JP2009008783A - Light emitting device and electronic equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light emitting device in which the power consumption is reduced and the reliability is improved. <P>SOLUTION: The light emitting device 10 comprises: n pieces of light emitting elements P1 to Pn; driving circuits Ua1 to Uan; a maximum voltage detection circuit 30 which detects the maximum voltage Vmax among voltages V1 to Vn; a maximum voltage holding circuit 40 which detects and holds the maximum in the temporal change of the maximum voltage Vmax as a peak voltage Vpeak; a power source control circuit 50 which produces a control voltage Vctl based on the peak voltage Vpeak; and a power source circuit 60 which produces a power source voltage Vdd based on a control voltage Vctl. The light emitting device 10 detects the maximum voltage Vmax of the n pieces of light emitting elements P1 to Pn spatially arranged and, further, detects the peak voltage Vpeak which is the temporally maximum voltage. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a technique for reducing power consumption of a light emitting device.

There is a light-emitting device in which current-driven light-emitting elements, such as EL (Electro Luminescent) elements, having a gradation corresponding to a flowing current are arranged. Examples of this type of light emitting device include a printer head and a display device. The organic EL used for the light emitting element emits light with a light emission intensity proportional to the value of the drive current flowing therethrough. For this reason, it is usual to drive a light emitting element with a constant current.
However, the voltage required for the organic EL to flow the same current increases with time (see, for example, Patent Document 1). On the other hand, a minimum operation voltage is required for a drive circuit that supplies a drive current to the light emitting element to operate normally. For this reason, when the power supply voltage supplied to the drive circuit is Vdd, the operating voltage is Va, and the voltage of the light emitting element is Vb, it is necessary to satisfy Vdd> Va + Vb.
Here, assuming that the value of the drive current is Id, the loss W of the drive circuit is given by the following equation.
W = {Vdd- (Va + Vb)} Id
Since the loss W becomes heat, it is not preferable in terms of reduction of power consumption. Further, unnecessary temperature increase damages the light emitting element, shortens the life, and decreases reliability.

  For this reason, Patent Document 2 discloses a technique for detecting the voltage of a specific light emitting element and adjusting the power supply voltage in accordance with the detected voltage.

Japanese Patent Laying-Open No. 2006-47668 (see paragraph number 0010 and FIG. 3) JP 2003-233342 A

  However, the technique disclosed in Patent Document 2 has a problem in that the driving state of other light emitting elements cannot be detected because the point at which the voltage is detected is a specific light emitting element. Further, Patent Document 1 describes the point of detecting the voltages of a plurality of light emitting elements, but does not describe the point of reflecting the detected voltages in the adjustment of the power supply voltage. It was unknown whether it would be reduced.

  The present invention has been made in view of the above-described circumstances, and in a plurality of light-emitting elements, even when there is deterioration due to aging, a necessary light emission intensity can be obtained and power consumption can be reduced. Let it be a solution issue.

  A light-emitting device according to the present invention includes a plurality of light-emitting elements, a plurality of transistors that receive a supply voltage from each of the plurality of light-emitting elements to supply a driving current, and each of the plurality of light-emitting elements. A maximum voltage detecting means for detecting a maximum voltage among the applied voltages applied by supplying the driving current as a first voltage; and detecting a maximum temporal change in the first voltage as a second voltage. Maximum voltage holding means for holding the power supply voltage, and power supply means for supplying the power supply voltage to the plurality of transistors and adjusting the power supply voltage based on the second voltage.

  According to the present invention, the first voltage is constantly monitored in order to detect the voltage when the light emitting element having the greatest deterioration over time among the voltages applied to the plurality of light emitting elements emits light at the maximum luminance. The first voltage is compared in time series, the maximum voltage is detected as the second voltage, and the power supply voltage is adjusted based on the detection result. As a result, even if the degree of deterioration with time of the light emitting elements varies, the light emitting element with the greatest deterioration can emit light with the maximum luminance, and the loss of the driving transistor can be minimized. Therefore, the accuracy of light emission luminance can be improved and power consumption can be reduced. Further, by reducing the heat generation of the driving transistor, the lifetime of the light emitting element can be extended and the reliability can be improved.

