JP2007290190A - Driving circuit, light emitting device, electronic equipment and driving method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To correct an amount of light of an OLED element and to correct driving current on the basis of a pulse width. <P>SOLUTION: A driving unit Ui comprises a reference potential setting section X which can set a reference potential Vref on the basis of reference data Dref and a driving current generating section Y which generates a pulse width-modulated driving current Ioled in accordance with gradation data Dg and sets the size of the driving current Ioled on the basis of the gradation data Dg and the reference potential Vref. The conditions of switches SW 11 to SW13 are controlled on the basis of the reference data Dref and the conditions of switches Swa and SWb are controlled on the basis of a first pulse width modulating signal PWMa and a second pulse width controlling signal PWMb. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a drive circuit using a light emitting material such as an organic EL (ElectroLuminescent) material,
The present invention relates to a light emitting device, an electronic device, and a driving method.

A printer as an image forming apparatus uses a light emitting device in which a large number of light emitting elements are arranged in an array as a head unit for forming an electrostatic latent image on an image carrier such as a photosensitive drum. The head portion is often composed of a single line in which a plurality of light emitting elements are arranged along the main scanning direction.
Patent Document 1 discloses a technique for selecting transistors connected in parallel in accordance with the variation in the amount of light emission in a current mirror circuit that generates a drive current in order to correct the variation in the amount of light emission of a plurality of light emitting elements. ing.
Japanese Patent No. 3296882 (refer to claim 1)

Incidentally, an electrostatic potential is recorded on the photosensitive drum in accordance with the amount of light, and a latent image having a density in accordance with the electrostatic potential is formed. When an electrostatic potential is generated by irradiating a photosensitive drum with light having a pulse width modulated, if the pulse width is narrow, the peak of the electrostatic potential is lowered and a target gradation cannot be obtained.
For this reason, there has been a problem that an accurate gradation cannot be obtained even if the light quantity of the light emitting element is corrected to be constant.
The present invention has been made in view of such circumstances, and performs correction of variations in light quantity of light emitting elements and correction according to gradation to be displayed independently and simultaneously with a simple configuration. It is a problem to be solved.

In order to solve this problem, a drive circuit according to the present invention drives a plurality of light emitting elements that emit light with a light amount corresponding to the magnitude of a drive current, and corresponds to each of the plurality of light emitting elements. A plurality of unit circuits for generating the drive current, each of the plurality of unit circuits showing a reference potential setting unit capable of setting a reference potential based on reference data, and a gradation value to be displayed A driving current generating unit configured to generate the driving current pulse-modulated in accordance with gradation data and to set the magnitude of the driving current based on the gradation data and the reference potential;

According to the present invention, since the drive current is pulse-width modulated based on the gradation data, the light emission power of the light emitting element corresponds to the gradation value by PWM control. The magnitude of the drive current is set based on the gradation data and the reference potential, and the reference potential is set based on the reference data. Therefore, according to the present invention, the magnitude of the drive current can be controlled based on two parameters such as reference data and gradation data. Here, since the time during which the light is emitted from the light emitting element depends on the gradation data, the magnitude of the drive current is controlled in accordance with the pulse width. Also, when this light emitting device is used as an optical head and a latent image is formed on an image bearing member coated with a photosensitive material, the magnitude of the drive current can be controlled according to the pulse width, so that the electrostatic potential at each pulse width can be controlled. It is possible to adjust the peak.
The light-emitting element may be any element that emits light with a light amount corresponding to the magnitude of the drive current. For example, a light-emitting diode such as an OLED element or an inorganic light-emitting diode is applicable. Furthermore, it is preferable that the reference potential setting unit and the drive current generation unit are configured by a current mirror circuit.

In the driving circuit described above, the reference potential setting unit is provided in a plurality of reference transistors, each drain of which is connected to a connection point, and all or a part of the plurality of reference transistors, and the gate and the source are short-circuited. One state or a plurality of reference switches controlled based on the reference data so as to be in any one of the second state in which the gate and the drain are short-circuited, and connected to the connection point to supply a reference current It is preferable that a potential of the connection point is output as the reference potential, and the reference data is determined so that the light emission amounts of the plurality of light emitting elements are uniform. According to the present invention, by setting the state of the reference switch based on the reference data, it is possible to select which of the reference transistors included in the reference potential setting unit is valid. The reference potential can be adjusted by setting an equivalent transistor size according to the reference data. Note that the uniform light emission amount is not only a perfect match, but also includes an error due to measurement convenience, an error due to the resolution of the reference data, and the like as long as it is within a certain allowable range. For example, it is acceptable if it is within ± 5% of the average value of the light emission amount.

In the drive circuit described above, the drive current generator is provided in each of the plurality of drive transistors, each drain of which is connected to the light emitting element, and each of the plurality of drive transistors.
It is preferable to include a plurality of drive switches that are in one of a first state in which the gate and the connection point are short-circuited and a second state in which the gate and drain are short-circuited. According to this invention,
Since it is possible to select which of the drive transistors included in the drive current generation unit is valid, the size of the drive current can be adjusted by setting an equivalent transistor size of the drive current generation unit.

More specifically, a predetermined drive switch is selected from the plurality of drive switches according to the gradation value indicated by the gradation data, and the predetermined drive switch is selected for a time corresponding to the gradation value. It is preferable to include selection means (for example, corresponding to the decoder 14 of the embodiment) that controls to be in the first state and controls drive switches other than the predetermined drive switch to be in the second state. According to the present invention, the predetermined switch selected based on the gradation value is controlled so as to be in the first state only for the time corresponding to the gradation value, and thus the on / off of the driving transistor is controlled by the switch. Thus, the drive current can be subjected to pulse width modulation, and an effective drive transistor is selected according to the gradation value, so that the magnitude of the drive current can be controlled according to the pulse width.

