Patent Grant
10761470
The present invention relates to a printing apparatus and a light-emitting element driving device.
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.
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).
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
The light-emitting element 110 has an anode connected to a power supply voltage VCC, and a cathode connected to the terminal T1. 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 T1, 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 T2. 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 T2 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 SW1 using control signals sig1, sig2, and sig3.
In accordance with the control signal sig1 output from the control unit 170, the current generation unit 150 generates a standard current T1 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 T1 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 T2. The reference current generation unit 160 receives the standard current T1 from the current generation unit 150, and supplies, to the current path CP, a reference current I2 of a value obtained by multiplying the value of the standard current T1 by a predetermined ratio. In other words, the reference current generation unit 160 supplies the reference current I2 to a node connected to the anode terminal of the light-receiving element 120. The reference current I2 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 I2 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 I1 according to the reference current I2 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 M1 and M2 that are NMOS transistors.
Here, a node to which the standard current I1 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 n1. In addition, the ground node of the current mirror circuit of the reference current generation unit 160 is defined as a node n2. Furthermore, a node to which the reference current I2 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 n3. That is, the node n3 is connected to the current path CP and connected to the anode terminal of the light-receiving element 120.
The transistor M1 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 n1, and the source is connected to the node n2. In addition, the transistor M2 that forms the current mirror circuit of the reference current generation unit 160 is arranged such that the gate is connected to the node n1, the source is connected to the node n2, and the drain is connected to the node n3. The transistor M2 supplies, to the current path CP, the reference current I2 of a value obtained by multiplying the value of the standard current I1 flowing to the transistor M1 by a size ratio of the transistor M1 and the transistor M2. The size ratio of the transistor M1 and the transistor M2 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 I1 and the reference current I2 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 I1 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 I2 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 I2 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 T2, as will be described later in detail. The reference voltage control unit 180 includes resistors R1, R2, and R3, switch elements SW2 and SW3, a differential input amplifier 190, and an inverter INV1.
The resistors R1, R2, and R3 are connected in series between the power supply voltage VCC and a ground voltage VSS. One terminal of the switch element SW2 is connected to a node n4 that is the connection point between the resistors R1 and R2, and the other terminal is connected to the noninverting input terminal of the differential input amplifier 190. One terminal of the switch element SW3 is connected to a node n5 that is the connection point between the resistors R2 and R3, 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 n6 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 n6 as a reference voltage VR. The control signal sig2 is input to the switch element SW3 and the inverter INV1, and a signal whose logic is inverted by the inverter INV1 is input to the switch element SW2.
In the reference voltage control unit 180, when the control signal sig2 output from the control unit 170 is L (low level), the switch element SW2 is turned on, and the switch element SW3 is turned off. Accordingly, a voltage obtained by buffering the voltage of the node n4 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 sig2 output from the control unit 170 is H (high level), the switch element SW2 is turned off, and the switch element SW3 is turned on. Accordingly, a voltage obtained by buffering the voltage of the node n5 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 SW2 and the switch element SW3 in response to the control signal sig2 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 R1 to R3 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 sig2 output from the control unit 170.
The comparison unit 130 compares the monitor current Im with the reference current I2, 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 n3 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 T2 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 I2 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 n6 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 I2 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 I2, 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−I2) between the monitor current Im and the reference current I2 (<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 I2, 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 I2, 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 (I2−Im) between the monitor current Im and the reference current I2 (>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 I2, 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 I2 by the above-described arrangement. Based on the output according to the comparison between the monitor current Im and the reference current I2 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 I2 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 I2.
