JP2008003456A - Electro-optical device, its control method, and electronic equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce electric power consumed in a circuit driving an electro-optical element. <P>SOLUTION: A power supply voltage VDD is applied between a power supply line LP and a grounding line LG. A node N is set on a route R connecting the power supply line LP and the grounding line LG. The electro-optical element E is arranged on the route R connecting the grounding line LG and the node N, and driven by a drive current IEL made to flow in the route R. A drive circuit 361 generates the drive current IEL by regarding a voltage VDR between the power supply line LP and the node N as a drive power supply voltage. In a voltage control circuit 12, a power supply voltage VDD1 when an element voltage VEL between the grounding line LG and the node N is a voltage value VEL1 in driving the electro-optical element E controls the power supply voltage VDD so that the element voltage VEL is less than a power supply voltage VDD2 when it is a voltage value VEL2 (VEL2>VEL1). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a technique for controlling a power supply voltage of an electro-optical device using an electro-optical element such as an organic light-emitting diode element.

  FIG. 12 is a block diagram illustrating a typical example of a structure for driving one electro-optical element in the electro-optical device (for example, Patent Document 1 and Patent Document 2). As shown in the figure, a drive circuit 61 and an electro-optic element 62 connected in series are arranged on a path connecting the power supply line LP and the ground line LG. The voltage (hereinafter referred to as “power supply voltage”) VDD between the power supply line LP and the ground line LG is maintained at a predetermined voltage value. The drive circuit 61 generates a drive current IEL using the voltage VDR between the power supply line LP and the node N as a power supply voltage (hereinafter referred to as “drive power supply voltage”). The electro-optic element 62 operates (for example, emits light) by supplying the driving current IEL.

By the way, the electrical characteristics of the electro-optic element 62 vary depending on various factors such as the temperature of the environment in which the electro-optic device is used and the elapsed time from the manufacturing. The voltage VEL between both ends of the electro-optic element 62 (which is a voltage between the ground line LG and the node N. Hereinafter referred to as “element voltage”) VEL varies depending on the electrical characteristics of the electro-optic element 62. . FIG. 13 is a graph showing the relationship between the power supply voltage VDD / drive power supply voltage VDR and the element voltage VEL. As shown in the figure, since the power supply voltage VDD is maintained at a fixed value (18 V in FIG. 13) regardless of the element voltage VEL, the drive power supply voltage VDR decreases as the element voltage VEL increases. Therefore, even when the element voltage VEL is sufficiently increased, the driving power supply of VDR0 is the minimum voltage value (3V in FIG. 13, hereinafter referred to as “required voltage value”) necessary for the proper operation of the driving circuit 61. The power supply voltage VDD is set so that the voltage VDR is secured.
JP 2005-3849 A JP 2005-122076 A

  However, in the configuration in which the power supply voltage VDD is set so as to satisfy the above conditions, the drive power supply voltage VDR exceeding the required voltage value VDR0 is supplied to the drive circuit 61 when the element voltage VEL decreases as shown in FIG. Applied. Therefore, there is a problem in that the drive circuit 61 consumes power that is not normally necessary for the drive circuit 61 to operate properly. In view of the above circumstances, an object of the present invention is to solve the problem of reducing power consumed by a circuit that drives an electro-optical element.

  In order to solve the above problems, an electro-optical device according to an aspect of the present invention includes a first power supply line (for example, the ground line LG in FIG. 2) and a second power supply line (for example, FIG. 2) to which a power supply voltage is applied. Power line LP) and a path between the first power supply line and the node, driven by a drive current flowing through the path, and a voltage between the second power supply line and the node. A drive circuit that generates a drive current as a power supply voltage, and a power supply when the element voltage between the node and the first power supply line when driving the electro-optical element is a first voltage value (for example, the voltage value VEL1 in FIG. 6) A power supply voltage (for example, power supply voltage VDD2 in FIG. 6) when the voltage (for example, power supply voltage VDD1 in FIG. 6) is a second voltage value (for example, voltage value VEL2 in FIG. 6) in which the element voltage exceeds the first voltage value. A voltage control circuit for controlling the power supply voltage to To Bei.

  In the above configuration, the first power supply line is set so that the power supply voltage when the element voltage at the time of driving the electro-optic element is the first voltage value is lower than the power supply voltage when the element voltage is the second voltage value. Since the power supply voltage between the first power supply line and the second power supply line is controlled, the power supply voltage is maintained at a fixed value (for example, a voltage value that allows the drive circuit to operate properly when the element voltage is the second voltage value). Compared with the configuration, the driving power supply voltage when the element voltage is the first voltage value is reduced. Therefore, there is an advantage that power consumed in the drive circuit is reduced.

  The electro-optical element according to the present invention changes the optical characteristics such as the amount of light and the transmittance by supplying a drive current, and the voltage between both ends (for example, between the anode and the cathode) when a predetermined drive current flows ( This is an element whose element voltage changes. For example, an organic light-emitting diode element whose element voltage changes in accordance with the temperature of the environment in which the electro-optical device is used and the time elapsed since manufacture is a typical example of an electro-optical element. The voltage control circuit may control the power supply voltage according to the voltage between the node and the first power supply line, or may control the power supply voltage according to the voltage between the node and the second power supply line. May be.