In the driving circuit described above, the wiring to which the reference data and the gradation data are exclusively supplied, and a period during which each of the plurality of unit circuits should capture the reference data or the gradation data are designated. Each of the plurality of unit circuits includes a reference data storage unit that stores the reference data in a period specified by the specification signal, and a period specified by the specification signal. The data fetched from the wiring based on the gradation data storage means for storing the gradation data and the identification signal indicating whether the data supplied to the wiring is the reference data or the gradation data It is preferable to have distribution means for distributing the data to the reference data storage means and the gradation data storage means. According to the present invention, since the wiring for supplying the reference data and the gradation data can be shared, the drive circuit can be easily configured as compared with the case where separate wiring is provided.

In the drive circuit described above, the drive current generation unit may compare the pulse width of the drive current determined according to the gradation data with a width wider than the predetermined time when the pulse width of the drive current is smaller than the predetermined time. The drive current is preferably set to be large.
When this light emitting device is used as an optical head and a latent image is formed on an image bearing member coated with a photosensitive material, the peak of the electrostatic potential is lowered when the pulse width is narrow, and it is difficult to accurately express gradation. However, according to the present invention, when the pulse width is narrow, the magnitude of the drive current can be increased, so that the peak of the electrostatic potential can be appropriately adjusted.

Next, the light-emitting device according to the present invention preferably includes the drive circuit described above and a plurality of light-emitting elements that emit light with a light amount corresponding to the magnitude of the drive current. According to this light emitting device, the magnitude of the drive current can be controlled based on two parameters such as reference data and gradation data.
Next, an electronic apparatus according to the present invention includes the above-described light emitting device. Such electronic devices include image forming apparatuses such as printers, copiers, multifunction machines, and facsimiles.

Next, in the driving method according to the present invention, the amount of light corresponding to the magnitude of the driving current is applied to the image carrier coated with the photosensitive material that generates the electrostatic potential having the magnitude corresponding to the period of light irradiation. A method of driving a plurality of light emitting elements that irradiate light, corresponding to each of the plurality of light emitting elements,
Each reference potential is generated so that the amount of light emission corresponding to a predetermined gradation value approaches, and the driving current pulse-modulated according to the gradation data indicating the gradation value to be displayed is generated. The magnitude is set based on the pulse width of the driving current and the reference potential. According to this invention, the variation in the light amount of the light emitting element is corrected by setting the reference potential,
The peak of the electrostatic potential can be corrected by setting the magnitude of the drive current according to the pulse width of the drive current. Here, the magnitude of the drive current is preferably adjusted according to the pulse width so that the peak of the electrostatic potential is constant. The image carrier corresponds to a carrier coated with a photosensitive material such as a photosensitive drum or a photosensitive belt.

<1. Embodiment>
A preferred embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a perspective view illustrating a partial configuration of an image forming apparatus using the light emitting device according to the embodiment. As shown in the figure, the image forming apparatus includes a light emitting device 1 and a condensing lens array 1.
50 and the photosensitive drum 110. The light emitting device 1 has a large number of light emitting elements arranged in an array. These light emitting elements selectively emit light according to an image to be printed on a recording material such as paper. For example, an organic light emitting diode (Organic Light Emitti)
ng Diode (hereinafter abbreviated as “OLED” where appropriate) is used. The condensing lens array 150 is disposed between the light emitting device 1 and the photosensitive drum 110. The condensing lens array 150 includes a large number of gradient index lenses arranged in an array with each optical axis directed to the light emitting device 1. An example of such a condensing lens array 150 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 1 passes through each gradient index lens of the condensing lens array 150 and passes through the photosensitive drum 11.
Reach zero surface. By this exposure, a latent image corresponding to a desired image is formed on the surface of the photosensitive drum 110.

The photosensitive drum 110 is coated with a photosensitive material, and an electrostatic potential having a magnitude corresponding to the time of light irradiation is generated. Then, a latent image having a density corresponding to the magnitude of the electrostatic potential is formed on the photosensitive drum 110. The photosensitive drum 110 is irradiated with light for a period corresponding to the pulse width. As shown in the figure, when the pulse width is narrow, the waveform of the electrostatic potential falls before it rises sufficiently, so that the peak of the electrostatic potential becomes low and it becomes difficult to accurately express the gradation. Therefore, in this embodiment, the amount of light is adjusted according to the pulse width as will be described later.

FIG. 2 is a block diagram including the light emitting device 1 according to the embodiment and the peripheral configuration thereof. As shown in this figure, the light emitting device 1 includes a driver circuit A, a counter 210, an external drive circuit 220, a memory 230, and a control circuit 300 for controlling them. In addition, n OLED elements 10 are connected to the driver circuit A.

The control circuit 300 generates gradation data Dg that specifies various control signals and the light emission power (light quantity) of each OLED element 10 in order to control the driver circuit A. In the light emitting device 1, each OLED element 10 is driven by the PWM method. For PWM method, each OLED
The light emission power of the element 10 is determined by the product of the drive current Ioled and the light emission time. In the present embodiment, by adjusting the magnitude of the drive current Ioled, the variation in the light emission characteristics of the plurality of OLED elements 10 is compensated, and the light emission period is varied to thereby change the OLED element 1 at a desired gradation.
0 is emitted. In addition, by adjusting the magnitude of the drive current Ioled according to the pulse width, the peaks of the electrostatic potential are made uniform.

