Patent Publication Number: US-10761470-B2

Title: Printing apparatus and light-emitting element driving device

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a printing apparatus and a light-emitting element driving device. 
     Description of the Related Art 
     An electrophotographic printing apparatus (a laser printer or the like) includes a light-emitting element configured to irradiate a photosensitive drum with a laser beam. Among printing apparatuses, there is a printing apparatus having an auto power control (APC) function of controlling driving of a light-emitting element such that a laser beam is maintained at an appropriate light amount (target value). Japanese Patent Laid-Open No. 2017-63110 discloses a printing apparatus having an APC function, which includes a light-emitting element, a light-receiving element configured to output a monitor current corresponding to a light emission amount of the light-emitting element, a determination unit configured to compare the monitor current with a reference current, and a driving unit configured to drive the light-emitting element based on a comparison result by the determination unit. 
     SUMMARY OF THE INVENTION 
     In the arrangement of Japanese Patent Laid-Open No. 2017-63110, the monitor current and the reference current are input to the inverting input terminal of a comparator used in the determination unit, and a reference voltage is input to the noninverting input terminal. When performing APC, the comparator operates such that the voltage of the inverting input terminal equals the reference voltage. Hence, a reverse bias voltage applied to the light-receiving element at the time of the APC operation is decided by the difference between the reference voltage and a power supply voltage, which are constant voltages. Since the reverse bias voltage applied to the light-receiving element influences the characteristics of the light-receiving element such as a response speed and a dark current amount, the controllability of APC can be improved by controlling the reverse bias voltage. 
     Some embodiments of the present invention provide a technique advantageous in improving the controllability of APC. 
     According to some embodiments, a printing apparatus comprising: a light-emitting element; a light-receiving element including a first terminal and a second terminal, driven by a reverse bias voltage applied between the first terminal and the second terminal, and configured to detect a light emission amount of the light-emitting element; a reference current generation unit configured to supply a reference current to a node connected to the second terminal; a comparison unit configured to compare a monitor current with the reference current, the light-receiving element supplying the monitor current to the second terminal in accordance with the light emission amount; a driving unit configured to drive the light-emitting element based on an output of the comparison unit; and a reference voltage control unit configured to control a voltage of the second terminal, wherein the comparison unit includes a first input terminal connected to the second terminal, and a second input terminal, and the reference voltage control unit is configured to supply a reference voltage selected from at least two voltage values to the second input terminal, and to control the voltage of the second terminal to be a voltage according to the reference voltage, is provided. 
     According to some other embodiments, a printing apparatus comprising: a light-emitting element; a light-receiving element including a first terminal and a second terminal, driven by a reverse bias voltage applied between the first terminal and the second terminal, and configured to detect a light emission amount of the light-emitting element; a reference current generation unit configured to supply a reference current to a current path; a comparison unit configured to compare a monitor current with the reference current, the monitor current being supplied to the current path based on a detection amount of the light-receiving element according to the light emission amount; a driving unit configured to drive the light-emitting element based on an output of the comparison unit; a reference voltage control unit configured to generate a reference voltage selected from at least two voltage values to control a voltage of the second terminal; and a reverse bias voltage control unit arranged between the second terminal and the comparison unit and configured to receive the reference voltage from the reference voltage control unit and to control the second terminal to a voltage according to the reference voltage, wherein the comparison unit comprises a first input terminal connected to the current path, is provided. 
     According to still other embodiments, a light-emitting element driving device comprising: a driving terminal configured to output a driving signal used to drive a light-emitting element; a monitor terminal configured to receive a monitor current output from a light-receiving element configured to detect a light emission amount of the light-emitting element; a reference current generation unit configured to supply a reference current to a node connected to the monitor terminal; a comparison unit configured to compare the monitor current input from the light-receiving element to the monitor terminal with the reference current; a driving unit configured to generate the driving signal based on an output of the comparison unit; and a reference voltage control unit configured to control a voltage of the monitor terminal, wherein the comparison unit includes a first input terminal connected to the monitor terminal, and a second input terminal, and the reference voltage control unit is configured to supply a reference voltage selected from at least two voltage values to the second input terminal, and to control the voltage of the monitor terminal to be a voltage according to the reference voltage, is provided. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing an example of the arrangement of a printing apparatus according to the embodiment of the present invention; 
         FIGS. 2A and 2B  are timing charts showing an example of the operation of the printing apparatus shown in  FIG. 1 ; 
         FIG. 3  is a circuit diagram showing a modification of the printing apparatus shown in  FIG. 1 ; and 
         FIG. 4  is a circuit diagram showing a modification of the printing apparatus shown in  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     A detailed embodiment of a printing apparatus according to the present invention will now be described with reference to the accompanying drawings. Note that in the following description and drawings, common reference numerals denote common components throughout a plurality of drawings. Hence, the common components will be described by cross-referencing the plurality of drawings, and a description of components denoted by common reference numerals will appropriately be omitted. 