  In a preferred aspect of the present invention, the voltage control circuit controls the power supply voltage so that the power supply voltage decreases as the element voltage decreases. According to this aspect, power consumption in the drive circuit can be reduced even when the element voltage varies over a wide range. In addition, if the drive power supply voltage of the drive circuit is maintained at a predetermined value (for example, a required voltage value necessary for the operation of the drive circuit or a voltage value exceeding it) regardless of the element voltage, the power consumed by the drive circuit Is maintained at a predetermined value regardless of the element voltage.

  In the above, the present invention has been specified by focusing on only one electro-optical element, but an electro-optical device including a plurality of electro-optical elements is naturally included in the scope of the present invention. In a preferred aspect of the electro-optical device including a plurality of electro-optical elements and a plurality of drive circuits corresponding to the electro-optical elements, the voltage control circuit has a maximum element voltage among the plurality of electro-optical elements. The power supply voltage is controlled so that the drive power supply voltage of the drive circuit corresponding to the electro-optic element is equal to or higher than a required voltage value necessary for the operation of each drive circuit (for example, the required voltage value VDR0 in FIG. 6). According to the above aspect, since the power supply voltage is controlled so that the drive power supply voltage of the drive circuit corresponding to the electro-optic element having the maximum element voltage is equal to or higher than the required voltage value, all the drive circuits are driven. It becomes possible to reliably maintain the power supply voltage at or above the required voltage value.

  As a means for variably generating the power supply voltage in the voltage control circuit, either a switching regulator or a series regulator can be employed. However, in a configuration using a series regulator, the power consumption of the drive circuit is only consumed by the series regulator, and the overall power consumption of the electro-optical device is the same as when the power supply voltage is fixed. Absent. However, compared with a configuration in which the power supply voltage is fixed, heat generation in the drive circuit is suppressed (the suppression is radiated by the series regulator), so the series regulator is placed at a position away from the electro-optic element. According to the above configuration, the effect of suppressing the deterioration of the electro-optical element due to the radiant heat from the drive circuit and the fluctuation of the characteristic (for example, the amount of light emission) of the electro-optical element according to the temperature is surely achieved. For example, a configuration in which the drive circuit and the electro-optic element are disposed on the substrate and the series regulator is mounted on a wiring substrate separate from the substrate is suitably employed. Of course, even when a switching regulator is employed, a configuration in which the voltage control circuit is arranged on a separate wiring board from the board on which the driving circuit and the electro-optical element are arranged is preferable. The wiring board may be installed on the board indirectly via a component such as a flexible wiring board, or may be installed directly on the board.

  An electro-optical device according to a preferred aspect of the present invention includes a reference setting circuit that variably generates a reference voltage, a plurality of electro-optical elements, a plurality of drive circuits corresponding to each electro-optical element, and A plurality of comparison circuits (for example, the comparison circuit 365 in FIG. 8) for comparing the element voltage and the reference voltage are provided, and the voltage control circuit has a maximum value among the element voltages according to the result of comparison by each comparison circuit. And the power supply voltage is controlled so that the drive power supply voltage of the drive circuit corresponding to the maximum value is equal to or higher than the required voltage value necessary for the operation of each drive circuit. According to the above aspect, since the power supply voltage is controlled according to the comparison of the size of each element voltage and the reference voltage, for example, compared with the configuration that controls the power supply voltage according to the measured value of each element voltage, The circuit scale of the electro-optical device can be reduced. A specific example of the above aspect will be described later as a second embodiment.

  In a further preferred aspect, the voltage control circuit selects a predetermined number of element voltages according to the result of comparison by each comparison circuit, and the drive power supply voltage of the drive circuit corresponding to the maximum value among the predetermined number of element voltages is The power supply voltage is repeatedly controlled so as to be a predetermined value. According to the above aspect, there is an advantage that the process of analyzing the comparison result by each comparison circuit is simplified as compared with the configuration in which all the element voltages are constantly monitored.

  The electro-optical device according to the invention is used in various electronic apparatuses. A typical example of the electronic apparatus according to the present invention is an electrophotographic image forming apparatus in which the electro-optical device according to each of the above embodiments is used for exposure of an image carrier such as a photosensitive drum. This image forming apparatus is realized by adding an image carrier on which a latent image is formed by exposure, the electro-optical device of the present invention that exposes the image carrier, and a developer (for example, toner) to the latent image on the image carrier. And a developing unit for forming an image. However, the use of the electro-optical device according to the present invention is not limited to the exposure of the image carrier. For example, in an image reading apparatus such as a scanner, the electro-optical device according to the present invention can be used for illuminating a document. The image reading apparatus includes an electro-optical device according to each of the above aspects, and a light-receiving device (for example, a CCD (Charge Coupled Device) element that converts light emitted from the electro-optical device and reflected by a reading target (original) into an electric signal. Etc.). Furthermore, an electro-optical device in which electro-optical elements are arranged in a matrix is also used as a display device for various electronic devices such as a personal computer and a mobile phone.