The memory 230 is a non-volatile storage unit that stores reference data Dref that specifies the magnitude of the drive current Ioled of each OLED element 10. Further, the control circuit 300 can be connected to the measurement system 20. The measurement system 20 individually measures the light amount of each OLED element 10 and generates reference data Dref. The control circuit 300 stores the reference data Dref acquired from the measurement system 20 in the memory 230. The counter 210 counts the second clock signal CK2 supplied from the control circuit 300, and generates count data CNT indicating the count value. The external drive circuit 220 includes a current source capable of adjusting a current value. This external drive circuit 2
The reference current Iref is supplied to the driver circuit A by 20.

FIG. 3 shows a block diagram of the driver circuit A and its peripheral circuits. The driver circuit A includes a shift register 100 and n drive units U1 to Un. The drive units U1 to Un are n
The drive current Iol provided corresponding to each of the OLED elements 10 and subjected to pulse width modulation
supply ed. That is, the light emission duty that is the ratio between the lighting period and the extinguishing period is adjusted according to the light emission amount (light emission power) that the OLED element 10 should emit.

The shift register 100 sequentially shifts the start pulse SP according to the first clock signal CK1, and generates latch pulses LAT1 to LATn. Latch pulses LAT1 to LATn
Becomes exclusively high (active). The latch pulses LAT1 to LATn receive the gradation data Dg or the reference data Dref supplied via the wiring L1 for each drive unit U1 to U1.
Designates the timing for importing into Un.

Further, a selection signal SEL (identification signal) is supplied through the wiring L2. The selection signal SEL designates an initialization mode at a high level, and designates a reference current setting mode, a power adjustment mode, and a light emission mode at a low level. In the reference current setting mode, the magnitude of the reference current Iref is set. In the power adjustment mode, the reference data Dref is set so that the light emission powers of the respective OLED elements 10 approach uniformly. In the initialization mode, the reference data Dref stored in the memory 230
Is transferred to each of the drive units U1 to Un. In the light emission mode, the drive current Ioled is supplied to each OLED element 10.
Further, the count data CNT is supplied through the wiring L3, and the reference current Iref is supplied from the external driving circuit 220 to the driving units U1 to Un through the wiring L4.

FIG. 4 shows a block diagram of the i-th drive unit Ui (i is an integer satisfying 1 ≦ i ≦ N). The other drive units are similarly configured. The drive unit Ui uses the reference data Dref
The reference data latch 12 stores the gradation data Dg, and the gradation data latch 13 stores the gradation data Dg. The distribution circuit 11 supplies the reference data Dref taken in via the wiring L1 to the reference data latch 12 when the selection signal SEL is high level, and takes it in via the wiring L1 when the selection signal SEL is low level. The gradation data Dg is supplied to the gradation data latch 13. The selection signal SEL functions as an identification signal indicating whether the data supplied to the wiring L1 is the reference data Dref or the gradation data Dg. The reference data latch 12 and the gradation data latch 13 capture and hold data when the latch pulse LATi becomes active. The gradation data Dg in this example is 3 bits. The reference data latch 12 and the gradation data latch 13 are volatile memories.

The correction circuit 15 includes a reference potential setting unit X and a drive current generation unit Y, and functions as a current mirror circuit. The reference potential setting unit X includes transistors Q0, Q11, Q12, and Q13.
, And switches SW11, SW12, and SW13, and a reference current Iref at the connection point Z
So that the reference potential Vref at the connection point Z is adjusted. Transistors Q0, Q11, Q
12 and the source of Q13 are connected to the connection point Z. The connection point Z is connected to the external drive circuit 220. The switches SW11, SW12, and SW13 are connected to the transistor Q
Each of 11, Q12, and Q13 is in one of a first state in which the source and the gate are connected and a second state in which the drain and the gate are connected. The transistor is diode-connected in the first state, and is turned off in the second state. Specifically, the first state is set when the bit of the reference data Dref indicates “1”, and the second state is set when “0”. The channel width of the transistor is W, the channel length is L, and the transistor size is W
/ L. In this example, the transistor size ratio of the transistors Q0, Q11, Q12, and Q13 is 1: 0.5: 0.25: 0.125. Switch SW11, S
When either W12 or SW13 is in the first state, the transistor size of the transistor Q0 is equivalently increased. In this example, the transistor size of the reference potential setting unit X changes in the range of 1-1.825 with respect to the transistor Q0.

On the other hand, the drive current generator Y includes transistors Qa and Qb and switches SWa and Sb.
Wb is provided. The sources of the transistors Qa and Qb are connected to the anode of the OLED element 10. In each of the transistors Qa and Qb, the switches SWa and SWb are in one of a first state in which the connection point Z and the gate are connected and a second state in which the drain and the gate are connected. Specifically, the switches SWa and SWb are in the first state when the pulse width modulation signals PWMa and PWMb are at the high level, and the switches SWa and SWb are in the first state when the first and second pulse width modulation signals PWMa and PWMb are at the low level. There are two states. In this example, the transistor size ratio of the transistors Q0, Qa and Qb is 1: 1: 0.5. For example, when the switches SW11 and SWa are in the first state, the switches SW12 and SW13
If SWb is in the second state, the equivalent transistor size ratio of the reference potential setting unit X and the drive current generation unit Y is 1.5: 1. In this case, the relationship between the drive current Ioled and the reference current Iref is Ioled = Iref / 1.5. In such a configuration, the drive current Ioled is given as the sum of the first current Ia and the second current Ib, and the magnitudes of the first current Ia and the second current Ib are in accordance with the reference potential Vref. Since the reference potential Vref is determined by the equivalent transistor size of the reference potential setting unit X, the magnitude of the drive current Ioled can be adjusted according to the states of the switches SW11 to SW13. Since the states of the switches SW11 to SW13 are determined by the reference data Dref, the reference potential setting unit X functions as means for adjusting the magnitude of the drive current Ioled by setting the reference potential Vref based on the reference data Dref.