     The structures and operations of a printing apparatus according to this embodiment and a light-emitting element driving device included in the printing apparatus will be described with reference to  FIGS. 1, 2A, and 2B .  FIG. 1  is a circuit diagram showing an example of the arrangement of a printing apparatus  100  according to the first embodiment. The printing apparatus  100  includes a light-emitting element  110 , a light-receiving element  120 , a light-emitting element driving device  300  (to be sometimes referred to as a device  300  hereinafter), and a photosensitive drum  400 . The device  300  includes a comparison unit  130 , a driving unit  140 , a current generation unit  150 , a reference current generation unit  160 , a control unit  170 , a reference voltage control unit  180 , and a switch element SW 1 . In addition, the device  300  includes a terminal T 1  (electrode pad) configured to output a driving signal used to drive the light-emitting element  110 , and a terminal T 2  (electrode pad) configured to receive a current output from the light-receiving element  120  that detects the light emission amount of the light-emitting element  110 . 
     The light-emitting element  110  has an anode connected to a power supply voltage VCC, and a cathode connected to the terminal T 1 . The light-emitting element  110  may be, for example, a laser diode or the like. The light-emitting element  110  emits light when driven by a driving signal supplied from the driving unit  140  via the terminal T 1 , and the photosensitive drum  400  is irradiated with the emitted light (for example, a laser beam). 
     The light-receiving element  120  has a cathode terminal (first terminal) connected to the power supply voltage VCC, and an anode terminal (second terminal) connected to the terminal T 2 . The light-receiving element  120  may be, for example, a photoelectric conversion element such as a photodiode. The light-receiving element  120  is driven by a reverse bias voltage applied between the cathode terminal and the anode terminal, and receives the light from the light-emitting element  110 , thereby detecting the light emission amount of the light-emitting element  110 . The light-receiving element  120  outputs a monitor current Im corresponding to the light emission amount of the light-emitting element  110  to the terminal T 2  via the anode terminal. 
     Constituent elements included in the device  300  will be described next. The control unit  170  may be, for example, a CPU or a processor configured to control a printing operation. The control unit  170  controls the current generation unit  150 , the reference voltage control unit  180 , the comparison unit  130 , and the switch element SW 1  using control signals sig 1 , sig 2 , and sig 3 . 
     In accordance with the control signal sig 1  output from the control unit  170 , the current generation unit  150  generates a standard current T 1  that is a constant current according to the target value of the light emission amount of the light-emitting element  110 . The current generation unit  150  supplies the standard current T 1  to the reference current generation unit  160 . 
     The reference current generation unit  160  is connected to the current generation unit  150  and a current path CP connected to the terminal T 2 . The reference current generation unit  160  receives the standard current T 1  from the current generation unit  150 , and supplies, to the current path CP, a reference current I 2  of a value obtained by multiplying the value of the standard current T 1  by a predetermined ratio. In other words, the reference current generation unit  160  supplies the reference current I 2  to a node connected to the anode terminal of the light-receiving element  120 . The reference current I 2  may be referred to as a “target current” in correspondence with the target value of the light emission amount of the light-emitting element  110 . In other words, the reference current generation unit  160  supplies, to the current path CP, the reference current I 2  used to control the light emission amount of the light-emitting element  110  to a target value. In addition, the above-described current generation unit  150  supplies the standard current I 1  according to the reference current I 2  to the reference current generation unit  160 . The reference current generation unit  160  may be formed by, for example, NMOS transistors. In this embodiment, the reference current generation unit  160  includes a current mirror circuit formed by transistors M 1  and M 2  that are NMOS transistors. 
     Here, a node to which the standard current I 1  from the current generation unit  150  flows and which corresponds to the input terminal of the current mirror circuit of the reference current generation unit  160  is defined as a node n 1 . In addition, the ground node of the current mirror circuit of the reference current generation unit  160  is defined as a node n 2 . Furthermore, a node to which the reference current I 2  flows and which corresponds to the output terminal of the current mirror circuit of the reference current generation unit  160  is defined as a node n 3 . That is, the node n 3  is connected to the current path CP and connected to the anode terminal of the light-receiving element  120 . 
     The transistor M 1  that forms the current mirror circuit of the reference current generation unit  160  is arranged such that the drain and the gate are connected to the node n 1 , and the source is connected to the node n 2 . In addition, the transistor M 2  that forms the current mirror circuit of the reference current generation unit  160  is arranged such that the gate is connected to the node n 1 , the source is connected to the node n 2 , and the drain is connected to the node n 3 . The transistor M 2  supplies, to the current path CP, the reference current I 2  of a value obtained by multiplying the value of the standard current I 1  flowing to the transistor M 1  by a size ratio of the transistor M 1  and the transistor M 2 . The size ratio of the transistor M 1  and the transistor M 2  corresponds to the current conversion ratio of the reference current generation unit  160 , and can also be expressed as the mirror ratio of the current mirror circuit. 