  The present invention is also specified as a method of controlling the electro-optical device. The method of the present invention is arranged on a path connecting the first power supply line and the second power supply line to which the power supply voltage is applied, and the first power supply line and the node, and is driven by a drive current flowing through the path. A method for controlling an electro-optical device comprising: an element; and a drive circuit that generates a drive current using a voltage between a second power supply line and a node as a drive power supply voltage. The power supply voltage is set so that the power supply voltage when the element voltage between the first power supply line is the first voltage value is lower than the power supply voltage when the element voltage is the second voltage value exceeding the first voltage value. Control. According to the above method, the electric power consumed in the drive circuit is reduced for the reason described above for the electro-optical device of the invention.

<A: First Embodiment>
FIG. 1 is a block diagram showing the configuration of the electro-optical device according to the first embodiment of the invention. The electro-optical device H is employed in an electrophotographic image forming apparatus as an exposure device (line head) that exposes a photosensitive drum. As shown in FIG. 1, the electro-optical device H includes a wiring board 10 on which a voltage control circuit 12 is disposed, a head module 30 that irradiates a photosensitive drum with a light beam according to a desired image, and the wiring board 10 and the head. The flexible wiring board 15 which electrically connects the module 30 is provided.

  The voltage control circuit 12 is means for variably generating the power supply voltage VDD, and includes a control circuit 121 and a power supply circuit 123. The power supply circuit 123 is a DC-DC converter that generates a power supply voltage VDD from a voltage (hereinafter referred to as “input voltage”) VIN supplied from an external power supply. The control circuit 121 controls the voltage value of the power supply voltage VDD generated by the power supply circuit 123. The power supply voltage VDD generated by the power supply circuit 123 is supplied to the head module 30 via the power supply line LP.

  The head module 30 is an element in which an element unit 34 and an IC chip 36 are arranged on the surface of a flat substrate 32. The element section 34 includes n (n is a natural number) electro-optic elements E arranged in a single row or a plurality of rows in a straight line along the main scanning direction. Each electro-optical element E is an organic light-emitting diode element in which a light-emitting layer of an organic EL (Electroluminescence) material is interposed between an anode and a cathode facing each other.

  The IC chip 36 includes n unit circuits U each corresponding to a separate electro-optical element E. In FIG. 1, only one IC chip 36 is shown for the sake of convenience, but a configuration in which n unit circuits U are distributed over a plurality of IC chips is also employed. Each unit circuit U may be constituted by a large number of active elements (for example, a thin film transistor in which a semiconductor layer is formed of low-temperature polysilicon) formed on the surface of the substrate 32 together with the element portion 34.

  FIG. 2 is a block diagram focusing on the structure of one unit circuit U. As shown in FIGS. 1 and 2, each unit circuit U includes a drive circuit 361 and a measurement circuit 363. The power supply voltage VDD generated by the power supply circuit 123 is commonly supplied to the drive circuit 361 of each unit circuit U via the power supply line LP. The drive circuit 361 and the electro-optic element E are arranged in series on a path R connecting the power supply line LP and the ground line LG (ground voltage VSS). More specifically, the drive circuit 361 is interposed between the node N on the path R and the power supply line LP. The electro-optic element E has an anode connected to the node N and a cathode connected to the ground line LG. Therefore, the drive power supply voltage VDR (VDR = VDD−VEL) corresponding to the difference value between the power supply voltage VDD and the element voltage VEL is applied to the drive circuit 361. The element voltage VEL is a voltage between the anode and the cathode of the electro-optical element E (voltage between the ground line LG and the node N).

  The drive circuit 361 is means for generating and outputting a drive current IEL based on the drive power supply voltage VDR. The electro-optical element E emits light when supplied with the driving current IEL. FIG. 3 is a graph showing the relationship between the drive current IEL flowing through the electro-optic element E and the element voltage VEL at different temperatures. As shown in the figure, even when the same drive current IEL is supplied from the drive circuit 361, the element voltage VEL changes according to the temperature of the environment in which the electro-optical device H is used. Although FIG. 3 illustrates the change in the element voltage VEL according to the temperature, the element voltage VEL varies depending on various factors such as the elapsed time from the manufacture of the electro-optical element E.

  As shown in FIG. 2, the drive circuit 361 includes a p-channel type drive transistor QDR and an n-channel type period control transistor QCT. The source of the driving transistor QDR is connected to the power supply line LP, and the drain is connected to the drain of the period control transistor QCT. The source of the period control transistor QCT is connected to the node N (the anode of the electro-optic element E).

