JP2004288675A - Light emitting element driving device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light emitting element driving device suitable for use to drive a laser element used as a light source for a laser xerography. <P>SOLUTION: A semiconductor laser driving device automatically controls the quantity of light of a semiconductor laser LD by detecting the quantity of light of the semiconductor laser LD, a quantity-of-light detecting circuit 13 and imparting a quantity-of-light control voltage Vcont based on the quantity-of-light detection voltage Vdet as a means for supplying drive current to the semiconductor laser LD, that is, a current source 114 as its control voltage. When the quantity-of-light control voltage Vcont exceeds a limited voltage Vlim, the limited voltage Vlim is selected instead of the quantity-of-light control voltage Vcont, and applied to the current source 114 as its control voltage to limit the current. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a driving device for a light emitting device, and more particularly to a driving device for a light emitting device suitable for driving a laser device used as a light source for laser xerography.
[0002]
[Prior art]
In the field of laser xerography using a laser element as a light source, demands for higher resolution and higher speed are increasing. There is a limit to the speed at which on / off control of the driving of the laser element is performed according to the input image data (hereinafter, referred to as modulation speed). When the number of laser beams is one, the modulation speed must be sacrificed in order to increase not only the resolution in the main scanning direction but also the resolution in the sub-scanning direction. Therefore, the only way to increase the resolution in the sub-scanning direction without increasing the modulation speed is to increase the number of laser light beams. When the number of laser beams is, for example, four, assuming that the modulation speed is the same as one, the resolution in the main scanning and sub-scanning directions can be doubled.
[0003]
A semiconductor laser used as a light source for laser xerography includes an edge-emitting laser element having a structure in which laser light is extracted in a direction parallel to the active layer (hereinafter, referred to as an edge-emitting laser), and a laser beam perpendicular to the active layer. Surface emitting laser elements (hereinafter referred to as surface emitting lasers) having a structure to be taken out in various directions. Conventionally, in laser xerography, an edge emitting laser has generally been used as a laser light source.
[0004]
However, from the viewpoint of increasing the number of laser light beams, edge emitting lasers are considered to be technically difficult, and structurally, surface emitting lasers increase the number of laser light beams more than edge emitting lasers. It is advantageous. For these reasons, in recent years, in the field of laser xerography, in order to respond to the demand for higher resolution and higher speed, an apparatus using a surface emitting laser capable of emitting a large number of laser light beams as a laser light source. Is being developed.
[0005]
Incidentally, semiconductor lasers are vulnerable to overcurrent. In addition, the surface emitting laser exhibits a so-called thermal rollover phenomenon in which the light amount increases as the drive current increases up to the peak light amount, and when the drive current increases, the light amount decreases conversely as the drive current increases. Has characteristics. Therefore, considering automatic light amount control for a surface emitting laser, if light amount control is performed in a region exceeding the peak light amount, the current-light amount characteristic is negative in that region. The control for increasing the drive current is performed and the control becomes impossible. As a result, an excessive current flows to the surface emitting laser.
[0006]
In the light amount control in the region exceeding the peak light amount, in order to prevent an excessive current from flowing to the surface emitting laser, a current limiting function is conventionally provided so that a current exceeding a predetermined current value does not flow to the surface emitting laser. 2. Description of the Related Art A laser driving device having a provided configuration is known (for example, see Patent Document 1).
[0007]
Further, the comparator detects that the control voltage for controlling the light quantity of the semiconductor laser exceeds a predetermined value, and cuts off the DC drive current of the semiconductor laser by setting the control voltage to zero in response to the detection output. A known semiconductor laser drive circuit is also known (for example, see Patent Document 2).
[0008]
Furthermore, there is also known an optical output control circuit in which a limiter circuit, which is often used in a power supply circuit, is applied to a laser drive circuit so that when a base current drive current for controlling a series regulation transistor increases, the current is extracted and the output current is controlled. (For example, see Patent Document 3).
[0009]
[Patent Document 1]
JP-A-10-119350 (particularly paragraph 0028 and FIG. 4)
[Patent Document 2]
JP-A-61-144087 (particularly, right column on page 2, left column on page 3, and FIG. 1)
[Patent Document 3]
JP-A-58-200445 (particularly, lower right column of page 2, upper left column of page 3, and FIG. 3)
[0010]
[Problems to be solved by the invention]
However, in the related art disclosed in Patent Literature 1, a current equal to or greater than a predetermined current value is merely prevented from flowing to the surface emitting laser. Further, in the related art according to Patent Literature 2, when the light amount control voltage exceeds a predetermined value, the control voltage is set to zero to cut off the DC drive current of the semiconductor laser. When the voltage becomes lower than the predetermined value, the current limit is released and the control voltage exceeds the predetermined value again, so that oscillation occurs, and depending on the circuit system, the semiconductor laser may be destroyed by overshoot when the control voltage returns. There is a problem that it can be.
[0011]
It is also conceivable to adopt a method of setting the reference value of the light quantity control to zero instead of setting the control voltage to zero at the time of the current limitation as in the related art according to Patent Document 2. However, if the reference value of the light quantity control is zero, it is impossible to escape from the current limit state forever, so some kind of recovery means is required. In this case, even when the current is limited by a slight noise or the like, the laser is turned off and the image is not formed with a latent image on the photosensitive member, so that the entire page is wasted. Further, the conventional technique according to Patent Literature 3 employs a circuit system in which a constant current operation is performed until the drive current reaches a certain value (limit value) by feedback control, and the operation shifts to a constant current operation when the drive current exceeds a certain value. It is necessary to apply enough negative feedback to shift to constant current operation at the limit value.However, applying enough negative feedback makes it easier to oscillate, and in some cases, overcurrent occurs due to oscillation and shortens the life of the laser Conversely, in order to prevent oscillation, the response of the negative feedback may be slowed down. However, in this case, it takes time from the flow of the overcurrent to the actual current limiting operation, so that the life of the laser is also shortened. In order to prevent oscillation, it is conceivable to reduce the gain of the negative feedback. However, in this case, even if the limit value is exceeded, the current limit is slow and the current value greatly exceeds the limit value and becomes constant. When the laser drive circuit oscillates, there is a problem that an unnecessary peak current is easily generated in the semiconductor laser.