FIG. 5 shows a circuit diagram of the decoder 14. The decoder 14 includes a comparator 14A, an SR flip-flop 14B, an inverter 14C, and a NOR circuit 14D. Comparator 14
A compares the count data CNT and the gradation data Dg, and generates a comparison signal CMP that goes high when the two values match. The set terminal of the SR flip-flop 14B is supplied with the light emission start pulse FS that becomes high level at the start of light emission, and the reset terminal thereof is supplied with the comparison signal CMP. The light emission start pulse FS designates the start of light emission. The SR flip-flop 14B outputs a pulse width modulation signal PWM that becomes low level (active) during a period from when the light emission start pulse FS changes to high level to when the comparison signal CMP changes to high level. The low level period in which the pulse width modulation signal PWM is active is a time corresponding to the gradation value of the gradation data Dg. The inverter 14C supplies the first pulse width modulation signal PWMa obtained by inverting the pulse width modulation signal PWM to the switch SWa. Therefore, on / off of the transistor Qa is controlled by the first pulse width modulation signal PWMa.

Further, the pulse width modulation signal PWM is supplied to one input terminal of the NOR circuit 14D, and the most significant bit of the gradation data Dg is supplied to the other terminal. When the most significant bit is “0”,
The NOR circuit 14D acts as an inverting gate, inverts the pulse width modulation signal PWM, and outputs it as the second pulse width modulation signal PWMb. On the other hand, when the most significant bit is “1”, the NOR circuit 1
4D fixes the logic level of the output signal to a low level.

As a result, the transistor Qa is turned on for a time corresponding to the gradation value regardless of the gradation value of the gradation data Dg. On the other hand, in the transistor Qb, the gradation value of the gradation data Dg is “1”.
If it is from “7” to “7”, it is in the on state only for the time corresponding to the gradation value, and if it is from “8” to “15”, it is always in the off state. The light emission amount (light emission power) of the OLED element 10 is determined by the product of the time during which the drive current Ioled is supplied and the magnitude of the drive current Ioled. Here, the time during which the drive current Ioled is supplied varies depending on the gradation value. On the other hand, the magnitude of the drive current Ioled is determined by which transistor is enabled in the reference potential setting unit X and the drive current generation unit Y.

In the reference potential setting unit X, a valid transistor is set based on the reference data Dref, and in the drive current generation unit Y, a valid transistor is set based on the most significant bit of the gradation data Dg. Since the pulse width is determined by the gradation data Dg, the drive current generator Y functions as means for adjusting the magnitude of the drive current Ioled according to the pulse width. More specifically, when the pulse width of the drive current Ioled determined according to the gradation data Dg is narrower than a predetermined time (in this example, a time corresponding to the gradation value “8”), the drive current generation unit Y It functions as a means for setting the magnitude of the drive current Ioled to be larger than when the pulse width is wider than a predetermined time. Further, since the decoder 14 selects the switches SWa and SWb of the reference current generation unit Y based on the most significant bit of the gradation data Dg, a plurality of switches SWa, SWb are selected according to the gradation value indicated by the gradation data Dg. Means for selecting a predetermined switch from SWb, controlling the predetermined switch to be in the first state for a time corresponding to the gradation value, and controlling switches other than the predetermined switch to be in the second state Function as.

FIG. 6 is a flowchart showing operations of the light emitting device 1 and the measurement system 20 in the reference current setting mode. In the reference current setting mode, first, in accordance with a command from the measurement system 20, the control circuit 300 switches the switches SW11 to 1 in each drive unit U1 to Un.
3. Set SWa and SWb to the initial state (step S1). Specifically, switch S
W11 and SWa are set to the first state, and switches SW12, SW13, and SWb are set to the second state.
Next, the control circuit 300 controls the external drive circuit 220 so that the magnitude of the reference current Iref becomes the initial value I1. The initial value I1 is a current value obtained in advance by experiments or the like in order to obtain a necessary light emission power.

Next, the control circuit 300 causes the n OLED elements 10 to sequentially emit light, and the light emission power is measured in the measurement system 20 (step S3). And the measurement system 20
It is determined whether or not the measurement is completed for the LED element 10 (step S4), and the determination is repeated until the measurement is completed. When the measurement is completed for all the OLED elements 10, the measurement system 20 calculates the average value of the light emission power (step S5) and determines whether the average value of the light emission power is within a predetermined range from the target value. (Step S6). More specifically, when the average value of the light emission power is Pave and the target value is Px, it is determined whether or not the following expression is satisfied.
Px−ΔP ≦ Pave ≦ Px + ΔP
However, ΔP is light emission power allowed as an error in gradation.