  The drive transistor QDR is a constant current source that generates a drive current IEL according to the gate voltage VG. The period control transistor QCT is selectively turned on or off according to the gate voltage VON. The time density at which the control transistor QCT is turned on is controlled according to the gradation specified for the electro-optic element E. In a period in which the period control transistor QCT is kept on, the electro-optical element E emits light by supplying the drive current IEL via the period control transistor QCT. In the period in which the period control transistor QCT is maintained in the OFF state, the supply of the drive current IEL is stopped, so that the electro-optical element E is turned off. Therefore, the electro-optical element E is controlled to a designated gradation (light emission amount). In order for the drive circuit 361 to properly generate the drive current IEL corresponding to the gate voltage VG, it is necessary to maintain the drive power supply voltage VDR between the power supply line LP and the node N at a required voltage value VDR0 or more. In the present embodiment, the case where the gradation of the electro-optical element E is controlled according to the pulse width of the driving current IEL is exemplified. However, the electro-optical element E according to the current value (gate voltage VG) of the driving current IEL. The gradation may be controlled.

  The measurement circuit 363 measures the element voltage VEL at the node N. The measurement circuit 363 of this embodiment is an A / D converter that outputs digital data (hereinafter referred to as “measurement data”) DV corresponding to the element voltage VEL. The control circuit 121 generates a reference voltage VREF corresponding to the n pieces of measurement data DV output from the measurement circuit 363 of each unit circuit U and outputs the reference voltage VREF to the power supply circuit 123. A method for selecting the reference voltage VREF will be described later.

  FIG. 4 is a circuit diagram showing a configuration of the power supply circuit 123. The power supply circuit 123 of this embodiment is a step-down switching regulator that generates a power supply voltage VDD under the control of the switching element 21. The switching element 21 in FIG. 4 switches between conduction and non-conduction between the terminal TIN to which the input voltage VIN is supplied and the terminal TOUT to which the power supply voltage VDD is output. The capacitor C1 stabilizes the input voltage VIN. The smoothing circuit 22 including the coil L, the capacitor C2, and the diode D smoothes the ripple of the power supply voltage VDD synchronized with the operation of the switching element 21.

  The comparison circuit 23 outputs a binary signal CMP according to the magnitude of the feedback voltage VFB obtained by dividing the power supply voltage VDD by the resistors R1 and R2 and the reference voltage VREF supplied from the control circuit 121. The signal CMP is supplied to the reset terminal (R) of the flip-flop (RS type) 25. A triangular wave CLK is supplied from the oscillator 24 to the set terminal (S) of the flip-flop 25. The signal output from the output terminal (Q) of the flip-flop 25 is supplied to the gate of the switching element 21 via the buffer, so that the switching element 21 has a time corresponding to the magnitude of the feedback voltage VFB and the reference voltage VREF. Turns on intermittently at density. Therefore, the power supply voltage VDD corresponding to the reference voltage VREF is smoothed by the smoothing circuit 22 and then output from the output terminal TOUT to the power supply line LP.

  Next, a specific operation of the control circuit 121 will be described with reference to FIG. The processing in FIG. 5 is repeatedly executed at a predetermined cycle within the period when the electro-optical device H is powered on. First, the control circuit 121 initializes the reference voltage VREF output to the power supply circuit 123 to a predetermined voltage value (for example, the same voltage value as the input voltage VIN) (step S11). The power supply circuit 123 generates a power supply voltage VDD corresponding to the reference voltage VREF and outputs it to each drive circuit 361. The drive circuit 361 drives the electro-optical element E by supplying a drive current IEL generated according to the power supply voltage VDD. The control circuit 121 acquires the measurement data DV corresponding to the element voltage VEL at the time of driving from the measurement circuit 363 of each unit circuit U (step S12).

  Next, the control circuit 121 specifies the maximum value VEL_max of the element voltage VEL based on the n pieces of measurement data DV acquired from each measurement circuit 363 (step S13). Then, the control circuit 121 sets the reference voltage VREF so that the difference value (VDD−VEL_max) between the power supply voltage VDD and the maximum value VEL_max is equal to the required voltage value VDR0 (step S14). Output to the circuit 123 (step S15). Since the difference value between the power supply voltage VDD and the element voltage VEL corresponds to the drive power supply voltage VDR, the drive power supply voltage VDR of the drive circuit 361 corresponding to the electro-optical element E with the element voltage VEL having the maximum value VEL_max is n drives. It becomes the minimum value in the circuit 361. Therefore, it can be said that the process of step S14 is a process of selecting the reference voltage VREF so that the drive power supply voltage VDR becomes the minimum value within a range (VDR> VDR0) in which all the drive circuits 361 can be properly operated. .

  Next, FIG. 6 is a graph showing the relationship between the drive power supply voltage VDR / power supply voltage VDD and the maximum value VEL_max of the element voltage VEL when the power supply voltage VDD is controlled by the above procedure. As shown in the figure, the control circuit 121 controls the power supply circuit 123 so that the power supply voltage VDD decreases as the maximum value VEL_max is lower. That is, the power supply voltage VDD1 when the element voltage VEL is the voltage value VEL1 is lower than the power supply voltage VDD2 when the element voltage VEL is the voltage value VEL2 exceeding the voltage value VEL1. More specifically, the power supply voltage VDD in this embodiment is controlled such that the drive power supply voltage VDR is maintained at the required voltage value VDR0 regardless of the element voltage VEL.