[0012]
[Means for Solving the Problems]
In order to solve the above problem, a light emitting element driving device according to claim 1 includes a driving current supply unit that supplies a driving current corresponding to a control voltage to a light emitting element, and a first control voltage corresponding to a maximum allowable current. Generating means for generating, a comparing means for comparing a second control voltage obtained in accordance with a light emission amount of the light emitting element with the first voltage, and the first and the second signals based on a comparison result of the comparing means. And selecting means for selecting one of the two control voltages and providing the selected one as the control voltage to the drive current supply means.
[0013]
In the light-emitting element driving device having the above configuration, when the second control voltage exceeds the first control voltage, the output of the comparing means is inverted. In response to this, the selection means selects the first control voltage instead of the second control voltage which has been selected up to that time, and supplies the first control voltage to the drive current supply means as the control voltage. As a result, the drive current of the light emitting element can be limited to a current corresponding to the first control voltage. With this current limiting function, for example, when driving a light emitting element having a characteristic that indicates a thermal rollover phenomenon, control is performed to control the light amount in a region exceeding the peak light amount, and to increase the drive current as the light amount decreases. Even if the control is performed, the current can be limited when the second control voltage exceeds the first control voltage, so that it is possible to reliably prevent the thermal rollover phenomenon from causing the control to become uncontrollable.
[0014]
According to a second aspect of the present invention, in the light emitting device driving device according to the first aspect, the light emitting device driving device further includes a bias current supply unit configured to supply a bias current to the light emitting device. In a case where a circuit system for supplying the light emitting element is employed, the limit current is determined according to a total current flowing through the light emitting element.
[0015]
In the light-emitting element driving device having the above-described configuration, when a circuit method in which a modulation current is superimposed on a bias current during modulation and supplied to the light-emitting element is adopted, a limit current is determined in accordance with a total current flowing through the light-emitting element. The limit current corresponding to the current flowing through the light emitting element can be set. Therefore, since the current can be limited based on the limit current corresponding to the current actually flowing to the light emitting element, it is possible to reliably prevent the control failure due to the thermal rollover phenomenon during the automatic light amount control.
[0016]
According to a third aspect of the present invention, in the light emitting element driving device according to the first aspect, the generating means includes a first transistor having a gate and a drain connected in common, and a current supply means. Two MOS transistors, and a size ratio (channel width / channel width / channel width) of the first MOS transistor and the second MOS transistor so that a supply current to a common connection point of the gate and the drain is smaller than a drive current. (Length of channel length) is set.
[0017]
In the light-emitting element driving device having the above-described configuration, by setting the first control voltage for determining the limit current of the light-emitting element according to the size ratio of the MOS transistors forming the current mirror circuit, the current flowing through the light-emitting element is small. However, the means for generating the first control voltage can handle a much larger current. Therefore, the first control voltage can be set based on a very large current compared to a very small current flowing to the light emitting element, so that current consumption can be suppressed and a transistor size is small, so that a chip can be downsized.
[0018]
According to a fourth aspect of the present invention, in the light emitting element driving device according to the third aspect, a signal line for transmitting the first control voltage to the selecting unit and the second control voltage are transmitted to the selecting unit. And a capacitor connected between at least one of the signal lines and the source of the MOS transistor constituting the current supply means.
[0019]
In the light emitting element driving device having the above configuration, the source of the MOS transistor forming the current supply means is connected to the power supply line. Therefore, the capacitor connected between at least one of the signal line of the first control voltage and the signal line of the second control voltage and the source of the MOS transistor constituting the current supply means is used for the fast inversion operation of the comparison means. When a potential change occurs in the signal line due to this, the function of absorbing the change is provided. Accordingly, the first control voltage corresponding to the limit current and the second control voltage corresponding to the amount of emitted light can be prevented from being AC-coupled during the switching operation of the switching unit based on the inversion operation of the comparing unit. The operation of limiting the current to the limit current can be reliably performed even at the timing of the switching.
[0020]
According to a fifth aspect of the present invention, in the light emitting element driving device according to the fourth aspect, the selection means includes a first switch element for selecting the second control voltage and a first switching voltage for selecting the second control voltage. And a second switch element to be selected, and the switching control is performed so that the ON periods of the first and second switch elements do not overlap.
[0021]
In the light-emitting element driving device having the above-described configuration, switching control is performed on the first switch element for selecting the second control voltage and the second switch element for selecting the first voltage so that both ON periods do not overlap. Accordingly, it is possible to prevent capacitors provided respectively for the first control voltage and the second voltage from being directly connected and to prevent discharge, and thus to prevent unstable operation at the time of switching.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0023]
FIG. 1 is a circuit diagram illustrating a configuration example of a light emitting element driving device according to an embodiment of the present invention. In the present embodiment, as a light emitting element to be driven, for example, a semiconductor laser, particularly a GaN blue laser or a single mode surface emitting laser having a large internal resistance is used. Hereinafter, these laser elements are collectively referred to as a semiconductor laser LD. In the semiconductor laser LD, for example, a cathode is grounded, and an anode is a driving end.
[0024]
The light emitting element driving device according to the present embodiment has at least a driving current control circuit 11, a driving voltage control circuit 12, a light amount detection circuit 13, an error detection circuit 14, and a control circuit 15.
[0025]
The drive current control circuit 11 includes an analog inverter 111, a limit voltage generation circuit 112, a comparator 113, a current source 114, a capacitor C11, and switches SW11, SW12, and SW13. In the drive current control circuit 11, the drive current of the semiconductor laser LD is controlled so that the light emission amount of the semiconductor laser LD becomes a specified light amount. The drive current control circuit 11 is supplied with a light quantity control voltage (second control voltage) Vcont to be described later from the error detection circuit 14 via the drive voltage control circuit 12.
[0026]
The light amount control voltage Vcont is inverted by the analog inverter 111 via the switch SW21 and applied to the input side terminal of the switch SW11. The capacitor C11 is connected between the power supply line L of the power supply voltage VDD, which is the source of the P-MOS transistor constituting the current source 114, and the output side terminal of the switch SW11, thereby forming a sample and hold circuit together with the switch SW11. Make up. Of course, when the current source 114 is not a source but a sink, the capacitor C11 is connected between the current source 114 and GND because the current source 114 is formed of an N-MOS transistor. This sample-and-hold circuit samples and holds the light amount control voltage Vcont that is applied after being inverted by the analog inverter 111.