If the average value Pave of the light emission power is within a predetermined range from the target value Px, the measurement system 20 determines the magnitude of the reference current Iref at this time as the reference value I2. On the other hand, if the average value Pave of the light emission power is outside the predetermined range, the measurement system 20 controls the external drive circuit 220 so as to change the magnitude of the reference current Iref by the predetermined value ΔI via the control circuit 20. More specifically, if the average value Pave of the light emission power exceeds the predetermined range (Pave> Px + ΔP), it is decreased by the predetermined value ΔI from the previous reference current Iref, and the average value of the light emission power falls below the predetermined range. (Pave <Px−ΔP), the value is increased by a predetermined value ΔI from the previous reference current Iref. Then, the process returns to step S3, and the process is repeated until the average value Pave of the light emission power falls within the predetermined range. The predetermined value ΔI may be changed so as to decrease according to the number of trials. As a result, the light emission power is roughly adjusted at the beginning, and the light emission power can be adjusted in fine increments as the light emission power approaches the target value Px, so compared with the case where the light emission power is adjusted in fine increments from the beginning, The number of trials can be reduced.

In this way, the magnitude of the reference current Iref is determined to be the reference value I2. The reference value I2 is the magnitude of the reference current Iref at which the average value Pave of the light emission power falls within a predetermined range from the target value Px when the switch SW11 is in the first state and the switches SW12 and SW13 are in the second state. . As described above, the transistor sizes of the transistors Q11 to Q13 are 0.5: 0.2.
5: 0.125. By controlling the states of the switches SW11 to SW13, the transistor size of the reference potential setting unit X is “1”, “1.125”, “1.25”, “1.375” with respect to the transistor size of the transistor Q1. ”,“ 1.5 ”,“ 1.6 ”
25 ”,“ 1.75 ”, and“ 1.875 ”. Here, when the switch SW11 is in the first state and the switches SW12 and SW13 are in the second state, the transistor size is “1.
5 ”, which corresponds to the case where the transistor size of the reference potential setting unit X is a median value. That is, the reference value I2 is the magnitude of the reference current Iref where the average value Pave of the light emission power falls within a predetermined range from the target value Px when the transistor size of the reference potential setting unit X is the median value.

After determining the reference value I2, the mode shifts to the power adjustment mode. FIG. 7 is a flowchart showing the processing contents of the power adjustment mode. First, according to the command from the measurement system 20, the control circuit 300 sets the switches SW11 to SW13, SWa, and SWb to the initial state in each of the drive units U1 to Un (step S11). Specifically, the switches SW11 and SWa are set to the first state, and the switches SW12, SW13, and SWb are set to the second state.

Next, the control circuit 300 controls the external drive circuit 220 so that the magnitude of the reference current Iref becomes the reference value I2. In this case, the average value Pave of the light emission power is the target value Px of the light emission power.
Is approximately the same.
Thereafter, the measurement system 20 measures the light emission power of the OLED element 10 to be measured (step S13), and determines whether the measured light emission power is within a predetermined range from the target value Px (step S14).
When the measured light emission power is Pm and the target value is Px, it is determined whether or not the following expression is satisfied.
Px−ΔP ≦ Pm ≦ Px + ΔP
However, ΔP is light emission power allowed as an error in gradation.

In the reference current setting mode described above, the reference current Iref is set so that the average value Pave of the light emission power is within a predetermined range from the target value Px. However, even if the average value Pave substantially coincides with the target value Px, the light emission characteristics of the individual OLED elements 10 vary, and thus the light emission power Pm of the individual OLED elements 10 is not always within the predetermined range from the target value Px. Absent.
When the measured light emission power Pm is not within the predetermined range, the measurement system 20 controls the control circuit 3
Control is performed to change the state of the switches SW11 to SW13 of the reference potential setting unit X via 00 (step S15), and the process returns to step S13. Then, the processes in steps S13 to S15 are repeated until the light emission power Pm falls within the predetermined range. When the light emission power Pm falls within the predetermined range, the reference data Dref indicating the state of the switches SW11 to SW13 is stored in the memory 230.

Thereafter, it is determined whether or not the measurement has been completed for all the OLED elements 10 (step S
17) If there is an unmeasured OLED element, the process returns to step S13, and all OLs are
The process is repeated until the measurement of the ED element 10 is completed (step S17). In this way, the reference data Dref is stored in the memory 230 for each OLED element 10.
When the average value Pave of the light emission power measured in the reference current setting mode, the maximum value of the light emission power is Pmax, and the minimum value is Pmin, the light emission characteristics of each OLED element 10 are shown in FIG.
). In this case, if the average value Pave of the light emission power is equal to the target value Px, the light emission power of the OLED element 10 becomes the target value Px as shown in FIG. 8B by performing correction based on the reference data Dref. Get closer.

Next, the initialization mode and the light emission mode will be described. FIG. 9 shows a timing chart of the light emitting device 1 in the initialization mode and the light emission mode. When the signal PW becomes high level, the power switch is turned on, while when the signal PW becomes low level, the power switch is turned off.
After the signal PW changes from the low level to the high level and the power source of the light emitting device 1 is turned on, the initialization mode is entered. In the initialization mode, the selection signal SEL is at a high level. At this time, the control circuit 300 sequentially reads the reference data Dref from the memory 230 and supplies it to the driver circuit A. Dref (1) to Dref (n) shown in the figure are reference data Dref corresponding to the drive units U1 to Un, respectively. During the period when the selection signal SEL is at a high level, the distribution circuit 11 of the drive unit Ui shown in FIG. 4 supplies the data fetched from the wiring L2 to the reference data latch 12. When the latch signal LATi becomes active, the reference data latch 12 holds the reference data Dref (i) and supplies it to the switches SW11 to SW13. Similarly, the reference data Dref is taken in the other drive units, and the states of the switches SW11 to SW13 of the reference potential setting unit X are set.