  As described above, according to the configuration in which the power supply voltage VDD is controlled in accordance with the element voltage VEL, the power consumed by the drive circuit 361 compared to the conventional configuration in which the power supply voltage VDD is fixed to a predetermined voltage value. There is an advantage that is reduced. This effect will be described in detail as follows.

First, the power consumed in each part of the electro-optical device H when driving one electro-optical element E is examined. Since a common drive current IEL flows through the electro-optic element E and the drive circuit 361, the power PEL consumed by the electro-optic element E and the power PDR consumed by the drive circuit 361 are expressed by the following equations.
PEL = VEL × IEL (1)
PDR = VDR × IEL = (VDD−VEL) × IEL (2)
Therefore, the power PMD consumed by the head module 30 is expressed by the following equation (3).
PMD = PEL + PDR = VDD × IEL (3)

If the power supply circuit 123 operates with an efficiency η (0 <η ≦ 1), the power PPC consumed by the power supply circuit 123 is expressed by the following equation (4). Since the power supply circuit 123 of this embodiment is a switching regulator, the efficiency η is a substantially fixed value regardless of the power supply voltage VDD.
PPC = VDD × IEL × (1-η) / η (4)
Accordingly, the total sum PT of the electric power required for driving one electro-optical element E is expressed by Equation (5).
PT = VIN × IIN = PMD + PPC = VDD × IEL / η (5)
The current IIN in equation (5) is a current that flows into the power supply circuit 123 from an external power supply.

  Next, FIG. 7 is a graph showing the relationship between the power calculated by the above equations (2) to (5) and the element voltage VEL (maximum value VEL_max). In the figure, each power (VDD control) in the configuration of this embodiment for controlling the power supply voltage VDD according to the element voltage VEL and the conventional power supply voltage VDD maintained at a fixed value regardless of the element voltage VEL. Each power in the configuration (fixed to VDD) is also shown. In FIG. 7, it is assumed that the drive current IEL is 1A for convenience and the efficiency η is 0.8.

  As shown in FIG. 7, the power PDR consumed by the drive circuit 361 under the conventional configuration increases as the element voltage VEL decreases. On the other hand, the power PDR in the present embodiment is maintained at a fixed value regardless of the element voltage VEL. In other words, according to the present embodiment, it is possible to reduce the power ΔP consumed by the drive circuit 361 in the conventional configuration. Therefore, the power PT consumed in the entire electro-optical device H and the power PMD consumed in the head module 30 are also reduced as compared with the conventional configuration.

  Further, since the heat generation in the drive circuit 361 is suppressed by suppressing the power PDR, there is also an effect that fluctuation and deterioration of the gradation (light emission amount) of the electro-optical element E due to heating can be suppressed. Further, in the present embodiment, since the power supply circuit 123 is mounted on the wiring board 10 separate from the head module 30 (substrate 32), the amount of heat reaching the head module 30 from the power supply circuit 123 is reduced, and as a result, However, deterioration of the electro-optic element E due to heating is suppressed.

<B: Second Embodiment>
Next, a second embodiment of the present invention will be described. In the present embodiment, elements having the same functions and functions as those of the first embodiment are denoted by the same reference numerals as above, and detailed descriptions thereof are omitted as appropriate.

  FIG. 8 is a block diagram illustrating a configuration of the electro-optical device H according to the present embodiment. As shown in the figure, the control circuit 121 includes a reference setting circuit 125 and a determination circuit 126. The reference setting circuit 125 is a circuit that variably generates a reference voltage VREF1 and outputs it to each unit circuit U. The reference setting circuit 125 of this embodiment sequentially increases the reference voltage VREF1 at a predetermined cycle.

  Each unit circuit U includes a drive circuit 361 and a comparison circuit 365. The drive circuit 361 has the same configuration as in the first embodiment. The comparison circuit 365 is a means for outputting a signal C corresponding to the magnitude of the reference voltage VREF1 output from the reference setting circuit 125 and the element voltage VEL. More specifically, the comparison circuit 365 sets the signal C to a low level when the reference voltage VREF1 is lower than the element voltage VEL, and transitions the signal C to a high level when the reference voltage VREF1 exceeds the element voltage VEL. Since each element voltage VEL differs depending on the characteristics of each electro-optical element E, the high-level signal C gradually increases as the reference voltage VREF1 increases.

  The determination circuit 126 determines whether or not the result of the comparison by each comparison circuit 365 satisfies a predetermined condition. The determination circuit 126 of the present embodiment is a means for outputting an instruction to the reference setting circuit 125 when all (n-system) signals C have transitioned to a high level (for example, a circuit that outputs a logical product of n-system signals C). ). The reference setting circuit 125 outputs the reference voltage VREF1 output to each unit circuit U to the power supply circuit 123 as the reference voltage VREF2 when the determination circuit 126 outputs an instruction.

  Since all the signals C have transitioned to the high level means that the reference voltage VREF1 has reached the maximum value VEL_max of the element voltage VEL, the reference setting circuit 125 is similar to step S15 in the first embodiment. The reference voltage VREF2 corresponding to the maximum value VEL_max of the element voltage VEL is output. The power supply circuit 123 is a switching regulator similar to that in FIG. 4, and generates a power supply voltage VDD corresponding to the reference voltage VREF2. For example, the power supply circuit 123 generates the power supply voltage VDD so that the difference value (drive power supply voltage VDR) between the power supply voltage VDD and the maximum value VEL_max is equal to the required voltage value VDR0.