[0027]
The limit voltage generating circuit 112, the comparator 113, and the switch SW12 constitute a current limiting circuit (limiter) for limiting a drive current flowing through the semiconductor laser LD. The limit voltage generation circuit 112 generates a limit voltage (first control voltage) Vlim corresponding to a limit current that limits a drive current flowing through the semiconductor laser LD. The comparator 113 has a limit voltage Vlim generated by the limit voltage generation circuit 112 as an inverted (-) input, and a non-inverted (+) input as a light amount control voltage Vcont supplied through the analog inverter 111 and the switch SW11. When the voltage Vcont exceeds the limit voltage Vlim, the comparison output is inverted.
[0028]
The switch SW12 has one input of a light quantity control voltage Vcont applied through the analog inverter 111 and the switch SW11, and the other input of the limit voltage Vlim generated by the limit voltage generation circuit 112. When Vcont is selected and the comparison output of the comparator 113 is inverted, the limit voltage Vlim is selected instead of the light amount control voltage Vcont in response to the inversion of the comparison output.
[0029]
The light amount control voltage Vcont or the limit voltage Vlim selected by the switch SW12 is supplied to the current source 114 as the control voltage. One end of the current source 114 is connected to the power supply line L of the power supply voltage VDD. The switch SW13 has one end connected to the other end of the current source 114 and the other end connected to one end of the switch SW24 of the drive voltage control circuit 12, respectively.
[0030]
In the drive current control circuit 11 having such a configuration, the light quantity control voltage Vcont supplied from the error detection circuit 15 via the drive voltage control circuit 12 is inverted by the inverter 111, and thereafter, a sample-and-hold circuit including the switch SW11 and the capacitor C11. Is sampled and held. The sampled and held light quantity control voltage Vcont is supplied to the current source 114 as a control voltage by being selected by the switch SW12, and the drive current supplied from the current source 114 to the semiconductor laser LD through the switches SW13 and SW24. Control.
[0031]
Thus, the drive current control circuit 11 controls the drive current of the semiconductor laser LD such that the light amount of the semiconductor laser LD becomes a specified light amount (target light amount) determined by the reference voltage Vref of the error detection circuit 15. This is APC (Automatic Power Control; automatic light intensity control) for controlling the laser power of the semiconductor laser LD to be a power specified by the reference voltage Vref.
[0032]
The light quantity control voltage Vcont after the sample hold is further compared with the limit voltage Vlim generated by the limit voltage generation circuit 112 in the comparator 113. The comparator 113 inverts the comparison output when the light amount control voltage Vcont exceeds the limit voltage Vlim, and controls the switch SW12. As a result, the switch SW12 selects the limit voltage Vlim instead of the light amount control voltage Vcont which has been selected up to that time, and supplies the same to the semiconductor laser LD as the control voltage. As a result, the drive current control circuit 11 limits the current so that the drive current flowing through the semiconductor laser LD becomes a constant limit current corresponding to the limit voltage Vlim.
[0033]
The drive voltage control circuit 12 includes four switches SW21 to SW25, two capacitors C21 and C22, and a differential amplifier 121. The drive voltage control circuit 12 controls the drive voltage applied to the semiconductor laser LD at the time of lighting based on the terminal voltage of the semiconductor laser LD when the drive current control circuit 11 controls the light amount to the specified amount.
[0034]
The switch SW21 has one end connected to the output terminal of the error detection circuit 14 and the other end connected to the non-inverting input terminal of the operational amplifier 121. The capacitor C21 is connected between the other end of the switch SW21 and the ground to form a sample and hold circuit together with the switch SW21. This sample and hold circuit samples and holds the output voltage of the error detection circuit 14.
[0035]
The differential amplifier 121 operates as a buffer when the inverting input terminal and the output terminal are short-circuited by the switch SW25. The switch SW22 has one end connected to the output terminal of the differential amplifier 121. The capacitor C22 is connected between the other end of the switch SW22 and the ground to form a sample and hold circuit together with the switch SW22.
[0036]
The switch SW23 has one end connected to the other end of the switch SW22 and the other end connected to the anode of the semiconductor laser LD. The switch SW24 has one end connected to the other end of the switch SW23 and the anode of the semiconductor laser LD, and the other end connected to the output terminal (inverting input terminal) of the differential amplifier 121.
[0037]
In the drive voltage control circuit 12 having such a configuration, at the time of controlling the drive current of the semiconductor laser LD, that is, at the time of APC (at the time of light amount control), the switches SW22, SW23, and SW25 are both off, and both the switches SW21 and SW24 are on. become. Then, the output voltage of the error detection circuit 14, that is, the light amount control voltage Vcont is applied to the semiconductor laser LD via the switch SW21, the differential amplifier 121, and the switch SW24, and a feedback loop is formed.
[0038]
When the semiconductor laser LD is modulated after the light quantity control, the switches SW22 and SW25 are turned on, and the switches SW21, SW23 and SW24 are all turned off. Then, the output voltage of the differential amplifier 121, that is, the terminal voltage of the semiconductor laser LD at the end of the light quantity control is held by the capacitor C22. When the switch SW23 is turned on when the semiconductor laser LD is turned on, the hold voltage of the capacitor C22 is applied as a drive voltage to the drive end (anode) of the semiconductor laser LD via the switch SW23.
[0039]
The light quantity detection circuit 13 uses, for example, a photodiode PD as a photodetector, and detects the quantity of laser light emitted from the semiconductor laser LD by the photodiode PD. The photodiode PD has a cathode connected to the power supply line L. One end of a resistor R is connected to the anode of the photodiode PD. The other end of the resistor R is grounded. Here, when the photodiode PD detects the laser light of the semiconductor laser LD, a current corresponding to the light amount flows through the resistor R. As a result, a light amount detection voltage Vdet corresponding to the light amount of the semiconductor laser LD is generated between both ends of the resistor R.
[0040]
This light amount detection voltage Vdet is supplied to a non-inverting input terminal of the amplifier 131. The output terminal and the inverting input terminal of the amplifier 131 are directly connected. The output voltage of the amplifier 131, that is, the light amount detection voltage Vdet of the light amount detection circuit 13 is supplied to the drive current control circuit 11 via the error detection circuit 14 and the drive voltage control circuit 12. That is, a feedback system for performing APC (automatic light amount control) is configured by feeding back the light amount detection voltage Vdet of the light amount detection circuit 13 to the drive current control circuit 11.