     In this embodiment, the reference current generation unit  160  configured to perform current/current conversion between the standard current I 1  and the reference current I 2  by the simple current mirror circuit with a gain of 1 has been described. However, the present invention is not limited to this. For example, the reference current generation unit  160  may have a circuit arrangement that includes a plurality of current mirror circuits having mirror ratios different from each other and can convert the standard current I 1  by a plurality of current conversion ratios (gains). In this case, the reference current generation unit  160 , for example, selects a setting of a gain from the plurality of gains in accordance with the control signal output from the control unit  170 , and outputs the reference current I 2  according to the target value of the light emission amount of the light-emitting element  110 . In addition, the reference current generation unit  160  may use, for example, the arrangement of a cascode current mirror circuit to improve the accuracy of the reference current I 2  to be output. 
     The reference voltage control unit  180  controls the voltage of the anode terminal of the light-receiving element  120  via the terminal T 2 , as will be described later in detail. The reference voltage control unit  180  includes resistors R 1 , R 2 , and R 3 , switch elements SW 2  and SW 3 , a differential input amplifier  190 , and an inverter INV 1 . 
     The resistors R 1 , R 2 , and R 3  are connected in series between the power supply voltage VCC and a ground voltage VSS. One terminal of the switch element SW 2  is connected to a node n 4  that is the connection point between the resistors R 1  and R 2 , and the other terminal is connected to the noninverting input terminal of the differential input amplifier  190 . One terminal of the switch element SW 3  is connected to a node n 5  that is the connection point between the resistors R 2  and R 3 , and the other terminal is connected to the noninverting input terminal of the differential input amplifier  190 . The differential input amplifier  190  has an arrangement of a voltage follower circuit in which the noninverting input terminal and a node n 6  that is the output terminal are connected, and outputs a voltage input to the noninverting input terminal of the differential input amplifier  190  to the node n 6  as a reference voltage VR. The control signal sig 2  is input to the switch element SW 3  and the inverter INV 1 , and a signal whose logic is inverted by the inverter INV 1  is input to the switch element SW 2 . 
     In the reference voltage control unit  180 , when the control signal sig 2  output from the control unit  170  is L (low level), the switch element SW 2  is turned on, and the switch element SW 3  is turned off. Accordingly, a voltage obtained by buffering the voltage of the node n 4  by the differential input amplifier  190  is output as a reference voltage VR. The reference voltage VR in this case will sometimes be referred to as a reference voltage VRH hereinafter. Additionally, in the reference voltage control unit  180 , when the control signal sig 2  output from the control unit  170  is H (high level), the switch element SW 2  is turned off, and the switch element SW 3  is turned on. Accordingly, a voltage obtained by buffering the voltage of the node n 5  by the differential input amplifier  190  is output as the reference voltage VR. The reference voltage VR in this case will sometimes be referred to as a reference voltage VRL hereinafter. 
     As described above, the reference voltage control unit  180  includes a voltage generation unit that generates at least two voltages of different voltage values, and a voltage follower circuit that receives the output from the voltage generation unit. The reference voltage control unit  180  selectively turns on one of the switch element SW 2  and the switch element SW 3  in response to the control signal sig 2  output from the control unit  170 , and outputs one of the reference voltages VRH and VRL. The one of the reference voltages VRH and VRL is supplied to the noninverting input terminal (second input terminal) of the comparison unit  130  to be described later. In other words, the reference voltage control unit  180  supplies the reference voltage VR selected from at least two (two types of) voltage values to the noninverting input terminal of the comparison unit  130 . 
     In this embodiment, an example in which as the voltage generation unit of the reference voltage control unit  180 , a voltage-dividing circuit that divides the power supply voltage VCC by the three resistors R 1  to R 3  to generate two voltages having voltage values different from each other has been described. However, the arrangement of the reference voltage control unit  180  is not limited to this, and the arrangement need only supply or internally generate a plurality of voltages of different voltage values and output one of the voltages in accordance with the control signal sig 2  output from the control unit  170 . 
     The comparison unit  130  compares the monitor current Im with the reference current I 2 , the light-receiving element  120  supplying the monitor current Im to the anode terminal in accordance with the light emission amount of the light-emitting element  110 . The comparison unit  130  includes an inverting input terminal INN (first input terminal) connected to the current path CP, and a noninverting input terminal INP to which the reference voltage VR is supplied. More specifically, the node n 3  corresponding to the output terminal of the current mirror circuit of the reference current generation unit  160  is connected to the inverting input terminal INN via the terminal T 2  and the current path CP via the anode terminal of the light-receiving element  120  and the current path CP. Accordingly, the monitor current Im that flows from the light-receiving element  120  and the reference current I 2  that flows from the reference current generation unit  160  are input to the inverting input terminal INN of the comparison unit  130 . In addition, the node n 6  corresponding to the output terminal of the voltage follower circuit of the reference voltage control unit  180  is connected to the noninverting input terminal INP, and the reference voltage VR is supplied from the reference voltage control unit  180 . 