  As described above, since the power supply voltage VDD corresponding to the element voltage VEL (maximum value VEL_max) is generated also in the present embodiment, the same effects as those of the first embodiment can be obtained. Furthermore, in the present embodiment, it is sufficient to arrange the comparison circuits 365 that are easier to simplify the configuration than the measurement circuit 363 (A / D converter) of the first embodiment in each unit circuit U. There is an advantage that the scale of each unit circuit U (and also the IC chip 36) is reduced as compared with the embodiment.

  In the above description, the configuration in which the reference voltage VREF1 is increased is illustrated, but a configuration in which the reference setting circuit 125 sequentially decreases the reference voltage VREF2 is also employed. In this configuration, the signal C from each comparison circuit 365 is initially maintained at a high level, and the determination circuit 126 sets the reference when the voltage is lower than the element voltage VEL (that is, the maximum value VEL_max) in any unit circuit U. An instruction is output to the circuit 125. Accordingly, it is possible to quickly determine the reference voltage VREF2 as compared with the configuration in which the reference voltage VREF1 is increased. In addition, regardless of the direction in which the reference voltage VREF1 is changed, the determination circuit 126 monitors the transition of the signal C from each comparison circuit 365 so that the determination circuit 126 detects an abnormality in each unit circuit U (for example, an anode and a cathode It is also possible to detect the presence or absence of a short circuit, insulation, an increase in element voltage VEL due to manufacturing defects, and the like.

<C: Modification>
Various modifications can be made to each of the above embodiments. An example of a specific modification is as follows. In addition, you may combine each following aspect suitably.

(1) Modification 1
In the first embodiment, the configuration in which the element voltage VEL is measured for all the electro-optical elements E is exemplified. However, the element voltage VEL is measured for only a part of the electro-optical elements E in the element unit 34, and this measurement is performed. The power supply voltage VDD may be controlled based on the value. Therefore, a configuration in which the measurement circuit 363 is provided in all the unit circuits U is not always necessary.

  Further, a predetermined number of electro-optic elements E (hereinafter referred to as “sample elements”) are selected in order from the highest element voltage VEL, and the element voltage VEL of each sample element E is operated during operation of the electro-optic device H. The power supply voltage VDD may be controlled based on the above. For example, immediately after the manufacture of the electro-optical device H, the determination circuit 126 in FIG. 9 selects a predetermined number of element voltages VEL in order from the comparison result by each comparison circuit 365, and each element voltage selected here is selected. A sample element E corresponding to VEL is specified. Then, at the stage where the electro-optical device H is used, the maximum value VEL_max is specified from the predetermined number of element voltages VEL based on the comparison result by the comparison circuit 365 corresponding to the sample element E, and the maximum value VEL_max is obtained. Based on this, the power supply voltage VDD is controlled. According to the above configuration, there is an advantage that the process of monitoring each element voltage VEL is simplified.

(2) Modification 2
In each of the above embodiments, the configuration in which the switching regulator is employed in the power supply circuit 123 is illustrated, but a series regulator such as a three-terminal regulator may be used as the power supply circuit 123. Note that the series regulator generates the power supply voltage VDD by lowering the input voltage VIN by releasing Joule heat. Therefore, even if the power consumption PDR of the drive circuit 361 is reduced by controlling the power supply voltage VDD according to the element voltage VEL, this reduction is only released as Joule heat in the power supply circuit 123, and the electro-optical device H The power PT consumed as a whole is the same as the conventional configuration in which the power supply voltage VDD is fixed. However, since the reduction in the power consumption PDR in the drive circuit 361 is consumed in the power supply circuit 123 mounted on the wiring board 10 separate from the head module 30, the heat generation in the drive circuit 361 and the electric power caused by this are generated. The effect that the deterioration of the optical element E is suppressed is certainly exhibited.

(3) Modification 3
The drive power supply voltage VDR is not necessarily maintained at the required voltage value VDR0. For example, in FIG. 9, the power supply voltage VDD is controlled so that the difference value (drive power supply voltage VDR) between the power supply voltage VDD and the maximum value VEL_max of the element voltage VEL is equal to or higher than the required voltage value VDR0 (VDD−VEL_max ≧ VDR0). 6 is a graph showing the relationship between the power supply voltage VDD / drive power supply voltage VDR and the element voltage VEL. In the figure, the drive power supply voltage VDR (straight line VDR in FIG. 13) in the conventional configuration in which the power supply voltage VDD is fixed is also shown by a two-dotted line. As shown in the figure, even if the drive power supply voltage VDR is not maintained at the required voltage value VDR0, the power supply voltage VDD is fixed as long as the power supply voltage VDD is controlled according to the maximum value VEL_max. The desired effect of reducing the drive power supply voltage VDR (and the power PDR consumed by the drive circuit 361) as compared with the configuration is achieved.