[0041]
The error detection circuit 14 includes a differential amplifier 141, switches SW31 and SW32, and a capacitor C31. The error detection circuit 14 detects a difference between the light amount detection voltage Vdet detected by the light amount detection circuit 13 and the reference voltage Vref and outputs the difference as a light amount control voltage Vcont. I do. The differential amplifier 141 has the reference voltage Vref as a non-inverting input and the light amount detection voltage Vdet supplied from the light amount detecting circuit 13 via the switch SW31 as an inverting input. Here, the light amount reference voltage Vref is set so that its voltage value corresponds to the target light amount (laser power) of the semiconductor laser LD. The switch SW32 and the capacitor C31 are connected in series between the inverting input terminal and the output terminal of the differential amplifier 141.
[0042]
In the error detection circuit 14 having such a configuration, the differential amplifier 141 sets a light amount detection voltage Vdet corresponding to the light amount of the semiconductor laser LD detected by the light amount detection circuit 13 in accordance with the target light amount of the semiconductor laser LD. The reference voltage Vref is compared with the reference voltage Vref, and the difference, that is, the error voltage is output as the light amount control voltage Vcont.
[0043]
The control circuit 15 turns on (closes) the switches SW11 and SW13 of the drive current control circuit 11, the switches SW21 to SW25 of the drive voltage control circuit 12, and the switches SW31 and SW32 of the error detection circuit 14, based on the input data that is the pulse data. / Off (open) control. Further, in the semiconductor laser driving device according to the present embodiment, the control circuit 15 controls the voltage to be always driven during the lighting period / light-out period in the modulation period in which the driving for turning on / off the semiconductor laser LD is repeated according to the input data. Is performed.
[0044]
Next, the circuit operation of the light emitting element driving device according to the present embodiment having the above configuration, that is, the semiconductor laser driving device will be described.
[0045]
When the power is turned on, first, the APC mode is entered. In the APC mode, the switch SW31 and the switch SW32 in the error detection circuit 14 are both turned on, the switches SW21 and SW24 in the drive voltage control circuit 12 are both turned on, and the switches SW11, SW25 and the switch in the drive current control circuit 11 are turned on. SW13 are both turned on. Accordingly, the current of the current source 114 flows as a drive current to the semiconductor laser LD via the switch SW13, and the semiconductor laser LD is turned on.
[0046]
When the semiconductor laser LD is turned on, the laser light is received by the photodiode PD of the light amount detection circuit 13, and a current corresponding to the light amount flows through the photodiode PD. The current flowing through the photodiode PD is converted into a voltage by the resistor R, amplified by the amplifier 131, and output as a light amount detection voltage Vdet corresponding to the laser power (light amount) of the semiconductor laser LD.
[0047]
This light amount detection voltage Vdet is supplied to the error detection circuit 14, and becomes an inverted input of the differential amplifier 131 via the switch SW31. The differential amplifier 131 amplifies the difference (error voltage) between the detection voltage Vdet and the reference voltage Vref and outputs the result as the light amount control voltage Vcont. This light quantity control voltage Vcont is input to the non-inverting terminal of the differential amplifier 121 via the switch SW21 of the drive voltage control circuit 12. On the other hand, the anode voltage of the semiconductor laser LD is input to the inverting input terminal via the SW 24. The differential amplifier 121 amplifies the light amount control voltage Vcont and the anode voltage of the semiconductor laser LD and supplies the amplified light amount control voltage Vcont to the drive current control circuit 11.
[0048]
In the drive current control circuit 11, the switch SW12 selects the switch SW11 in the APC mode, so that the output of the differential amplifier 121 in the drive voltage control circuit 12 is output via the analog inverter 111 through the switches SW11 and SW12. The current source 114 is provided as its control voltage. By controlling the current source 114 with the light amount control voltage Vcont, the driving current of the semiconductor laser LD supplied from the current source 114 is controlled, and as a result, the light amount of the semiconductor laser LD is controlled.
[0049]
By the control of the negative feedback loop according to the light amount detection voltage Vdet described above, finally, the detection voltage Vdet matches the reference voltage Vref set corresponding to the specified light amount, and the anode voltage of the semiconductor laser LD and The terminal voltage of the capacitor C21 matches and converges. As a result, the light amount of the semiconductor laser LD becomes the specified light amount. The above series of feedback control is APC (automatic light amount control). The operation of the APC may be completed once, or may be repeatedly performed a plurality of times.
[0050]
Further, in the drive current control circuit 11 in the APC mode, the comparator 113 constantly monitors whether or not the light quantity control voltage Vcont exceeds the limit voltage generated by the limit voltage generation circuit 112. When the voltage exceeds Vlim, the comparison output is inverted. Then, in response to the inversion of the comparison output of the comparator 113, the switch SW12 selects the limit voltage Vlim instead of the light quantity control voltage Vcont selected up to that time, and gives the limit voltage Vlim as the control voltage to the semiconductor laser LD. . As a result, when the light quantity control voltage Vcont exceeds the limit voltage Vlim, the drive current flowing through the semiconductor laser LD is limited to a certain limit current corresponding to the limit voltage. Further, the comparator 113 always compares the light amount control voltage Vcont with the limit voltage Vlim even in a mode other than the APC mode. Therefore, when the switch SW11 is turned on due to a malfunction after the APC is completed, or when the light amount control voltage Vcont Is adjusted (not shown), when the limit current is reduced, the comparator 113 is inverted to prevent overcurrent to the laser.
[0051]
After the APC operation is completed, the switch SW11 of the drive current control circuit 11 and the switch SW32 of the error detection circuit 14 are both turned off, and the switches SW22, SW23, and SW25 of the drive voltage control circuit 12, which were off during the APC, are all turned on. State. Then, the output voltage of the differential amplifier 141 immediately before that, that is, the light amount control voltage Vcont when the semiconductor laser LD is controlled to the specified light amount is held by the capacitors C31 and C21, and the drive current when the light amount is further controlled to the specified light amount. Is held by the capacitor C11.
[0052]
Further, the capacitor C22 of the drive voltage control circuit 12 is charged with the light quantity control voltage Vcont held in the capacitor C21. At this time, the voltage held by the capacitor C11 is a control voltage for setting a drive current when the semiconductor laser LD emits light with a specified light amount, and the voltage held by the capacitors C21 and C22 is the semiconductor laser at the specified light amount. This is the terminal voltage of the LD.