More specifically, the power supply unit includes a power supply control unit and a power supply unit, and the power supply control unit AD-converts the detected second voltage and outputs it as held data, and the held data Comparing the second voltage with the third voltage, and the second voltage value is the second voltage value. Preferably, the power supply unit adjusts the power supply voltage based on the third voltage. The update means updates the retained data stored in the storage means when the voltage becomes larger than three voltages.
In this case, since the stored data is stored in the storage means, the maximum voltage can be detected in a longer time, and the power supply voltage can be adjusted based on this. The storage means is preferably non-volatile.

  In addition, the updating means compares the second voltage with the third voltage in a state where the plurality of light emitting elements emit light, and the value of the second voltage is greater than the value of the third voltage. When it becomes larger, it is preferable to update the held data stored in the storage means. If the light emitting element is not lit, the maximum voltage is not reached. In such a case, the updating operation is stopped.

  The power supply unit generates the power supply voltage obtained by adding a voltage obtained by multiplying a control voltage by a first coefficient and a predetermined voltage, and the power supply control unit multiplies the third voltage by a reciprocal of the first coefficient. It is preferable to provide a multiplication circuit for generating the control voltage. In this case, the power supply voltage can be adjusted more accurately.

  An electronic apparatus according to the present invention includes the above-described light emitting device. Examples of such an electronic device include an image forming apparatus such as a printer, a display device, and the like.

<1. Appearance of light emitting device>
FIG. 1 is a perspective view showing a partial configuration of an image forming apparatus using a light emitting device 10 according to an arrangement pattern of the present invention as an optical head (exposure device). As shown in the figure, the image forming apparatus includes a light emitting device 10, a condensing lens array 15, and a photosensitive drum 110. The light emitting device 10 includes n (n is a natural number of 2 or more) light emitting elements. Light is emitted from the light emitting element. This emission is selectively performed according to the form of an image to be printed on a recording material such as paper. These lights travel to the condensing lens array 15. The photosensitive drum 110 is supported by a rotating shaft extending in the main scanning direction, and rotates in the sub-scanning direction (direction in which the recording material is conveyed) with the outer peripheral surface facing the light emitting device 10.

  The condensing lens array 15 is disposed in the gap between the light emitting device 10 and the photosensitive drum 110. The condensing lens array 15 includes a large number of gradient index lenses arranged in an array with each optical axis directed toward the light emitting device 10. Light emitted from each light emitting element of the light emitting device 10 passes through each refractive index distribution type lens of the condensing lens array 15 and then reaches the surface of the photosensitive drum 110. By this exposure, a latent image (electrostatic latent image) corresponding to a desired image is formed on the surface of the photosensitive drum 110.

<2. Electrical configuration of light emitting device>
FIG. 2 is a block diagram showing an electrical configuration of part of the light emitting device 10. As shown in FIG. 2, the light emitting device 10 includes drive circuits Ua1 to Uan that supply drive currents to n light emitting elements P1 to Pn, respectively. The light-emitting elements P1 to Pn are current-driven elements that emit light with a luminance corresponding to the magnitude of the drive current. Typically, a light-emitting layer made of an organic EL material is interposed between an anode and a cathode. It is an OLED element. Examples of current-driven light emitting elements other than OLED elements include inorganic EL elements.

  The control circuit 20 includes a shift register, and generates selection signals G1 to Gn by transferring a start pulse according to a clock signal. The selection signals G1 to Gn are exclusively active.