Next, in the light emission mode, the selection signal SEL becomes low level. The operation in the light emission mode is roughly divided into a writing period Tw for fetching the gradation data Dg into each of the drive units U1 to Un and a light emission period Tel for causing the OLED element 10 to emit light based on the gradation data Dg. First, in the writing period Tw, the control circuit 300 sequentially outputs the gradation data Dg (1) to Dg (n) to the wiring L1. At this time, since the selection signal SEL is at the low level, the distribution circuit 11 supplies the data fetched from the wiring L1 to the gradation data latch 13. The gradation data latch 13 latches the gradation data Dg (i) according to the latch signal LATi. In the writing period Tw, since the latch signals LAT1 to LATn are sequentially activated, the gradation data Dg (1) to Dg (n) are stored in the gradation data latch 13 in each of the drive units U1 to Un.

Next, in the light emission period Tel, the counter 210 counts the clock signal CK2, and outputs the result as count data CNT to the wiring L3. For example, gradation data Dg
Assume that the data value of “3” is “3”. In this case, since the MSB of the gradation data Dg is at a low level, the NOR circuit 14D shown in FIG. 5 inverts the pulse width modulation signal PWM to generate the second
It functions as an inverter that outputs the pulse width modulation signal PWMb. Here, the pulse width modulation signal PWM becomes low level during the period from when the light emission start signal FS becomes high level to when the output signal CMP of the comparator 14A becomes active. The comparator 14A transitions the output signal to a high level when the value of the count data CNT matches the value of the gradation data Dg. Therefore, in this example, the first pulse width signal PWMa and the second pulse width modulation signal PWMb are active during the period from time t0 to time t1. As a result, FIG.
The transistors Qa and Qb shown in FIG.

When the data value of the gradation data Dg is “15”, the MSB of the gradation data Dg
Becomes the high level, the NOR circuit 14D fixes the logic level of the second pulse width modulation signal PWMb to the low level. On the other hand, the first pulse width modulation signal PWMa becomes active during the period from when the light emission start signal FS becomes high level until the output signal CMP of the comparator 14A becomes active, that is, from time t0 to time t2. . As a result, FIG.
The transistor Qa shown in FIG. 5 is turned on only for a period corresponding to the gradation data Dg, while the transistor Qb is turned off.

In this way, whether or not to enable the transistor Qb is controlled according to the MSB value of the gradation data Dg. As a result, as shown in FIG. 10, when the gradation to be displayed is small, the transistor Qb is enabled to increase the light emission power, and when the gradation is large, the transistor Qb
The light emission power can be reduced by disabling.

As described above, by correcting the variation in the light emission characteristics of the OLED element 10 by the reference potential setting unit X, a latent image can be formed on the photosensitive drum 110 with uniform light emission power.
Further, although the peak of the electrostatic potential of the photosensitive drum 110 varies depending on the gradation value, the light-emitting device 1 activates which of the transistors constituting the drive current generation unit Y is effective according to the value of the gradation data Dg. Therefore, it is possible to correct the sensitivity of the photosensitive drum 110 and print with accurate gradation.

In the above-described embodiment, the light emitting device using the OLED element 10 is illustrated, but the present invention is not limited to this. For example, any light emitting element that emits light in response to a drive signal may be used. As such a light emitting element, an inorganic EL element can be exemplified. Also,
An LED (Light Emitting Diode) can also be employed as the light emitting layer. In other words, the light-emitting element in the present invention only needs to be made of a material that emits light upon application of electrical energy.

<2. Modification>
The present invention is not limited to the above-described embodiments, and for example, the following modifications are possible.
(1) In the embodiment described above, the external drive circuit 220 includes a current source, and the reference current Ire
Although f is generated and supplied to the driver circuit A, a voltage source may be provided instead of the current source, and the reference current Iref may be generated by converting the voltage into a current inside the driver circuit A.

(2) Although the switch corresponding to the transistor Q0 is not provided in the reference potential generation unit X of the above-described embodiment, a switch corresponding to the transistor Q0 may be provided.
In this case, the switch may be controlled so that at least one of the transistors Q0 and Q11 to Q13 is diode-connected.

(3) The drive current generation unit Y of the above-described embodiment includes two transistors Qa and Qb, and the transistor Qb has a time corresponding to the gradation value only when the gradation value is smaller than a predetermined value. Although control is performed so as to be in one state, a transistor that includes three or more transistors and contributes to pulse width modulation may be selected according to the pulse width of the drive current Ioled. In this case, the magnitude of the drive current Ioled can be controlled in more detail according to the pulse width. More specifically, it is preferable to select a transistor that is effective according to the pulse width so that the peak of the electrostatic potential generated on the photosensitive drum 110 is constant.
<3. Image forming apparatus>
As shown in FIG. 1, the light emitting device 1 according to each of the above embodiments can be used as a line type optical head for writing a latent image on an image carrier in an image forming apparatus using an electrophotographic system. Examples of the image forming apparatus include a printer, a printing part of a copying machine, and a printing part of a facsimile.
FIG. 11 is a longitudinal sectional view showing an example of an image forming apparatus using the light emitting device 1 as a line type optical head. 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 organic EL array exposure heads 1K, 1C, 1 having the same configuration are used.
M and 1Y are four photosensitive drums (image carriers) 110K, 110C, and 1 having the same configuration.
10M and 110Y are arranged at the exposure positions, respectively. Organic EL array exposure head 1K
, 1C, 1M, and 1Y are the light emitting device 1 according to any one of the above-illustrated embodiments.