[0053]
When the APC operation is completed, a modulation mode is entered in which the driving of the semiconductor laser LD is controlled to be turned on (lighting) / off (light off) according to the input data. In this modulation mode, when the semiconductor laser LD is turned on, the switch SW23 of the drive voltage control circuit 12 is turned on. Then, the hold voltage of the capacitor C22, that is, the terminal voltage of the semiconductor laser LD at the time of the specified light amount is applied to the drive end (anode) of the semiconductor laser LD through the switch SW23. Further, since the switch SW13 of the drive current control circuit 11 is turned on, a drive current is supplied from the current source 114 to the semiconductor laser LD via the switch SW13.
[0054]
As described above, the light amount of the semiconductor laser LD is detected by the light amount detection circuit 13, and the light amount control voltage Vcont based on the light amount detection voltage Vdet is supplied to the means for supplying a drive current to the semiconductor laser LD, that is, the current source 114. In the semiconductor laser driving device that automatically controls the light amount of the semiconductor laser LD by giving it as a voltage, when the light amount control voltage Vcont exceeds the limit voltage Vlim, the limit voltage Vlim is selected instead of the light amount control voltage Vcont. By giving the control voltage to the current source 114, the drive current of the semiconductor laser LD can be limited to a current (limit current) corresponding to the limit voltage Vlim.
[0055]
Thereby, as the semiconductor laser LD, in particular, the thermal rollover phenomenon in which the light amount increases as the drive current increases up to the peak light amount, and conversely decreases as the drive current increases when the peak light amount is exceeded. In the case of using a surface emitting laser having the characteristics shown in the figure, when performing light amount control in a region exceeding the peak light amount, even if control is performed to increase the drive current as the emitted light amount decreases, the light amount control voltage Since the current can be limited when Vcont exceeds the limit voltage Vlim, it is possible to reliably prevent the control from being disabled due to the thermal rollover phenomenon.
[0056]
Here, the circuit operation of the current limiting circuit (limiter) in the drive current control circuit 11 will be described in more detail. Here, the description will be made on the assumption that the limit voltage Vlim is constant irrespective of the passage of time and the light amount of the semiconductor laser LD is increased with the passage of time until the limit value (light amount corresponding to the limit current) is exceeded. I do.
[0057]
As shown in FIG. 2, when the light amount of the semiconductor laser LD is increased, the light amount control voltage Vcont according to the light amount detection voltage Vdet increases with time. Then, when the light quantity control voltage Vcont exceeds the limit voltage Vlim, the output of the comparator 113 is inverted. Then, in response to the inverted output of the comparator 113, the switch SW12 selects a constant value limit voltage Vlim instead of the light amount control voltage Vcont selected up to that time. As a result, the control voltage of the current source 114 is fixed at the limit voltage Vlim.
[0058]
At this time, the light amount control voltage Vcont increases as the light amount of the semiconductor laser LD increases. Therefore, the switch SW12 lowers the target value of the light amount of the semiconductor laser LD and maintains the state until the light amount control voltage Vcont becomes equal to or lower than the limit voltage Vlim. Then, when the light amount control voltage Vcont becomes equal to or lower than the limit voltage Vlim, the output of the comparator 113 is inverted again. In response to this, the switch SW12 selects the light amount control voltage Vcont instead of the limit voltage Vlim and sends it to the current source 114. Give as control voltage. Thereby, the operation returns to the APC operation again.
[0059]
Thus, in the current limiting circuit having the above configuration, when the light amount control voltage Vcont exceeds the limit voltage Vlim, the control voltage of the current source 114 that supplies the drive current to the semiconductor laser LD corresponds to the limit current of the semiconductor laser LD. A circuit according to the related art that shifts from a constant voltage operation to a constant current operation when the drive current exceeds a predetermined value due to feedback control because there is no unstable state such as oscillation because the configuration is switched to the limit voltage Vlim. As in the case of the system, the controllability does not deteriorate near the limit value.
[0060]
Furthermore, since the APC operation can be promptly restored once the overcurrent state is released, even if the current limiting circuit operates due to noise or the like, its effect is limited to a very short period of the order of microseconds. You. Therefore, in the case where the present invention is applied to a driving device of a semiconductor laser which is a light source of laser xerography, even if the current limiting circuit operates due to noise or the like, an image during printing is not damaged by the influence.
[0061]
In the semiconductor laser drive device according to the embodiment, the drive current control circuit 11 supplies the current of the current source 114 to the semiconductor laser LD as the drive current. However, in the modulation mode, even when the semiconductor laser LD is turned off. In order to perform high-speed modulation, in the case of an edge emitting laser, it is preferable to keep the bias current Ibias near the emission threshold current flowing to the semiconductor laser LD.
[0062]
Specifically, as shown in FIG. 3, a current source 16 is connected between a power supply line L of a power supply voltage VDD and a driving end (anode) of the semiconductor laser LD, and the current source 16 emits light of the semiconductor laser LD. By applying a bias voltage Vbias corresponding to a current near the threshold current and constantly flowing a bias current Ibias near the light emission threshold current to the semiconductor laser LD, even when the semiconductor laser LD is turned off, the terminal voltage of the semiconductor laser LD is turned on. Since the voltage can be maintained close to the voltage, high-speed modulation can be performed.
[0063]
As another example, as shown in FIG. 4, a switch SW41 is inserted between the current source 16 and the drive end (anode) of the semiconductor laser LD, and the switch SW41 is turned on only when the semiconductor laser LD is turned off. In this way, it is also possible to adopt a circuit system for switching between the modulation current from the current source 114 and the bias current Ibias from the current source 16 as the current flowing through the semiconductor laser LD.
[0064]
As described above, by adopting a circuit system for switching between the modulation current and the bias current Ibias, the variable range of the modulation current can be narrowed. It is possible to suppress the drive current from being shifted between the light amount control and the modulation due to the influence.
[0065]
In any of the above two circuit systems, a modulation current is superimposed on a bias current Ibias and supplied as a drive current to the semiconductor laser LD during modulation. Therefore, when these circuit systems are employed, the above-described limit current is not determined in accordance with only the current supplied from the current source 114, but is determined by the total current flowing through the semiconductor laser LD, that is, the bias current Ibias + the modulation current. It will be decided accordingly.
[0066]
Thereby, even in the case of a circuit system in which the modulation current is superimposed on the bias current Ibias and supplied to the semiconductor laser LD, the limit current is set in accordance with the current actually flowing through the semiconductor laser LD, and the limit current is set to Since the current can be limited as a reference, it is possible to reliably prevent control failure due to the thermal rollover phenomenon during APC.