Additionally, in this embodiment, the device 300 of the printing apparatus 100 includes the switch element SW1 configured to connect the inverting input terminal INN and the noninverting input terminal INP of the comparison unit 130, as shown in
The driving unit 140 generates a driving signal used to drive the light-emitting element 110 via the terminal T1 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 SW1 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
In
Referring to
Next, when the control signal sig3 changes to H, and a period P11 in which the APC operation of comparing the monitor current Im with the reference current I2 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 P11 in which the APC operation is performed, the switch element SW1 to which the inverted signal of the control signal sig3 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 I2 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 VT2 of the terminal T2 connected to the anode terminal of the light-receiving element 120. Immediately after the start of the period P11, 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 SW1 is turned off. For this reason, the terminal voltage VT2 lowers in the ground voltage direction from the reference voltage VRH via the transistor M2 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 VT2 rises to the target voltage (reference voltage VRH). Next, when the monitor current Im and the reference current I2 balance, and the terminal voltage VT2 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 P12 until the terminal voltage VT2 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
For this reason, when a period P21 (in this embodiment, the period P11 and the period P21 have the same length) in which the control signal sig3 changes to H, and the APC operation is performed starts, the APC operation is started like the operation shown in
Here, to avoid an influence on the APC operation, the timing of switching the control signal sig2 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 sig2 as needed in the period in which the control signal sig3 is L.
Here, referring back to
Hence, if the control signal sig2 is L, the voltage VDS2 is larger, as compared to a case in which the control signal sig2 is H (VRH>VRL). For this reason, the conversion accuracy of the reference current generation unit 160 may become high. More specifically, for example, if the voltage VDS2 equals the reference voltage VRL, the value of the voltage VDS2 between the drain and the source of the transistor M2 is low, the transistor M2 operates in a linear region, and a desired current ratio is not obtained in some cases. On the other hand, if the voltage VDS2 equals the reference voltage VRH larger than the reference voltage VRL, the transistor M2 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 VDS2 equals the reference voltage VRL. For this reason, if the control signal sig2 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 sig2 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 sig2. 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 VDS2 between the source and the drain of the transistor M2 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 sig2 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 sig2 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,
The structure and operation of a printing apparatus 100 according to this embodiment will be described with reference to
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 T2 is opposite to that in the first embodiment, a reference current generation unit 160 is formed by transistors M1 and M2 using PMOS transistors. In addition, the light-emitting element 110, a comparison unit 130 a driving unit 140, an inverter INV2, a switch element SW1, and a terminal T1 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 SW4. 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
As shown in
In the arrangement shown in
As shown in
More specifically, the inter-group switch element SW4 electrically connects the terminal T2 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 110a. Next, the inter-group switch element SW4 electrically connects the terminal T2 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 110b.
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
The structure and operation of a printing apparatus 100 according to this embodiment will be described with reference to
In this embodiment, a reverse bias voltage control unit 200 is arranged between a comparison unit 130 and a terminal T2 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 T2. 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 I2 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 M11 using a PMOS transistor, and transistors M12 and M13 using NMOS transistors. The transistors M12 and M13 form a current mirror circuit. That is, the reverse bias voltage control unit 200 includes the current mirror circuit formed by the transistors M12 and M13, and the transistor M11 arranged between the current mirror circuit and the terminal T2 connected to the anode terminal of the light-receiving element 120. One (source) of the main terminals of the transistor M11 is connected to the anode terminal of the light-receiving element 120 via the terminal T2, and the other (drain) is connected to the current mirror circuit. In addition, the control terminal (gate) of the transistor M11 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 T2 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 n11. That is, the node n11 is connected to the anode terminal of the light-receiving element 120. Furthermore, the ground node is defined as a node n12. 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 T2 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 n13. The node n13 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 M11 has a source connected to the node n11, a gate to which the reference voltage VR is supplied, and a drain to which the drain and the gate of the transistor M12 and the gate of the transistor M13 are connected. The transistor M12 has a source connected to the node n12, and the transistor M13 has a source connected to the node n12, and a drain connected to the node n13.
The transistor M13 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 M12 by the size ratio (mirror ratio) of the transistor M12 and the transistor M13. 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 M11, the transistor M11 performs a source follower operation. For this reason, using the reference voltage VR and a gate-to-source voltage VGS of the transistor M11, a terminal voltage VT2 of the terminal T2 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 VT2 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 M11, 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 I2 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 P22 shown in
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.
| Number | Date | Country | Kind |
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| 2018-172823 | Sep 2018 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
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