(4) Modification 4
In the above embodiment, the configuration in which the drive circuit 361 is arranged on the power supply line LP side and the electro-optical element E is arranged on the ground line LG side is illustrated. However, as shown in FIG. The optical element E may be disposed and the drive circuit 361 may be disposed on the ground line LG side. In the above embodiment, the configuration for controlling the potential of the power supply line LP is exemplified. However, instead of this configuration or together with this configuration, the potential (VSS) of the ground line LG is changed to the element voltage VEL (maximum value VEL_max). You may control according to it.

(5) Modification 5
The organic light emitting diode element is only an example of the electro-optical element E. Regarding the electro-optic element applied to the present invention, there is no distinction between a self-luminous type that emits light itself and a non-luminous type that changes the transmittance of external light (for example, a liquid crystal element). For example, inorganic EL elements, field emission (FE) elements, surface-conduction electron (SE) elements, ballistic electron surface emitting (BS) elements, and light emitting diode (LED) elements Various electro-optical elements such as a liquid crystal element, an electrophoretic element, and an electrochromic element can be used in the present invention.

<D: Application example>
Next, the configuration of the image forming apparatus using the electro-optical device H according to each of the above embodiments will be described.
FIG. 11 is a cross-sectional view illustrating a configuration of an image forming apparatus employing the electro-optical device H according to each of the above embodiments. The image forming apparatus is a tandem type full-color image forming apparatus, and the four electro-optical devices H (HK, HC, HM, and HY) according to the above embodiment and four photosensitive devices corresponding to the electro-optical devices H are used. Body drum 70 (70K, 70C, 70M, 70Y). One electro-optical device H is disposed so as to face the image forming surface (outer peripheral surface) of the corresponding photosensitive drum 70. Note that the subscripts “K”, “C”, “M”, and “Y” of each symbol are used for forming each visible image of black (K), cyan (C), magenta (M), and yellow (Y). Means.

  As shown in FIG. 11, an endless intermediate transfer belt 72 is wound around a driving roller 711 and a driven roller 712. The four photosensitive drums 70 are arranged around the intermediate transfer belt 72 at a predetermined interval from each other. Each photosensitive drum 70 rotates in synchronization with driving of the intermediate transfer belt 72.

  In addition to the electro-optical device H, a corona charger 731 (731K, 731C, 731M, 731Y) and a developing unit 732 (732K, 732C, 732M, 732Y) are arranged around each photosensitive drum 70. The corona charger 731 uniformly charges the image forming surface of the photosensitive drum 70 corresponding thereto. Each electro-optical device H exposes this charged image forming surface to form an electrostatic latent image. Each developing device 732 forms a visible image (visible image) on the photosensitive drum 70 by attaching a developer (toner) to the electrostatic latent image.

  As described above, the visible images of the respective colors (black, cyan, magenta, yellow) formed on the photosensitive drum 70 are sequentially transferred (primary transfer) to the surface of the intermediate transfer belt 72 to form a full-color visible image. Is done. Four primary transfer corotrons (transfer devices) 74 (74K, 74C, 74M, and 74Y) are arranged inside the intermediate transfer belt 72. Each primary transfer corotron 74 electrostatically attracts a visible image from the corresponding photosensitive drum 70, thereby developing a visible image on the intermediate transfer belt 72 that passes through the gap between the photosensitive drum 70 and the primary transfer corotron 74. Transcript.

  The sheets (recording material) 75 are fed one by one from the paper feed cassette 762 by the pickup roller 761 and conveyed to the nip between the intermediate transfer belt 72 and the secondary transfer roller 77. The full-color visible image formed on the surface of the intermediate transfer belt 72 is transferred (secondary transfer) to one side of the sheet 75 by the secondary transfer roller 77 and is fixed to the sheet 75 by passing through the fixing roller pair 78. . The paper discharge roller pair 79 discharges the sheet 75 on which the visible image is fixed through the above steps.

  Since the image forming apparatus exemplified above uses an organic light emitting diode element as a light source (exposure means), the apparatus is made smaller than a configuration using a laser scanning optical system. Note that the electro-optical device H can be applied to an image forming apparatus having a configuration other than those exemplified above. For example, a rotary development type image forming apparatus, an image forming apparatus that directly transfers a visible image from a photosensitive drum to a sheet without using an intermediate transfer belt, or an image forming that forms a monochrome image The electro-optical device H can also be used as the device.

  The use of the electro-optical device H is not limited to the exposure of the photosensitive drum (image carrier). For example, the electro-optical device H is employed in an image reading device as an illumination device that irradiates light to a reading target such as a document. As this type of image reading apparatus, there is a scanner, a copying machine or a reading part of a facsimile, a barcode reader, or a two-dimensional image code reader for reading a two-dimensional image code such as a QR code (registered trademark).

  In addition, the electro-optical device in which the electro-optical elements E are arranged in a matrix is also used as a display device for various electronic devices. Examples of the electronic device to which the present invention is applied include a portable personal computer, a mobile phone, a personal digital assistant (PDA), a digital still camera, a television, a video camera, a car navigation device, a pager, and an electronic notebook. , Electronic paper, calculators, word processors, workstations, videophones, POS terminals, printers, scanners, copiers, video players, devices with touch panels, and the like.