As shown in FIG. 11, the 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, there are four photosensitive drums 1 each having a photosensitive layer on the outer peripheral surface.
10K, 110C, 110M, and 110Y are arranged at predetermined intervals. 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. Photosensitive drum 110K, 1
10C, 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), a corona charger 111 (K, C,
M, Y), organic EL array exposure head 1 (K, C, M, Y), and developer 114 (K, C)
, M, Y) are arranged. The corona charger 111 (K, C, M, Y) uniformly charges the outer peripheral surface of the corresponding photosensitive drum 110 (K, C, M, Y). The organic EL array exposure head 1 (K, C, M, Y) writes an electrostatic latent image on the charged outer peripheral surface of the photosensitive drum. In each organic EL array exposure head 1 (K, C, M, Y), the arrangement direction of the plurality of OLED elements 10 is aligned with the bus (main scanning direction) of the photosensitive drum 110 (K, C, M, Y). Installed. The electrostatic latent image is written by irradiating the photosensitive drum with light by the plurality of light emitting elements 30 described above. The developing device 114 (K, C, M, Y) forms a visible image, ie, a visible image, on the photosensitive drum by attaching toner as a developer to the electrostatic latent image.

Black, cyan, magenta, and black formed by such a four-color monochromatic image forming station
The yellow visible images are sequentially transferred onto the intermediate transfer belt 120 to be superimposed on the intermediate transfer belt 120. As a result, a full-color visible image is obtained. Inside the intermediate transfer belt 120, four primary transfer corotrons (transfer units) 112 (K, C, M,
Y) is arranged. The primary transfer corotron 112 (K, C, M, Y) is disposed in the vicinity of the photosensitive drum 110 (K, C, M, Y), respectively, and the photosensitive drum 110 (K,
By electrostatically attracting the visible image from (C, M, Y), the visible image is transferred to the intermediate transfer belt 120 passing between the photosensitive drum and the primary transfer corotron.

A sheet 102 as an object on which an image is finally formed is fed one by one from the sheet feeding cassette 101 by the pickup roller 103, and between the intermediate transfer belt 120 and the secondary transfer roller 126 in contact with the driving roller 121. Sent to the nip. 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 by passing through a fixing roller pair 127 as a fixing unit. . Thereafter, the sheet 102 is discharged onto a paper discharge cassette formed on 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.
FIG. 12 is a longitudinal sectional view of another image forming apparatus using the light emitting device 1 as a line type optical head. This image forming apparatus is a rotary developing type full-color image forming apparatus using a belt intermediate transfer body system. In the image forming apparatus shown in FIG. 14, a corona charger 168, a rotary developing unit 161, an organic EL array exposure head 167, and an intermediate transfer belt 169 are provided around the photosensitive drum 165.

The corona charger 168 uniformly charges the outer peripheral surface of the photosensitive drum 165. The organic EL array exposure head 167 writes an electrostatic latent image on the charged outer peripheral surface of the photosensitive drum 165. The organic EL array exposure head 167 is the light emitting device 1 of each aspect exemplified above,
The plurality of light emitting elements 30 are arranged so that the arrangement direction thereof is along the bus line (main scanning direction) of the photosensitive drum 165. The electrostatic latent image is written by irradiating the photosensitive drum 165 with light from these light emitting elements 30.

The developing unit 161 includes four developing units 163Y, 163C, 163M, and 163K.
The drums are arranged at an angular interval of ° and can be rotated counterclockwise about the shaft 161a. The developing units 163Y, 163C, 163M, and 163K supply yellow, cyan, magenta, and black toners to the photosensitive drum 165, respectively, and attach the toner as a developer to the electrostatic latent image, thereby the photosensitive drum 165. A visible image, that is, 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 and the primary transfer roller 166 by electrostatically attracting the visible image from the photosensitive drum 165.

Specifically, in the first rotation of the photosensitive drum 165, an electrostatic latent image for a yellow (Y) image is written by the exposure head 167, and a developed image of the same color is formed by the developing unit 163Y.
Further, the image is transferred to the intermediate transfer belt 169. Further, in the next rotation, an electrostatic latent image for a cyan (C) image is written by the exposure head 167 and a developed image of the same color is formed by the developing device 163C, and an intermediate transfer is performed so as to overlap the yellow developed image. Transferred to the belt 169. In this way, while the photosensitive drum 165 rotates four times, yellow, cyan, magenta,
The black visible image is sequentially superimposed on the intermediate transfer belt 169, and as a result, a full-color visible image is formed on the transfer belt 169. 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 visible 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 obtained on the intermediate transfer belt 169 by transferring the next-color visible image.

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). And
The secondary transfer roller 171 moves the intermediate transfer belt 169 when transferring a full-color visible image 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.
75. 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. 11 and 12 uses the OLED element 10 as an exposure unit, the apparatus can be made smaller than when a laser scanning optical system is used. Note that the light-emitting device of the present invention can also be adopted in an electrophotographic image forming apparatus other than those exemplified above. For example, the light emitting device according to the present invention is 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. Is possible.