[0067]
Further, in the above-described embodiment, the current limiting circuit according to the present invention is applied to a semiconductor laser driving device in which the semiconductor laser LD is first driven by the drive voltage control circuit 12 and then driven by the drive current control circuit 11. Although the description has been given by taking the case of applying the technique described above as an example, the present invention is not limited to this application example, but is applicable to all semiconductor laser driving devices having a current driving configuration. For example, as shown in FIG. 5, a semiconductor laser driving device having a configuration in which a hold voltage in a sample and hold circuit 17 including a switch SW21 and a capacitor C21 is directly supplied to a drive current control circuit 11 can be cited as one of them.
[0068]
Next, a specific circuit configuration of the drive current control circuit 11 will be described. FIG. 6 is a circuit diagram showing an example of a specific circuit configuration of a main part of the drive current control circuit 11.
[0069]
In FIG. 6, a current source 114, which is a means for supplying a drive current to the semiconductor laser LD, is configured by, for example, a PchMOS transistor Q51. In a PchMOS transistor, control is performed so that the gate potential becomes minus the source potential. For this reason, when the Vcont potential is lower than the Vlim potential, the comparator 113 needs to be inverted, and the inverted and non-inverted inputs of the comparator 113 are reversed from those in FIGS. The gate of the MOS transistor Q51 is connected to the output terminal (the common drain connection point of the MOS transistors Q53 and Q54 described later) of the switch circuit 115 constituting the switch SW12, and the source is connected to the power supply line of the power supply voltage VDD. ing.
[0070]
The limit voltage generation circuit 112 has a so-called diode-connected PchMOS transistor Q52 having a source connected to the power supply line of the power supply voltage VDD and a gate and a drain connected in common, and a gate / drain common connection point of the PchMOS transistor Q52. Based on a circuit configuration having a current source I51 connected to the ground, a limit voltage Vlim is generated by a voltage drop in the MOS transistor Q52 at a node N which is a common connection point between the MOS transistor Q52 and the current source I51. Configuration.
[0071]
In the limit voltage generation circuit 112 having such a configuration, the gate and drain of the diode-connected MOS transistor Q52 are connected to the gate of the MOS transistor Q51 forming the current source 114 via the MOS transistor Q54, as is apparent from FIG. Therefore, a current mirror circuit is formed together with the MOS transistor Q51.
[0072]
The MOS transistor Q52 is set so that the channel width W has a predetermined relationship with the channel width W of the MOS transistor Q51 constituting the current source 114 under the condition that the transistor size, for example, the channel length L is common. . Assuming that the channel width of the MOS transistor Q51 is W11 and the channel width of the MOS transistor Q52 is W12, for example, the relationship is set such that W11: W12 = 100: 1.
[0073]
As described above, the current I51 that determines the limit current of the semiconductor laser LD is set by the size ratio (the ratio of channel width W / channel length L) between the MOS transistors Q51 and Q52, that is, the ratio of the channel width W in this example. As a result, even when the current flowing through the semiconductor laser LD is large, the current I51 that determines the limit current can be kept small. Therefore, since the limit current can be set with a smaller current than the large current flowing through the semiconductor laser LD, current consumption can be suppressed.
[0074]
The limit voltage generation circuit 112 further includes a capacitor C51 connected between the node N and the power supply line of the power supply voltage VDD, that is, connected in parallel with the MOS transistor Q52, in addition to the above basic components. Have. Here, when the comparison output of the comparator 113 is inverted at high speed, the potential of the inverting input terminal of the comparator 113 fluctuates due to the inversion operation due to the capacitive coupling, and accordingly, the potential of the node N, that is, The limit voltage Vlim also changes. The capacitor C51 has a function of absorbing a fluctuation of the limit voltage Vlim caused by the high-speed inversion operation of the comparator 113 to prevent a malfunction.
[0075]
As described above, the capacitor C51 is connected between the node N and the power supply line of the power supply voltage VDD to absorb the fluctuation of the limit voltage Vlim caused by the high-speed inversion operation of the comparator 113. When the switch SW12 is switched based on the operation, the limit voltage Vlim corresponding to the limit current and the light quantity control voltage Vcont corresponding to the light quantity can be prevented from being AC-coupled, so that the current is limited to the limit current even at the switching timing. The operation can be performed reliably.
[0076]
In this circuit example, a capacitor is connected between the signal line on the limit voltage Vlim side and the power supply line. However, a capacitor is connected between the signal line on the light quantity control voltage Vcont side and the power supply line, or both. Even in this case, when the switch SW12 is switched, the AC coupling between the limit voltage Vlim corresponding to the limit current and the light amount control voltage Vcont corresponding to the light amount can be suppressed. Furthermore, by arranging the signal lines of the light amount control voltage Vcont and the limit voltage Vlim so as not to be close to each other, when the switch SW12 is switched, the limit voltage Vlim corresponding to the limit current and the light amount control voltage Vcont corresponding to the light amount are obtained. Has the effect of suppressing AC coupling.
[0077]
Next, a switch circuit 115 configuring the switch SW12 of FIG. 1 includes a switch signal generation circuit 116 that generates first and second switch signals S1 and S2 having phases opposite to each other based on the comparison output of the comparator 113; A switch element, for example, a PchMOS transistor Q53 having the first switch signal S1 as a gate input and the light quantity control voltage Vcont as a source input, a second switch signal S2 as a gate input, and a limit voltage Vlim as a source input. In this configuration, a PchMOS transistor Q53 and a switching element whose drain is commonly connected, for example, a PchMOS transistor Q54 are provided.
[0078]
The switch signal generation circuit 116 generates first and second switch signals S1 and S2 for controlling ON / OFF of the MOS transistors Q53 and Q54 so that the MOS transistors Q53 and Q54 are not simultaneously turned on. Accordingly, switch signal generating circuit 116 and switch circuit 115 including MOS transistors Q53 and Q54 have a so-called break-before-make switch configuration in which MOS transistors Q53 and Q54 are not simultaneously turned on.
[0079]
In the switch signal generation circuit 116, a circuit portion that generates the first switch signal S1 includes an inverter 1161 that inverts the comparison output (a) of the comparator 113, and an inverter stage 1162 including even-numbered stages, for example, four inverters. , An output (b) of the inverter 1164 as one input, and an output (c) of the inverter stage 1162 as the other input. The output of the NAND gate 1163 is a first switch signal. Let it be S1.