1 is a block diagram illustrating a configuration of an electro-optical device according to a first embodiment. It is a block diagram which shows the structure of one unit circuit and its peripheral element. It is a graph which shows a mode that the relationship between element voltage VEL and drive current IEL changes according to temperature. It is a circuit diagram which shows the structure of a power supply circuit. It is a flowchart which shows the flow of operation | movement of a control circuit. It is a graph which shows the relationship between power supply voltage VDD and drive power supply voltage VDR, and element voltage VEL. It is a graph which shows the relationship between element voltage VEL and the electric power consumed by each part. FIG. 6 is a block diagram illustrating a configuration of an electro-optical device according to a second embodiment. It is a graph which shows the relationship between the power supply voltage VDD and the drive power supply voltage VDR in the modification, and the element voltage VEL. It is a block diagram which shows the relationship between the drive circuit and electro-optical element in a modification. It is a longitudinal cross-sectional view which shows the specific form (image forming apparatus) of an electronic device. It is a block diagram which shows the relationship between the drive circuit and electro-optical element in a prior art. It is a graph which shows the relationship between the power supply voltage VDD and the drive power supply voltage VDR in the prior art, and the element voltage VEL.

Explanation of symbols

H: electro-optical device, 10: wiring board, 12: voltage control circuit, 121: control circuit, 123: power supply circuit, 21: switching element, 30: head module, 32: substrate, 34 ...... Element part, E ... Electro-optic element, 36 ... IC chip, U ... Unit circuit, 361 ... Drive circuit, 363 ... Measurement circuit.

Claims (11)

A first power supply line and a second power supply line to which a power supply voltage is applied;
An electro-optic element disposed on a path connecting the first power supply line and the node and driven by a drive current flowing in the path;
A drive circuit that generates the drive current using a voltage between the second power supply line and the node as a drive power supply voltage;
The power supply voltage when the element voltage between the node and the first power supply line at the time of driving the electro-optical element is a first voltage value is a second voltage value in which the element voltage exceeds the first voltage value. And a voltage control circuit for controlling the power supply voltage so as to be lower than the power supply voltage in the case of The electro-optical device according to claim 1, wherein the voltage control circuit controls the power supply voltage so that the power supply voltage decreases as the element voltage decreases. The electro-optical device according to claim 1, wherein the voltage control circuit controls the power supply voltage so that a drive power supply voltage of the drive circuit is maintained at a predetermined value regardless of the element voltage. A plurality of the electro-optic elements;
A plurality of the drive circuits corresponding to the respective electro-optic elements,
In the voltage control circuit, a drive power supply voltage of a drive circuit corresponding to an electro-optic element having a maximum element voltage among the plurality of electro-optic elements is equal to or higher than a required voltage value necessary for the operation of each of the drive circuits. The electro-optical device according to any one of claims 1 to 3, wherein the power supply voltage is controlled as described above. The electro-optical device according to any one of claims 1 to 4, wherein the voltage control circuit includes a switching regulator that variably generates the power supply voltage. Comprising a substrate on which the drive circuit and the electro-optic element are disposed;
The electro-optical device according to claim 5, wherein the switching regulator is mounted on a wiring board separate from the board. Comprising a substrate on which the drive circuit and the electro-optic element are disposed;
The voltage control circuit includes a series regulator that variably generates the power supply voltage,
The electro-optical device according to claim 1, wherein the series regulator is mounted on a wiring board separate from the board. A reference setting circuit that variably generates a reference voltage;
A plurality of the electro-optic elements;
A plurality of the drive circuits corresponding to the electro-optical elements;
A plurality of comparison circuits for comparing the element voltage of each electro-optic element and the reference voltage;
The voltage control circuit specifies a maximum value from among the element voltages according to a comparison result by the comparison circuits, and the drive power supply voltage of the drive circuit corresponding to the maximum value is used for the operation of the drive circuits. The electro-optical device according to claim 1, wherein the power supply voltage is controlled to be equal to or higher than a necessary required voltage value. The voltage control circuit selects a predetermined number of element voltages according to the result of the comparison by each comparison circuit, and the drive power supply voltage of the drive circuit corresponding to the maximum value among the predetermined number of element voltages is a predetermined value. The electro-optical device according to claim 8, wherein the power supply voltage is repeatedly controlled so as to satisfy.   An electronic apparatus comprising the electro-optical device according to claim 1. An electro-optic element disposed on a path connecting the first power line and the second power line to which a power supply voltage is applied, the first power line and the node, and driven by a drive current flowing in the path; A method of controlling an electro-optical device comprising a drive circuit that generates the drive current using a voltage between two power supply lines and the node as a drive power supply voltage,
The power supply voltage when the element voltage between the node and the first power supply line at the time of driving the electro-optical element is a first voltage value is a second voltage value in which the element voltage exceeds the first voltage value. The control method of the electro-optical device, wherein the power supply voltage is controlled to be lower than the power supply voltage in the case of

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