The apparatus to which the light emitting device according to the present invention is applied is not limited to an image forming apparatus. For example, the light emitting device of the present invention is also used as a lighting device in various electronic devices. Examples of such electronic devices include facsimile machines, copiers, multifunction machines, and printers. For these electronic devices, light emitting devices in which a plurality of light emitting elements are arranged in a planar shape are suitably employed.

1 is a perspective view showing a partial configuration of an image forming apparatus using a light emitting device according to the present invention. It is a block diagram which shows the light-emitting device 1 and periphery structure. It is a block diagram of driver circuit A used for the device. It is a block diagram which shows the structure of the drive unit Ui. 3 is a circuit diagram showing a configuration of a decoder 14. FIG. It is a flowchart which shows operation | movement of a reference current setting mode. It is a flowchart which shows operation | movement of a power adjustment mode. It is explanatory drawing which shows the light emission power of an OLED element. It is a timing chart which shows the operation | movement in the initialization mode and light emission mode. It is explanatory drawing which shows the light emission power based on a gradation value. It is a longitudinal cross-sectional view which shows the structure of the image forming apparatus using the light-emitting device based on this invention. It is a longitudinal cross-sectional view which shows the structure of the other image forming apparatus using the light-emitting device based on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Light-emitting device, 10 ... OLED element (light emitting element), U1-Un ... Drive unit (unit circuit), 11 ... Distribution circuit (distribution means), 12 ... Reference data latch (reference data storage means) , 13 ... gradation data latch (gradation data storage means), 14 ... decoder (selection means), 220 ... external drive circuit (current source), 300 ... control circuit (designation signal generation means),
X: Reference potential setting unit, Y: Drive current generation unit, Z: Connection point, Ioled: Drive current, I
ref: reference current, Dref: reference data, Vref: reference potential, Dg: gradation data, Q
0, Q11 to Q13 ... Transistor (reference transistor), Qa, Qb ... Transistor (drive transistor), SW11 to SW13 ... Switch (reference switch), SWa,
SWb: switch (drive switch), LAT1 to LATn: latch pulse (designation signal), SEL: selection signal (identification signal).

Claims (9)

A drive circuit for driving a plurality of light emitting elements that emit light with a light amount corresponding to the magnitude of a drive current,
A plurality of unit circuits which are provided corresponding to each of the plurality of light emitting elements and generate the drive current;
Each of the plurality of unit circuits is
A reference potential setting unit capable of setting a reference potential based on reference data;
Generating the drive current pulse-modulated in accordance with gradation data indicating gradation values to be displayed;
A drive current generator configured to set the magnitude of the drive current based on the gradation data and the reference potential;
A drive circuit characterized by that. The reference potential setting unit includes:
A plurality of reference transistors, each drain connected to a connection point;
Provided in all or part of the plurality of reference transistors, and controlled based on the reference data so as to be in one of a first state in which the gate and the source are short-circuited and a second state in which the gate and the drain are short-circuited One or more reference switches,
A current source connected to the connection point for supplying a reference current;
The potential at the connection point is output as the reference potential, and the reference data is determined so that the light emission amounts of the plurality of light emitting elements are uniform.
The drive circuit according to claim 1. The drive current generator is
A plurality of drive transistors, each drain connected to the light emitting element;
A plurality of drive switches that are provided in each of the plurality of drive transistors and are in one of a first state in which the gate and the connection point are short-circuited and a second state in which the gate and drain are short-circuited;
The drive circuit according to claim 2. A predetermined drive switch is selected from the plurality of drive switches according to the gradation value indicated by the gradation data, and the predetermined drive switch is set to the first state only for a time corresponding to the gradation value. 4. The drive circuit according to claim 3, further comprising selection means for controlling the drive switches other than the predetermined drive switch to be in the second state. A wiring to which the reference data and the gradation data are supplied exclusively;
Each of the plurality of unit circuits includes a designation signal generation unit that generates a designation signal that designates a period in which the reference data or the gradation data is to be captured;
Each of the plurality of unit circuits is
Reference data storage means for storing the reference data in a period designated by the designation signal;
Gradation data storage means for storing the gradation data during a period designated by the designation signal;
Based on an identification signal indicating whether the data supplied to the wiring is the reference data or the gradation data, the data fetched from the wiring is transferred to the reference data storage means and the gradation data storage means. Distribution means for distributing,
The drive circuit according to claim 1, wherein the drive circuit is any one of claims 1 to 4. The drive current generation unit is configured such that when the pulse width of the drive current determined according to the gradation data is narrower than a predetermined time, the magnitude of the drive current is larger than when the pulse width is wider than the predetermined time. 6. The driving circuit according to claim 1, wherein the driving circuit is set to be large. The drive circuit according to any one of claims 1 to 6,
A plurality of light emitting elements that emit light with a light amount corresponding to the magnitude of the drive current,
A light emitting device comprising the light emitting device.   An electronic apparatus comprising the light emitting device according to claim 7. Driving a plurality of light emitting elements that irradiate an image carrier coated with a photosensitive material that generates an electrostatic potential having a magnitude corresponding to the period of light irradiation with a light amount corresponding to the magnitude of the drive current. A driving method comprising:
Corresponding to each of the plurality of light emitting elements, each reference potential is generated so that the light emission amount corresponding to a predetermined gradation value approaches,
Generating the drive current pulse-modulated in accordance with gradation data indicating gradation values to be displayed;
Setting the magnitude of the drive current based on the pulse width of the drive current and the reference potential;
A driving method characterized by that.

Images