FIG. 3 shows a circuit diagram of the drive circuit Ua1. The other drive circuits Ua2 to Uan are configured similarly to the drive circuit Ua1. As shown in this figure, the drive circuit Ua1 includes a drive transistor Tdr and a storage capacitor C provided between the source and gate thereof. Further, the gate of the driving transistor Tdr is electrically connected to a signal wiring to which the data signal VDATA is supplied via the switch SW. The switch SW is turned on when the selection signal G1 becomes active. At this time, the data signal VDATA is written into the storage capacitor C. The drive transistor Tdr supplies a drive current having a magnitude corresponding to the gate potential to the light emitting element P1.
Further, the power supply voltage Vdd is supplied to the source of the driving transistor Tdr via the power supply wiring. A voltage V1 at a connection point between the light emitting element P1 and the drain of the driving transistor Tdr is output.
In the above configuration, when the selection signal G1 becomes active, the switch SW is turned on and the data signal VDATA is supplied to the gate of the drive transistor Tdr. The drive transistor Tdr supplies a drive current corresponding to the gate potential to the light emitting element P1. Accordingly, the light emitting element P1 emits light with luminance according to the voltage of the data signal VDATA.

  Here, the power supply voltage Vdd is given as the sum of the voltage Va from the power supply terminal to the anode of the light emitting element P1 and the voltage Vb applied to the light emitting element P1. When the light emitting element P1 deteriorates with time, the voltage Vb increases even if the magnitude of the drive current is the same. That is, the drain voltage of the drive transistor Tdr increases. In order for the driving transistor Tdr to operate normally, the voltage between the source and the drain needs to be equal to or higher than a predetermined voltage. On the other hand, as the source-drain voltage increases, the loss of the drive transistor Tdr increases accordingly. Therefore, it is preferable that the voltage between the source and the drain is a constant voltage in consideration of a certain margin. This voltage can be specified from the electrical characteristics of the drive transistor Tdr. The voltage Va is set as such a voltage by a power supply circuit 60 described later.

As described above, the voltage Vb is determined by the deterioration over time of the light emitting element. However, when n light emitting elements P1 to Pn are used as in the present embodiment, the degree of deterioration over time varies. In this case, it is necessary to adjust the magnitude of the power supply voltage Vdd with reference to the voltage Vb when the light emitting element having the greatest deterioration with time emits light at the maximum luminance. However, not all the light emitting elements P1 to Pn always emit light with the maximum luminance, but each light emitting element emits light with the luminance specified by the data signal VDATA. However, in a long time, each light emitting element emits light with the maximum luminance at some timing.
Therefore, in the present embodiment, the maximum voltage among the voltages applied to the n light emitting elements P1 to Pn is maximized in order to detect the voltage Vb when the light emitting element having the greatest deterioration with time emits light with the maximum luminance. The voltage Vmax is constantly monitored, the maximum voltage Vmax is compared in time series, the maximum voltage is detected as the peak voltage Vpeak, and the power supply voltage Vdd is adjusted based on the detection result. The following will specifically describe these configurations.

  Returning to FIG. The light emitting device 10 includes a maximum voltage detection circuit 30, a maximum voltage holding circuit 40, a power supply control circuit 50, and a power supply circuit 60. The maximum voltage detection circuit 30 outputs the maximum voltage among the anode voltages V1 to Vn of the light emitting elements P1 to Pn as the maximum voltage Vmax. More specifically, the maximum voltage detection circuit 30 is configured as shown in FIG. As shown in this figure, the maximum voltage detection circuit 30 includes n diodes D1 to Dn whose voltages V1 to Vn are supplied to the respective anodes. The cathodes of the diodes D1 to Dn are connected to the other terminal of the resistance element 31 whose one terminal is grounded, and the maximum voltage Vmax is taken out from the other terminal. For example, when the voltage V2 is the maximum, the diode D2 is turned on, and the other diodes D1 and D3 to Dn are turned off. In addition, it is preferable from the viewpoint of reducing power consumption that the resistance value of the resistance element 31 is high and almost no current is input.

  Next, the maximum voltage holding circuit 40 holds the maximum of the temporal change of the maximum voltage Vmax and extracts it as the peak voltage Vpeak. Specifically, the maximum voltage holding circuit 40 is configured as shown in FIG. As shown in FIG. 5, the maximum voltage holding circuit 40 includes an operational amplifier 41, a diode 42, a capacitor 43, and a switch 44. When power is supplied to the maximum voltage holding circuit 40, the switch 44 is turned on, and the charge accumulated in the capacitor 43 is discharged and initialized. Thereafter, when the maximum voltage Vmax is supplied to the positive input terminal of the operational amplifier 41, the maximum voltage Vmax is compared with the voltage at the node 2, and if the maximum voltage Vmax exceeds the voltage at the node 2, the capacitor 43 is connected via the diode 42. The charge is accumulated in the. Therefore, the voltage of the node 2 becomes the maximum in the time series change of the maximum voltage Vmax, and this is held by the capacitor 43. Then, the voltage of the node 2 is taken out as the peak voltage Vpeak.