     The difference between the monitor current Im and the reference current I 2  is current/voltage-converted by the inverting input terminal INN of the comparison unit  130 . If the monitor current Im is larger than the reference current I 2 , the potential (voltage) of the inverting input terminal INN rises. It can be considered that the input capacitance of the inverting input terminal INN is charged by the difference (Im−I 2 ) between the monitor current Im and the reference current I 2  (&lt;Im). From another viewpoint, it may be considered that since the charge amount generated in the light-receiving element  120  per unit time is larger than the reference current I 2 , charges increase in the light-receiving element  120 , and the increased charges raise the potential of the inverting input terminal INN. 
     In addition, if the monitor current Im is smaller than the reference current I 2 , the potential (voltage) of the inverting input terminal INN lowers in the ground voltage direction. It can be considered that discharge from the input capacitance of the inverting input terminal INN is caused by the difference (I 2 −Im) between the monitor current Im and the reference current I 2  (&gt;Im). From another viewpoint, it may be considered that since the charge amount generated in the light-receiving element  120  per unit time is smaller than the reference current I 2 , charges decrease in the light-receiving element  120 , and the decreased charges lower the potential of the inverting input terminal INN. 
     In this embodiment, the comparison unit  130  compares the monitor current Im with the reference current I 2  by the above-described arrangement. Based on the output according to the comparison between the monitor current Im and the reference current I 2  by the comparison unit  130 , the driving unit  140  drives the light-emitting element  110 , and feedback control is performed to control the light emission amount of the light-emitting element  110  to the target value. Hence, when the current value of the monitor current Im and the current value of the reference current I 2  become equal to each other, the potential of the inverting input terminal INN can be equal to the reference voltage VR. The components of the device  300  may operate to determine that the light emission amount of the light-emitting element  110  becomes the target value when such a state occurs. Here, in feedback control, the potential of the inverting input terminal INN need not always equal the reference voltage VR, and it is only necessary to change the light emission amount of the light-emitting element  110  in accordance with the result of comparison between the monitor current Im and the reference current I 2 . 
     Additionally, in this embodiment, the device  300  of the printing apparatus  100  includes the switch element SW 1  configured to connect the inverting input terminal INN and the noninverting input terminal INP of the comparison unit  130 , as shown in  FIG. 1 . An inverted signal obtained by logic-inverting, by an inverter INV 2 , the control signal sig 3  output from the control unit  170  is input to the switch element SW 1 . The control signal sig 3  is a signal used to control the APC operation, and is supplied to the inverter INV 2 , the comparison unit  130 , and the driving unit  140 , as will be described later in detail. 
     The driving unit  140  generates a driving signal used to drive the light-emitting element  110  via the terminal T 1  based on the output of the comparison unit  130 . More specifically, the driving unit  140  includes, for example, an information holding unit (for example, a sampling circuit), and a driver unit. The driving unit  140  holds the output from the comparison unit  130  at the time of completion of APC in the information holding unit as information used to control the light emission amount of the light-emitting element  110  to the target value. In subsequent printing, the driver unit drives the light-emitting element  110  using the driving signal according to the information held in the information holding unit, and the light-emitting element  110  irradiates the photosensitive drum  400  with light in a light emission amount according to the driving signal. 
     As described above, the light-emitting element  110 , the light-receiving element  120 , the comparison unit  130 , the driving unit  140 , the current generation unit  150 , the reference current generation unit  160 , the reference voltage control unit  180 , and the switch element SW 1  constitute a feedback system configured to make the light emission amount of the light-emitting element  110  close to the target value. By feedback control using the feedback system, auto power control (APC) is implemented. In this embodiment, an example in which the anode driving type light-emitting element  110  is used has been described. However, an arrangement using a cathode driving type light-emitting element may be employed. 
     An APC operation according to this embodiment will be described next with reference to  FIGS. 2A and 2B .  FIGS. 2A and 2B  are timing charts showing an APC operation in a case in which one or more APC operations were already ended to control the light-emitting element  110  to a desired light amount, and the APC operation is further performed from this state. For example, when a laser printer performs printing, the APC operation is performed for each line space in some cases. In this case, the APC operation needs to be performed correctly within a predetermined time. 
     In  FIGS. 2A and 2B , the ordinate represents the voltage values of the control signals sig 2  and sig 3  and the terminal T 2 , and the abscissa represents time.  FIG. 2A  shows the APC operation performed when the control signal sig 2  output from the control unit  170  is L (low level) and, accordingly, the reference voltage control unit  180  outputs the reference voltage VRH.  FIG. 2B  shows the APC operation performed when the control signal sig 2  output from the control unit  170  is H (high level) and, accordingly, the reference voltage control unit  180  outputs the reference voltage VRL. 
