one line to the driver IC 302 (S906).

According to this configuration, since the contact point 701 serving as a reference can be aligned between adjacent lines, uneven density between the lines can be reduced. In the present embodiment, although the average value of the voltage drops of the two lines of the current line and the previous line is used, the voltage drops of three or more lines adjacent to each other including the current line are used. The average value of may be used.
[5] Modifications Although the present invention has been described above based on the embodiment, it goes without saying that the present invention is not limited to the above embodiment, and the following modifications can be implemented. .

  (1) In the above embodiment, although the case where the contact 701 is provided between the light emission blocks 402 is described as an example, it goes without saying that the present invention is not limited to this. Good. Further, it is not necessary to provide the contacts 701 at equal intervals on the power supply line 531. For example, since the amount of current is large at a position close to the constant voltage source AVDD, the voltage drop is large. If the distribution of the contacts 71 is made sparser as they move away from the source AVDD, the number of types of voltages supplied to the driver IC 302 can be increased while the total number of the contacts 701 is suppressed.

  Further, it goes without saying that the number of contacts 701 is not limited to 150, and may be more or less than 150. As the number of contact points 701 increases, it is possible to eliminate uneven density with high accuracy, but there is a disadvantage that the circuit scale of the TFT substrate 300 is increased. Conversely, if the number of contacts 701 is reduced, the circuit scale of the TFT substrate 300 can be reduced, but uneven density tends to remain. Therefore, it is desirable to determine the number of contact points 701 in consideration of uneven density and circuit scale.

(2) In the above embodiment, the case of selecting the common contact point 701 for all the DACs 400 has been described as an example, but it goes without saying that the present invention is not limited to this. It is also good.
For example, the contact 701 may be selected for each group of DACs 400 including one or more DACs 400, and the potential at the contact 701 may be used as the reference potential of the DACs 400 belonging to the group. In this case, the selector 700 may be provided for each group of DACs 400. In this way, uneven density can be further suppressed.

(3) In the above embodiment, the case where the power supply voltage is supplied to the driver IC 302 from the contacts 701 provided on the power supply line 531 has been described as an example, but it goes without saying that the present invention is not limited thereto. The following may be substituted for
That is, a variable voltage source for separately outputting a voltage corresponding to an intermediate value for each line calculated using Equation (1) or an intermediate value for a plurality of lines is separately provided, and the power supply voltage is supplied to the driver IC 302 from the variable voltage source. You may The variable voltage source may be, for example, a constant voltage source connected with a variable resistor, and may supply a desired power supply voltage by appropriately adjusting the electric resistance of the variable resistor. Even in this case, the same effect as that of the above embodiment can be obtained.

  (4) In the above embodiment, although the case where the image forming apparatus 1 is a tandem type color printer has been described as an example, it goes without saying that the present invention is not limited to this. The present invention may be applied to a printer. In addition, the same effects can be obtained even when the present invention is applied to a copying apparatus provided with a scanner, a facsimile apparatus further provided with a communication function, and a multifunction peripheral (MFP: Multi-Function Peripheral) having these functions. it can.

  The optical writing apparatus and the image forming apparatus according to the present invention are useful as an apparatus for reducing uneven density caused by voltage drop in a power supply line which supplies current to a light emitting element.

1 ... ... Image forming apparatus 100 ... Optical writing apparatus 101 ... Control unit 201 ... OLED
302: Driver IC
400 ... DAC
531 ... power supply line 700 ... selector 701 ... contact point

Claims (9)

An optical writing device which exposes a photosensitive member to form an electrostatic latent image line by line,
A plurality of current-driven light emitting element arranged in a line,
Extending along a plurality of the light emitting element, a power supply line for supplying power to a plurality of the light emitting element,
A constant voltage source connected to one end of the power supply line;
An instruction circuit for outputting a command voltage to instruct the light emission amount to the light emitting element by the potential difference from the reference potential,
A drive current input to the light emitting element is interposed between the power supply line and the light emitting element, according to the potential difference between the potential at the connection point with the power supply line and the indication potential output by the indication circuit. Drive circuits to control,
A plurality of contacts provided on the power supply line;
A selector configured to select one of the plurality of contacts in accordance with the lighting state of each light emitting element in the one line;
The optical writing device according to claim 1, wherein the reference potential of the indication circuit is given by the potential of the power supply line at the contact selected by the selection means. The optical writing device according to claim 1, wherein the selection means selects a contact farther from the constant voltage source on the power supply line as the number of light emitting elements to be lighted in the one line is larger. The said selection means is characterized by selecting the contact point distant from the said constant voltage source on the said power supply line, so that distribution of the light emitting element lighted in the said 1 line is far from the said constant voltage source on the said power supply line. The optical writing device according to 1 or 2. The selection means is characterized in that the contact point farther from the constant voltage source on the power supply line is selected as the voltage drop amount from the constant voltage source at the connection point farthest from the constant voltage source on the power supply line is larger. The optical writing device according to claim 1. 5. The optical writing device according to claim 4, wherein the selection means selects the contact point closest to half of the average value of the voltage drop amount of at least one line including at least the one line. The one or more lines, the optical writing device of claim 5, wherein the the one line are two lines between adjacent lines in said one line and the sub scanning direction. The optical writing device according to any one of claims 1 to 6, wherein the selection means selects the contact each time the one line is scanned. The optical writing device according to any one of claims 1 to 7, wherein the light emitting element is an OLED. An image forming apparatus comprising the optical writing device according to any one of claims 1 to 8.

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