  Next, the power supply control circuit 50 generates a control voltage Vctl that controls the magnitude of the power supply voltage Vdd output from the power supply circuit 60. FIG. 6 shows a circuit diagram of the power supply control circuit 50. As shown in this figure, the power supply control circuit 50 includes a comparator 51 that compares the peak voltage Vpeak and the adjustment voltage Vadj, an AD converter 52 that AD-converts the peak voltage Vpeak and generates retained data Dh, and retained data Dh. , A DA converter 54 that DA-converts the hold data Dh read from the memory 53 and outputs it as an adjustment voltage Vadj, and a multiplier that multiplies the adjustment voltage Vadj by a coefficient K and outputs a control voltage Vctl. 55.

The comparator 51 generates an enable signal EN that becomes a high level when the value of the peak voltage Vpeak exceeds the adjustment voltage Vadj, and supplies the enable signal EN to the memory 53. The memory 53 is a nonvolatile storage means, and updates the stored contents when the enable signal EN is at a high level. Therefore, when holding data Dh having a value larger than the stored holding data Dh is supplied from the AD converter 52, the memory 53 updates the stored contents.
The above-described maximum voltage holding circuit 50 outputs the peak voltage Vpeak during the period when the light emitting device 10 is powered on, but the memory 53 has a period from when the light emitting device 10 is shipped to the present. The maximum value of the peak voltage Vpeak is held. Thereby, it is possible to specify an applied voltage applied to the light emitting element having the greatest deterioration with time among the n light emitting elements P1 to Pn.

Next, the power supply circuit 60 generates the power supply voltage Vdd based on the control voltage Vctl. FIG. 7 shows a circuit diagram of the power supply circuit. As shown in this figure, the power supply circuit 60 includes resistance elements 61 to 63 and 65, an operational amplifier 64, a transistor 66, and a voltage source 67 that outputs a voltage Vcc. Here, Vcc> Vdd. The power supply voltage Vdd is given by the following equation (1) when the resistance values of the resistance elements 61 to 63 are R1 to R3.
Vdd = Vref (R1 / R2 + R2 / R3 + R3 / R1) / (R2 / R3) + Vctl / R1 / R3 ... (1)
= Va + Vb
Va = Vref (R1, R2 + R2, R3 + R3, R1) / (R2, R3)
Vb = Vctl ・ R1 / R3
However, Va is a voltage from a power supply terminal to the anode of a light emitting element, and Vb is a voltage from the anode of a light emitting element to a cathode (refer FIG. 3). Here, Vb = Vadj can be obtained by setting the coefficient K of the multiplication circuit 55 described above as K = R3 / R1. As a result, the power supply voltage Vdd can be accurately adjusted.

  According to this embodiment, in order to detect the voltage Vb when the light emitting element having the greatest deterioration with time among the voltages applied to the n light emitting elements P1 to Pn emits light at the maximum luminance, n light emitting elements are detected. The maximum voltage applied to P1 to Pn is always monitored as the maximum voltage Vmax, and the maximum voltage Vmax is compared in time series to detect the maximum voltage as the peak voltage Vpeak, based on the detection result. The power supply voltage Vdd was adjusted. As a result, even when the degree of deterioration with time of the light emitting element varies, the light emitting element with the greatest deterioration can emit light with the maximum luminance, and the loss of the driving transistor Tdr can be minimized. Therefore, according to the light emitting device 10, the accuracy of the light emission luminance can be improved and the power consumption can be reduced. Furthermore, by reducing the heat generation of the drive transistor Tdr, the lifetime of the light emitting elements P1 to Pn can be extended and the reliability can be improved.