     Referring to  FIG. 2A , first, when the control signal sig 3  before the comparison between the monitor current Im and the reference current I 2  is L, the driving unit  140  of the comparison unit  130  is inactive. The APC operation is not performed, and the light-emitting element  110  is not driven. Additionally, at this time, the switch element SW 1  to which the inverted signal of the control signal sig 3  is input is turned on, and the inverting input terminal INN and the noninverting input terminal INP of the comparison unit  130  are electrically connected. Hence, the terminal T 2  is electrically connected, via the switch element SW 1 , to the node n 6  that is the output terminal of the reference voltage control unit  180 . That is, the voltage of the anode terminal of the light-receiving element  120  connected to the terminal T 2  becomes the reference voltage VRH (a small voltage drop in the current path CP and the like is ignored here). Hence, a voltage (VCC−VRH) of a value obtained by subtracting the reference voltage VRH from the power supply voltage VCC applied between the cathode terminal and the anode terminal is applied as a reverse bias voltage VPDRH to the light-receiving element  120 . 
     Next, when the control signal sig 3  changes to H, and a period P 11  in which the APC operation of comparing the monitor current Im with the reference current I 2  is performed starts, the comparison unit  130  and the driving unit  140  become active. The driving unit  140  drives the light-emitting element  110  in accordance with the output of the comparison unit  130 . In addition, during the period P 11  in which the APC operation is performed, the switch element SW 1  to which the inverted signal of the control signal sig 3  is input is turned off, and the electrical connection between the inverting input terminal INN and the noninverting input terminal INP of the comparison unit  130  is released. 
     The light-emitting element  110  is driven by the driving unit  140 , the light-receiving element  120  outputs the monitor current Im according to the light emission amount of the light-emitting element  110 , and the comparison unit  130  outputs the result of comparison between the monitor current Im and the reference current I 2  to the driving unit  140 . Accordingly, a feedback loop is formed, and the APC operation is performed. 
     At this time, focus is placed on a terminal voltage VT 2  of the terminal T 2  connected to the anode terminal of the light-receiving element  120 . Immediately after the start of the period P 11 , the monitor current Im is not output due to the response delay of the light-receiving element  120 , and the like, and the switch element SW 1  is turned off. For this reason, the terminal voltage VT 2  lowers in the ground voltage direction from the reference voltage VRH via the transistor M 2  of the reference current generation unit  160 . 
     After that, when the monitor current Im is output from the light-receiving element  120 , the terminal voltage VT 2  rises to the target voltage (reference voltage VRH). Next, when the monitor current Im and the reference current I 2  balance, and the terminal voltage VT 2  converges to the reference voltage VRH by the feedback control, the APC operation is completed. 
     At this time, if the response speed of the light-receiving element  120  is low, and a long time is needed until the value according to the light emission amount of the light-emitting element  110  is output as the monitor current Im, a period P 12  until the terminal voltage VT 2  converges to the target voltage becomes long. In general, the response speed of the light-receiving element  120  changes depending on the voltage value of the reverse bias voltage applied to the light-receiving element  120  when the light-receiving element  120  is driven. The smaller the reverse bias voltage value is, the lower the response speed is. The larger the reverse bias voltage value is, the higher the response speed is. On the other hand, if the reverse bias voltage applied when the light-receiving element  120  is driven is large, the dark current amount of the light-receiving element  120  becomes large. Hence, it can be said that in the APC operation, an appropriate reverse bias voltage used to obtain a desired response speed or dark current amount changes depending on the light-receiving element  120  or the target value of the light emission amount of the light-emitting element  110 . 
     In the operation shown in  FIG. 2B , the control signal sig 2  is H, and the reference voltage control unit  180  outputs the reference voltage VRL. For this reason, when the control signal sig 3  is L, the terminal voltage VT 2  of the terminal T 2  connected to the anode terminal of the light-receiving element  120  changes to the reference voltage VRL via the switch element SW 1 . Hence, a voltage (VCC−VRL) of a value obtained by subtracting the reference voltage VRL from the power supply voltage VCC applied between the cathode terminal and the anode terminal is applied as a reverse bias voltage VPDRL to the light-receiving element  120 . Since the reference voltage VRL is smaller than the above-described reference voltage VRH, as shown in  FIG. 2B , the reverse bias voltage VPDRL applied to the light-receiving element  120  becomes larger than the above-described reverse bias voltage VPDRH. 
     For this reason, when a period P 21  (in this embodiment, the period P 11  and the period P 21  have the same length) in which the control signal sig 3  changes to H, and the APC operation is performed starts, the APC operation is started like the operation shown in  FIG. 2A . However, the value of the reverse bias voltage VPDRL used to drive the light-receiving element  120  is larger than the value of the reverse bias voltage VPDRH in the case shown in  FIG. 2A . As a result, the response speed of the light-receiving element  120  becomes high, and the time until the monitor current Im is output, or a period P 22  until the terminal voltage VT 2  converges to the target voltage (reference voltage VRL) after that becomes short. 
     Here, to avoid an influence on the APC operation, the timing of switching the control signal sig 2  may be in a period (APC non-operation period) in which the above-described feedback loop is not formed. That is, the control unit  170  switches the control signal sig 2  as needed in the period in which the control signal sig 3  is L. 