[0080]
The circuit portion that generates the second switch signal S2 includes an inverter stage 1165 having even-numbered stages, for example, four stages of inverters, and a comparison output (a) of the comparator 113 directly supplied as one input. And a NAND gate 1166 having the other output as the comparison output (e) via the other input terminal, and the output (f) of the NAND gate 1166 is used as a second switch signal S2.
[0081]
Next, a circuit operation of the switch signal generation circuit 116 having the above configuration will be described with reference to a timing chart of FIG. Here, it is assumed that the delay amount (delay time) in one stage of the inverter and one stage of the NAND gate is substantially the same, and is Δt.
[0082]
In the comparator 113, when the light amount control voltage Vcont exceeds the limit voltage Vlim and the comparison output (a) is inverted from the low level to the high level (time t1), the comparison output (a) is inverted by the inverter 1161, and Passing through stage 1162 causes a delay of 5Δt at its output (c). Then, the logical product of the output (b) of the inverter 1161 and the output (c) of the inverter stage 1162 is taken by the NAND gate 1163, and the output (d) of the NAND gate 1163 goes high during the period from time t2 to time t5. . The output (d) of the NAND gate 1163 is applied as the first switch signal S1 to the gate of the MOS transistor Q53, so that the MOS transistor Q53 is turned off in a period from time t2 to time t5.
[0083]
Further, when the comparison output (a) of the comparator 113 passes through the inverter stage 1165, the output (e) has a delay of 4Δt. Then, by performing a logical product of the comparison output (a) and the output (e) of the inverter stage 1165 at the NAND gate 1166, the output (f) of the NAND gate 1166 becomes low during the period from time t3 to time t4. . When the output (f) of the NAND gate 1166 is applied to the gate of the MOS transistor Q54 as the second switch signal S2, the MOS transistor Q54 is turned on during a period from time t3 to time t4.
[0084]
As described above, the MOS transistor Q53 that selects the light amount control voltage Vcont is turned off during the period from time t2 to time t5, and the MOS transistor Q54 that selects the limit voltage Vlim is turned on during the period from time t3 to time t4. That is, the MOS transistor Q53 shifts to the off state before the MOS transistor Q54 shifts to the on state, and the MOS transistor Q54 shifts to the off state before the MOS transistor Q53 shifts to the on state again. That's what it means.
[0085]
As described above, the first and second switch signals S1 and S2 having a time difference from each other are generated based on the comparison output (a) of the comparator 113, and the MOS transistors Q53 and Q54 are turned on by these switch signals S1 and S2. / Off control, that is, switching control by break-before-make so that the ON times of the MOS transistors Q53 and Q54 do not overlap, thereby limiting the discharge of the capacitor C51 when the Q53 and Q54 are simultaneously turned on. Since it is possible to prevent the limit voltage Vlim corresponding to the current and the light amount control voltage Vcont corresponding to the light amount from interacting with each other, it is possible to reliably perform the operation of limiting the current to the limit current even at the switching timing.
[0086]
In this circuit example, the first and second switch signals S1 and S2 having a time difference are generated to control the ON / OFF of the MOS transistors Q53 and Q54, so that the ON time of the MOS transistors Q53 and Q54 is reduced. Although the overlap is prevented, the mutual interference between Vlim and Vcont may be prevented even if the ON resistance of one of the MOS transistors Q53 and Q54 is increased as compared with the other. Alternatively, even if hysteresis is provided, the ON times of the MOS transistors Q53 and Q54 can be prevented from overlapping, and the same operation and effect as described above can be obtained.
[0087]
In the drive current control circuit 11 having the above-described configuration, an example in which Pch transistors are used as the MOS Q51 forming the current source 114, the MOS transistor Q52 forming the limit voltage generating circuit 112, and the MOS transistors Q53 and Q54 forming the switch circuit 115 is shown. However, it is also possible to use an Nch transistor by inverting the relationship of the potential.
[0088]
The semiconductor laser driving device according to one embodiment of the present invention described above, or the semiconductor laser driving device according to the modified example thereof, is suitable for use as a driving device of a multi-laser (surface emitting laser) having a large number of light emitting units. It is.
[0089]
FIG. 8 is a circuit diagram showing a configuration example of a multi-laser driving apparatus according to an application example of the present invention, which is driven by a surface-emitting laser 31 having, for example, 32 light emitting units LD1 to LD32.
[0090]
8, a multi-laser driving apparatus according to the application example includes a drive control circuit 22-1 to 22-32 for 32 channels provided corresponding to each of the light emitting units LD1 to LD32 of the surface emitting laser 21; A light amount detection circuit 23 for detecting the light amount of the light emitting laser 21, an error detection circuit 24 forming a feedback system for feeding back the detection result of the light amount detection circuit 23 to the drive control circuits 22-1 to 22-32, and a drive control circuit And a control circuit 25 that controls the circuits 22-1 to 22-32.
[0091]
The drive control circuits 22-1 to 22-32 for 32 channels all have the same circuit configuration, and these drive control circuits 22 (22-1 to 22-36) correspond to the drive current control circuit 11 and the drive voltage The control circuit 12 or the drive current control circuit 11 and the sample and hold circuit 17 shown in FIG. That is, the drive current control circuit 11 configuring each of the drive control circuits 22-1 to 22-32 has a current limit circuit (limiter) including the limit voltage generation circuit 112, the comparator 113, and the switch SW12.
[0092]
The error detection circuit 24 includes a switch 241, a differential amplifier 242, and switches SWfb1 to SWfb32 for 32 channels and capacitors Cfb1 to Cfb32. The amplifier 241 has a reference voltage Vref set corresponding to a target laser power as a non-inverting input, and an inverting input of a light amount detecting voltage Vdet supplied from the light amount detecting circuit 23 via the switch 241.
[0093]
The switches SWfb1 to SWfb32 and the capacitors Cfb1 to Cfb32 are provided corresponding to the light emitting units LD1 to LD32 of the surface emitting laser 21, respectively. Are connected in series between the inverting input terminal and the output terminal of the amplifier 241.
[0094]
The error voltage detected for each channel by the error detection circuit 24 is supplied to the corresponding circuit portion of the drive control circuits 22-1 to 22-32 as the light amount control voltage Vcont. As described above, the error detection circuit 24 detects the difference between the light amount detection voltage Vdet and the reference voltage Vref from the light amount detection circuit 23 for each channel, and uses the difference voltage as the light amount control voltage Vcont as the drive control circuits 22-1 to 22-. In the feedback system that returns to 32, the same operation as that of the semiconductor laser driving device according to the above-described embodiment is performed for each channel.