In the above-described embodiment, the update of the storage contents of the memory 53 may be executed while the light emitting element is emitting light. In this case, a signal indicating that the lighting is in progress may be acquired from a higher-level control device, and the update may be executed only when this signal is active. Specifically, an AND circuit is provided between the comparator 51 and the memory 53, and the output signal of the comparator 51 is supplied to one input terminal of the AND circuit, while the signal is supplied to the other input terminal to output the AND circuit. A signal may be supplied to the memory 53.
Moreover, although the drive circuits Ua1 to Uan described above are configured in a voltage programming format, it is needless to say that they may be configured in a current programming format.

<3. Entire image forming apparatus>
Next, the entire image forming apparatus will be described. Examples of the image forming apparatus include a printer, a copier printing portion, and a facsimile printing portion.
FIG. 8 is a longitudinal sectional view showing an example of an image forming apparatus using the light emitting device 10 as a line type exposure device. 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 exposure devices 10K, 10C, 10M, and 10Y having the same configuration are exposed positions of four photosensitive drums (image carriers) 110K, 110C, 110M, and 110Y having the same configuration. Respectively. The exposure apparatuses 10K, 10C, 10M, and 10Y are the light emitting apparatus 10 described above.

  As shown in this figure, this image forming apparatus is provided with a driving roller 121 and a driven roller 122, and an endless intermediate transfer belt 120 is wound around these rollers 121 and 122, as indicated by arrows. Thus, 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, photosensitive drums 110K, 110C, 110M, and 110Y having photosensitive layers on four outer peripheral surfaces 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. The photosensitive drums 110K, 110C, 110M, and 110Y are rotationally driven in synchronization with the driving of the intermediate transfer belt 120.

  Around each photosensitive drum 110 (K, C, M, Y), there is a corona charger 111 (K, C, M, Y), an exposure device 10 (K, C, M, Y), and a developing device. 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 exposure device 10 (K, C, M, Y) writes an electrostatic latent image on the charged outer peripheral surface of the photosensitive drum. Each exposure apparatus 10 (K, C, M, Y) is installed such that the arrangement direction of the plurality of EL elements is along the bus (main scanning direction) of the photosensitive drum 110 (K, C, M, Y). . The electrostatic latent image is written by irradiating the photosensitive drum with light from the plurality of EL elements. The developing device 114 (K, C, M, Y) forms a visible image, that is, a visible image on the photosensitive drum by attaching toner as a developer to the electrostatic latent image.

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

  A sheet 102 as an object on which an image is to be finally formed is fed one by one from the sheet feeding cassette 101 by the pickup roller 103, and 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 through the fixing roller pair 127 as a fixing unit. . Thereafter, the sheet 102 is discharged onto a paper discharge cassette formed in the upper part of the apparatus by a paper discharge roller pair 128.

  FIG. 9 is a longitudinal sectional view showing an example of another image forming apparatus using the light emitting device 10 as a line type exposure apparatus. 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 this figure, a corona charger 168, a rotary developing unit 161, an exposure device 167, and an intermediate transfer belt 169 are provided around a photosensitive drum (image carrier) 165.

  The corona charger 168 uniformly charges the outer peripheral surface of the photosensitive drum 165. The exposure device 167 writes an electrostatic latent image on the charged outer peripheral surface of the photosensitive drum 165. The exposure device 167 is the light emitting device 10 described above, and is installed so that the arrangement direction of the plurality of EL elements is along the bus line (main scanning direction) of the photosensitive drum 165. The electrostatic latent image is written by irradiating the photosensitive drum with light from the plurality of EL elements.