     Here, referring back to  FIG. 1 , the terminal voltage VT 2  of the terminal T 2  connected to the anode terminal of the light-receiving element  120  after the APC convergence can converge to the reference voltage output from the reference voltage control unit  180  because the feedback loop is formed. That is, it can also be said that the voltage of the anode terminal of the light-receiving element  120  after the APC convergence is controlled by the control signal sig 2 . In addition, a voltage VDS 2  that is the voltage between the drain and the source of the transistor M 2  of the reference current generation unit  160  has the same value as the terminal voltage VT 2 . For this reason, the voltage VDS 2  after the APC convergence can have the same value as the reference voltage VRH when the control signal sig 2  is L, and can have the same value as the reference voltage VRL when the control signal sig 2  is H. 
     Hence, if the control signal sig 2  is L, the voltage VDS 2  is larger, as compared to a case in which the control signal sig 2  is H (VRH&gt;VRL). For this reason, the conversion accuracy of the reference current generation unit  160  may become high. More specifically, for example, if the voltage VDS 2  equals the reference voltage VRL, the value of the voltage VDS 2  between the drain and the source of the transistor M 2  is low, the transistor M 2  operates in a linear region, and a desired current ratio is not obtained in some cases. On the other hand, if the voltage VDS 2  equals the reference voltage VRH larger than the reference voltage VRL, the transistor M 2  operates in a saturation region, and the possibility that a desired current ratio is obtained may be higher than in a case in which the voltage VDS 2  equals the reference voltage VRL. For this reason, if the control signal sig 2  is L, the conversion accuracy of the reference current generation unit  160  may become high. 
     In the above-described way, in this embodiment, the reverse bias voltage to be applied to the light-receiving element  120  can be controlled in accordance with the control signal sig 2  output from the control unit  170 . This makes it possible to control the response speed and the dark current of the light-receiving element  120  and also control the conversion accuracy of the reference current generation unit  160 . 
     This indicates that the reverse bias voltage used to drive the light-receiving element  120 , which changes depending on the target value of the light emission amount of the light-emitting element  110 , can be adjusted by the control signal sig 2 . That is, the controllability of APC can be improved. In addition, even if the characteristic of the light-receiving element  120 , and the like vary, an appropriate reverse bias voltage can be applied to the light-receiving element  120 . That is, the degree of freedom in designing the APC circuit can be improved. 
     For example, if the target value of the light emission amount of the light-emitting element  110  is large, the monitor current Im becomes large, and the response speed of the light-receiving element  120  relatively lowers. As a result, the APC convergence time can be long. In this case, control may be done to select a low voltage as the reference voltage VR to be output from the reference voltage control unit  180  and increase the reverse bias voltage of the light-receiving element. When a low voltage is selected as the reference voltage VR, the response speed of the light-receiving element  120  increases. In addition, if the target value of the light emission amount of the light-emitting element  110  is small, control may be done to select a high voltage as the reference voltage VR and increase the voltage VDS 2  between the source and the drain of the transistor M 2  of the reference current generation unit  160 . This can suppress the dark current generated in the light-receiving element  120 , raise the conversion accuracy of the reference current generation unit  160 , and raise the adjustment accuracy of the light emission amount even upon appropriate light emission of the light-emitting element  110 . 
     For example, to cause the light-emitting element  110  to emit light in a first light amount, the control unit  170  outputs the control signal sig 2  to the reference voltage control unit  180  such that the reference voltage control unit  180  supplies a first voltage as the reference voltage VR. On the other hand, to cause the light-emitting element  110  to emit light in a second light amount larger than the first light amount, the control unit  170  may output the control signal sig 2  to the reference voltage control unit  180  such that the reference voltage control unit  180  supplies, as the reference voltage VR, a second voltage that has an absolute value smaller than that of the first voltage and has the same polarity as the first voltage. 
     As described above,  FIGS. 2A and 2B  are timing charts showing an APC operation in a case in which one or more APC operations were already ended, and the APC operation is further performed from this state. However, this embodiment need not always be applied to this case. For example, this embodiment can also be applied when performing the APC operation in the first calibration step or the like after the printing apparatus is powered on. 
     The structure and operation of a printing apparatus  100  according to this embodiment will be described with reference to  FIG. 3 .  FIG. 3  is a circuit diagram showing an example of the arrangement of a light-emitting element  110 , a light-receiving element  120 , and a light-emitting element driving device  301  (to be sometimes referred to as a device  301  hereinafter) included in the printing apparatus  100  according to the second embodiment. 