[0095]
Then, by repeating this operation continuously for the number of light emitting portions LD of the surface emitting laser 21 (32 in this example), all the control voltages of the drive control circuits 22-1 to 22-32 for 32 channels are reduced. , 32 capacitors Cfb1 to Cfb32. When the 32 channel APC operation is completed, the switch 241 is turned off and the switch SWfb1 is turned on, and the control voltage at ch1 is used as the output voltage of the differential amplifier 242 to prepare for the next APC operation.
[0096]
Also in the multi-laser driving device having the above configuration, the emission of the surface emitting laser 21 is achieved by the drive current control circuit 11 constituting each of the drive control circuits 22-1 to 22-32 having a current limiting (limiter) function. For each of the units LD1 to LD32, when the light amount control voltage Vcont exceeds the limit voltage Vlim, a current limit is applied to limit the drive current of each of the light emitting units LD1 to LD32 of the surface emitting laser 21 to a certain limit current. Therefore, it is possible to surely prevent the thermal rollover phenomenon from causing the control to become uncontrollable.
[0097]
【The invention's effect】
As described above, according to the first aspect of the present invention, when the light amount control voltage exceeds the limit voltage, the limit voltage is selected instead of the light amount control voltage to be used as the control voltage of the drive current. When driving a light-emitting element that has the characteristic of a thermal rollover phenomenon, even if control is performed to increase the drive current as the amount of emitted light decreases, the amount of light is controlled even if the amount of emitted light decreases. Since the current limit can be applied when the control voltage exceeds the limit voltage, it is possible to reliably prevent the control from being disabled due to the thermal rollover phenomenon.
[0098]
According to the invention according to claim 2, when adopting a circuit system in which a modulation current is superimposed on a bias current during modulation and supplied to the light emitting element, a limit current is determined according to a total current flowing through the light emitting element, Since a limit current corresponding to the current actually flowing to the light emitting element can be set, and the current can be limited based on the limit current, it is possible to reliably prevent the control failure due to the thermal rollover phenomenon during the automatic light amount control. .
[0099]
According to the third aspect of the present invention, by setting the limit voltage for determining the limit current of the light emitting element by the size ratio (W / L ratio) of the MOS transistors constituting the current mirror circuit, the current flowing through the light emitting element is reduced. Even if it becomes larger, the limit voltage generating means can control with a smaller current, so that the current consumption can be reduced.
[0100]
According to the fourth aspect of the present invention, by connecting a capacitor between at least one of the light amount control voltage signal line and the limit voltage signal line and the power supply line, the capacitor is caused by the high-speed inversion operation of the comparing means. When the potential fluctuation occurs in the signal line, the fluctuation is absorbed, and when the switching means switches based on the reversing operation of the comparing means, the switching voltage corresponds to the limit voltage corresponding to the limit current and the light emission amount. Since the light quantity control voltage can be prevented from being AC-coupled, the operation of limiting the current to the limit current can be reliably performed even at the timing of the switching.
[0101]
According to the fifth aspect of the present invention, the first switch element for selecting the light amount control voltage and the second switch element for selecting the limit voltage are controlled to be switched so that both ON periods do not overlap. Since it is possible to prevent the limit voltage corresponding to the limit current and the light amount control voltage corresponding to the light amount of the light emitting element from interacting with each other, it is possible to reliably perform the operation of limiting the current to the limit current even at the switching timing. be able to.
[Brief description of the drawings]
FIG. 1 is a circuit diagram illustrating a configuration example of a light emitting element driving device according to an embodiment of the present invention.
FIG. 2 is an explanatory diagram of an operation of switching between a light amount control voltage and a limit voltage.
FIG. 3 is a circuit diagram illustrating a configuration example of a main part of a light emitting element driving device according to a modification.
FIG. 4 is a circuit diagram illustrating a configuration example of a main part of a light emitting element driving device according to another modification.
FIG. 5 is a circuit diagram showing a configuration example of a light emitting element driving device when a driving method using only current driving is employed.
FIG. 6 is a circuit diagram showing an example of a specific circuit configuration of a main part of a drive current control circuit.
FIG. 7 is a timing chart for explaining the circuit operation of the switch signal generation circuit.
FIG. 8 is a circuit diagram showing a configuration example of a multi-laser driving device according to an application example of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... Drive current control circuit, 12 ... Drive voltage control circuit, 13, 23 ... Light amount detection circuit, 14, 24 ... Error detection circuit, 15, 25 ... Control circuit, 21 ... Surface emitting laser, 22-1 to 22-32 ... Drive control circuit, 112 ... Limit voltage generation circuit, 113 ... Comparator, LD ... Semiconductor laser, LD1 to LD32 ... Light emitting part of surface emitting laser

Claims (5)

Drive current supply means for supplying a drive current corresponding to the control voltage to the light emitting element,
Generating means for generating a first control voltage corresponding to the maximum allowable current;
Comparing means for comparing a second control voltage obtained according to a light emission amount of the light emitting element with the first voltage;
Selecting means for selecting one of the first and second control voltages based on a comparison result of the comparing means, and providing the selected one as the control voltage to the drive current supply means. Device driving device. A bias current supply unit configured to supply a bias current to the light emitting element, wherein a circuit method of superimposing a modulation current on the bias current and supplying the bias current to the light emitting element during modulation is adopted.
The light emitting device driving device according to claim 1, wherein the limit current is determined according to a total current flowing through the light emitting device. The generation means includes a first transistor having a gate and a drain connected in common, and a second MOS transistor forming the current supply means, and a supply current to a common connection point between the gate and the drain is provided. 2. The light emitting device driving device according to claim 1, wherein a ratio of a channel width to a channel length of the first MOS transistor and the second MOS transistor is set so as to be smaller than a driving current. Between at least one of a signal line transmitting the first control voltage to the selection means and a signal line transmitting the second control voltage to the selection means, and a source of a MOS transistor constituting the current supply means; The light emitting device driving device according to claim 3, further comprising a connected capacitor. The selecting means has a first switch element for selecting the second control voltage, and a second switch element for selecting the first control voltage, and the first switch element selects the first control voltage. 5. The light emitting element driving device according to claim 4, wherein the switching control is performed so that the ON periods do not overlap.

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