  The developing unit 161 is a drum in which four developing units 163Y, 163C, 163M, and 163K are arranged at an angular interval of 90 °, and can rotate counterclockwise about the shaft 161a. The developing units 163Y, 163C, 163M, and 163K supply yellow, cyan, magenta, and black 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 device 167, and a developed image of the same color is formed by the developing device 163Y. The image is transferred to the transfer belt 169. Further, in the next rotation, an electrostatic latent image for a cyan (C) image is written by the exposure device 167, a developed image of the same color is formed by the developing device 163C, and intermediate transfer is performed so as to overlap the yellow developed image. Transferred to the belt 169. Then, during the four rotations of the photosensitive drum 9, the yellow, cyan, magenta, and black visible images are sequentially superimposed on the intermediate transfer belt 169. As a result, a full-color visible image is formed on the transfer belt 169. It is formed. When images are finally formed on both sides of a sheet as an object on which an image is to be formed, the same color images of the front and back surfaces are transferred to the intermediate transfer belt 169, and then the front and back surfaces are transferred to the intermediate transfer belt 169. A full-color visible image is obtained on the intermediate transfer belt 169 by transferring the visible image of the next color.

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

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

  Although the image forming apparatus has been exemplified above, the light emitting device 10 can be applied to other electrophotographic image forming apparatuses. For example, an image forming apparatus that transfers a visible image directly from a photosensitive drum to a sheet without using an intermediate transfer belt, an image forming apparatus that forms a monochrome image, and an image forming that uses a photosensitive belt as an image carrier. It can also be applied to devices. The light emitting device 10 can be applied to any electronic device. Examples of such an electronic device include a display device in addition to the image forming apparatus. Examples of the display device include a mobile personal computer adopting the light emitting device 10 as a display device, a mobile phone, a personal digital assistant (PDA), a television, and a video camera.

1 is a perspective view showing a partial configuration of an image forming apparatus using a light emitting device 10 according to an embodiment of the present invention as an optical head. 2 is a block diagram showing an electrical configuration of the light emitting device 10. FIG. It is a circuit diagram which shows the structure of a drive circuit. It is a circuit diagram which shows the structure of a maximum voltage detection circuit. It is a circuit diagram which shows the structure of a maximum voltage holding circuit. It is a circuit diagram which shows the structure of a power supply control circuit. It is a circuit diagram which shows the structure of a power supply circuit. 1 is a longitudinal sectional view illustrating an example of an image forming apparatus according to an embodiment. It is a longitudinal cross-sectional view which shows the other example of the image forming apparatus which concerns on this embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Light-emitting device, P1-Pn ... Light-emitting element, Ua1-Uan ... Drive circuit, Vmax ... Maximum voltage, 30 ... Maximum voltage detection circuit, Vpeak ... Peak voltage, 40 ... Maximum voltage holding circuit, 50 ... Power supply control circuit, Vdd ... power supply voltage, 60 ... power supply circuit.

Claims (5)

A plurality of light emitting elements;
Each of the plurality of light emitting elements is supplied with a power supply voltage, and a plurality of transistors for supplying a driving current;
Maximum voltage detection means for detecting, as a first voltage, a maximum voltage among applied voltages applied by supplying the driving current to each of the plurality of light emitting elements;
Maximum voltage holding means for detecting and holding the maximum of the temporal change of the first voltage as the second voltage;
Power supply means for supplying the power supply voltage to the plurality of transistors and adjusting the power supply voltage based on the second voltage;
A light emitting device characterized by that. The power supply means includes a power supply control unit and a power supply unit,
The power control unit
AD conversion means for AD-converting the detected second voltage and outputting it as retained data;
Storage means for storing the retained data;
DA conversion means for DA-converting the held data and outputting it as a third voltage;
Updating means for comparing the second voltage with the third voltage and updating the stored data stored in the storage means when the value of the second voltage is greater than the value of the third voltage;
The power supply unit adjusts the power supply voltage based on the third voltage;
The light-emitting device according to claim 1.   In the state where the plurality of light emitting elements emit light, the updating means compares the second voltage with the third voltage, and when the value of the second voltage becomes larger than the value of the third voltage, The light-emitting device according to claim 2, wherein the held data stored in the storage unit is updated. The power supply unit generates the power supply voltage obtained by adding a predetermined voltage to a voltage obtained by multiplying a control voltage by a first coefficient,
The power supply control unit includes a multiplication circuit that multiplies the third voltage by the reciprocal of the first coefficient to generate the control voltage.
The light emitting device according to any one of claims 1 to 3. An electronic apparatus comprising the light-emitting device according to claim 1.

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