     In this embodiment, the cathode terminal of the light-emitting element  110  and the anode terminal of the light-receiving element  120  are connected to a common ground voltage VSS, unlike the printing apparatus  100  according to the above-described first embodiment. That is, the light-emitting element  110  is the cathode driving type light-emitting element  110 . For this reason, since the polarity of the current of a monitor current Im output from the light-receiving element  120  to a terminal T 2  is opposite to that in the first embodiment, a reference current generation unit  160  is formed by transistors M 1  and M 2  using PMOS transistors. In addition, the light-emitting element  110 , a comparison unit  130  a driving unit  140 , an inverter INV 2 , a switch element SW 1 , and a terminal T 1  that outputs a driving signal used to drive the light-emitting element  110  form one group G. The device  301  includes a plurality of groups G. In addition, the device  301  includes an inter-group switch element SW 4 . The remaining components of the printing apparatus  100  may be similar to the components of above-described first embodiment. Hence, the device  301  different from that of the first embodiment will mainly be described here. In addition, for the descriptive convenience, two groups G are arranged on the device  301 , as shown in  FIG. 3 , and are referred to as a group Ga and a group Gb, respectively. 
     As shown in  FIG. 3 , the comparison unit  130 , the driving unit  140 , a current generation unit  150 , and the reference current generation unit  160  are arranged in correspondence with each of the groups Ga and Gb. In addition, the reference voltage control unit  180  may be arranged in correspondence with each of the groups Ga and Gb, like the comparison unit  130  and the driving unit  140 . On the other hand, since one light-receiving element  120  is arranged, the degree of freedom in designing the APC circuit is not greatly decreased even if a reference voltage VR is common to the groups Ga and Gb. To suppress the circuit scale, one reference voltage control unit  180  may be arranged, as shown in  FIG. 3 . 
     In the arrangement shown in  FIG. 3 , to make a discrimination of constituent elements such as the light-emitting element  110  between the group Ga and the group Gb, “a” or “b” is added to the end of each reference numeral or symbol if it is necessary to discriminate which group G a constituent element belongs to. For example, the light-emitting element  110  of the group Ga will be referred to as “light-emitting element  110   a ” (this also applies to the other constituent elements). 
     As shown in  FIG. 3 , the inter-group switch element SW 4  is arranged to connect an inverting input terminal INNa of a comparison unit  130   a  or an inverting input terminal INNb of a comparison unit  130   b  to the terminal T 2 . The inter-group switch element SW 4  selectively connects the terminal T 2  connected to the cathode terminal of the light-receiving element  120  and the comparison unit  130  included in one of the plurality of groups G in accordance with a control signal sig 4  output from a control unit  170 . When the device  301  has such an arrangement, the APC operation for the group Ga and the APC operation for the group Gb can sequentially be performed. 
     More specifically, the inter-group switch element SW 4  electrically connects the terminal T 2  and the inverting input terminal INNa to perform the APC operation of the group Ga and control the light amount of the light-emitting element  110   a . Next, the inter-group switch element SW 4  electrically connects the terminal T 2  and the inverting input terminal INNb to perform the APC operation of the group Gb and control the light amount of the light-emitting element  110   b.    
     According to this embodiment, for example, even if the cathode driving type light-emitting element  110  is used, the same effect as in the above-described first embodiment can be obtained. Additionally, even in the printing apparatus  100  (for example, the printing apparatus  100  compatible with multibeam) in which the plurality of groups G each including the light-emitting element  110 , the comparison unit  130 , and the driving unit  140  are arranged, the same effect as in the above-described first embodiment can be obtained for each light-emitting element  110 . Additionally, in the arrangement shown in  FIG. 3 , an example in which the two groups G including the group Ga and the group Gb are arranged on the device  301  has been described. However, three or more groups G may be arranged. 
     The structure and operation of a printing apparatus  100  according to this embodiment will be described with reference to  FIG. 4 .  FIG. 4  is a circuit diagram showing an example of the arrangement of a light-emitting element  110 , a light-receiving element  120 , and a light-emitting element driving device  302  (to be sometimes referred to as a device  302  hereinafter) included in the printing apparatus  100  according to the third embodiment. 
     In this embodiment, a reverse bias voltage control unit  200  is arranged between a comparison unit  130  and a terminal T 2  of the device  302  connected to the anode terminal of the light-receiving element  120 . The reverse bias voltage control unit  200  receives a reference voltage VR from a reference voltage control unit  180 , and controls the anode terminal of the light-receiving element  120  to a voltage according to the reference voltage via the terminal T 2 . In addition, a comparison voltage VC is supplied to a noninverting input terminal INP of the comparison unit  130 . Furthermore, a monitor current Im output from the reverse bias voltage control unit  200  is supplied to a current path CP to which a reference current I 2  used to control the light emission amount to a target value is supplied from a reference current generation unit  160 , unlike the device  300  according to the above-described first embodiment. The remaining components of the device  302  may be similar to the components of above-described device  300 , and a description thereof will be omitted here. 
     The reverse bias voltage control unit  200  will be described first. The reverse bias voltage control unit  200  includes a transistor M 11  using a PMOS transistor, and transistors M 12  and M 13  using NMOS transistors. The transistors M 12  and M 13  form a current mirror circuit. That is, the reverse bias voltage control unit  200  includes the current mirror circuit formed by the transistors M 12  and M 13 , and the transistor M 11  arranged between the current mirror circuit and the terminal T 2  connected to the anode terminal of the light-receiving element  120 . One (source) of the main terminals of the transistor M 11  is connected to the anode terminal of the light-receiving element  120  via the terminal T 2 , and the other (drain) is connected to the current mirror circuit. In addition, the control terminal (gate) of the transistor M 11  is connected to a terminal from which the reference voltage control unit  180  outputs the reference voltage VR. 
     A node corresponding to the input terminal of the reverse bias voltage control unit  200 , which is connected to the terminal T 2  to which a current Ip supplied from the light-receiving element  120  in accordance with the light emission amount of the light-emitting element  110  flows, is defined as a node n 11 . That is, the node n 11  is connected to the anode terminal of the light-receiving element  120 . Furthermore, the ground node is defined as a node n 12 . In addition, a node corresponding to the output terminal of the reverse bias voltage control unit  200 , through which the reverse bias voltage control unit  200  supplies a current according to the current Ip flowing through the terminal T 2  connected to the anode terminal of the light-receiving element  120  as the monitor current Im to the current path CP, is defined as a node n 13 . The node n 13  is connected to the reference current generation unit  160  and an inverting input terminal INN of the comparison unit  130  via the current path CP. The transistor M 11  has a source connected to the node n 11 , a gate to which the reference voltage VR is supplied, and a drain to which the drain and the gate of the transistor M 12  and the gate of the transistor M 13  are connected. The transistor M 12  has a source connected to the node n 12 , and the transistor M 13  has a source connected to the node n 12 , and a drain connected to the node n 13 . 
     The transistor M 13  supplies, to the current path CP, the monitor current Im of a value obtained by multiplying the value of the current Ip that flows from the light-receiving element  120  to the transistor M 12  by the size ratio (mirror ratio) of the transistor M 12  and the transistor M 13 . Hence, it can be said that the monitor current Im is a current supplied to the current path CP based on the detection amount of the light-receiving element  120  according to the light emission amount of the light-emitting element  110 . 
     Additionally, when the current Ip flows from the light-receiving element  120  to the transistor M 11 , the transistor M 11  performs a source follower operation. For this reason, using the reference voltage VR and a gate-to-source voltage VGS of the transistor M 11 , a terminal voltage VT 2  of the terminal T 2  connected to the anode terminal of the light-receiving element  120  is expressed as a voltage (VR+VGS). That is, the voltage applied to the anode terminal of the light-receiving element  120  via the terminal voltage VT 2  can be controlled by the reference voltage VR, and as a result, the reverse bias voltage applied when driving the light-receiving element  120  can be controlled. 
     The comparison voltage VC input to the noninverting input terminal INP of the comparison unit  130  may be a voltage set in advance to cause the current mirror circuits included in both the reference current generation unit  160  and the reverse bias voltage control unit  200  to accurately operate when performing the APC operation. In addition, the comparison voltage VC may be a voltage whose output is controlled by an arrangement similar to the reference voltage control unit  180 . For example, in a case in which the reverse bias voltage control unit  200  performs current/current conversion between the current Ip and the monitor current Im by the current mirror circuit with a gain of 1, a voltage having a value between the ground voltage and the voltage (for example, a power supply voltage VCC) of the cathode terminal of the light-receiving element may be supplied to the noninverting input terminal INP of the comparison unit  130 . Similarly, in a case in which current/current conversion is performed between the current Ip and the monitor current Im by the current mirror circuit with a gain of 1, a voltage according to the reference voltage VR may be supplied to the noninverting input terminal INP. In this case, the terminal from which the reference voltage control unit  180  outputs the reference voltage VR may be connected to the noninverting input terminal INP together with the gate of the transistor M 11 , and the reference voltage VR may be supplied to the noninverting input terminal INP. 
     In this embodiment, it is possible to control the reverse bias voltage of the light-receiving element  120  and improve the degree of freedom in designing the APC circuit while maintaining a state in which the monitor current Im and the reference current I 2  can accurately be adjusted. More specifically, if the target value of the light emission amount of the light-emitting element  110  using a laser diode or the like is small, and the current Ip output from the light-receiving element  120  is small, the voltage value of the reference voltage VR may be set large so the influence of the dark current of the light-receiving element  120  does not become large. Accordingly, the reverse bias voltage when driving the light-receiving element  120  becomes small, and generation of the dark current of the light-receiving element  120  is suppressed. On the other hand, if the target value of the light emission amount of the light-emitting element  110  is large, and the current Ip output from the light-receiving element is large, the voltage value of the reference voltage VR may be set small to increase the response speed of the light-receiving element  120 . Accordingly, the reverse bias voltage when driving the light-receiving element  120  becomes large, the response speed of the light-receiving element  120  increases, and a period P 22  shown in  FIG. 2B  is shortened. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2018-172823, filed Sep. 14, 2018 which is hereby incorporated by reference herein in its entirety.