JP4797418B2 - Light emitting element driving device - Google Patents

Description

  The present invention relates to a light emitting element driving apparatus and an image forming apparatus, and more particularly to a light emitting element driving apparatus suitable for driving a laser element used as a light source for laser xerography.

  In recent years, in the field of laser xerography, in order to meet the demand for higher resolution and higher speed, development of an apparatus using a surface emitting laser capable of emitting a large number of laser light beams as a laser light source has been advanced. .

A surface emitting laser generally has a higher internal resistance than an edge emitting laser . As a method for high-speed modulation of emitted light of a laser having a high internal resistance, there is a method using voltage driving as disclosed in Patent Document 1 below, for example. This configuration is shown in FIG. FIG. 1B shows a drive current waveform of the surface emitting laser 703 in the drive circuit shown in FIG. When the drive current becomes equal to or greater than the light emission threshold shown in FIG. 1B, the laser emits light with a light emission amount corresponding to the drive current.

  As shown in FIG. 1A, in Patent Document 1, in a driving circuit having a switching element 701, a diode 702, a constant voltage source 703, a semiconductor laser 704, and a constant current circuit 710, a direct current for bias is a constant current. A high-speed response is made possible by generating a pulse voltage using the switching element 701 during high-speed driving and generating the pulse voltage and applying this pulse voltage to the semiconductor laser 704. In such a configuration, in principle, the pulse characteristics are determined regardless of the internal resistance of the semiconductor laser 704, and an ideal undistorted waveform as shown by the dotted line in FIG. The integrated light quantity in b) (that is, the pulse width-integrated light quantity characteristic: this is called the Cin characteristic) can be brought close to the ideal straight line shown in FIG. The integrated light amount is a value obtained by integrating the light emission amount of the laser by the lighting time when the pulse is turned on.

However, in an apparatus using a surface emitting laser, it is necessary to lay out multi-channel (CH) wiring. Therefore, when the configuration disclosed in Patent Document 1 is applied to a surface emitting laser, a driving circuit (also referred to as a driver). As a result, the routing from the laser to the laser becomes longer, and the wiring parasitic capacitance including the adjacent capacitance between the wirings becomes larger. For this reason, even when voltage driving is performed, the pulse characteristic rounding due to the influence of the time constant (Rint + R1) × C of the wiring parasitic capacitance C and the output impedance (= switch ON resistance Rint + wiring resistance R1) cannot be ignored. As shown in FIG. 1B, the integrated light amount is smaller than that of the ideal waveform, so that the Cin characteristic is substantially deviated from the ideal straight line as shown by the dotted line in FIG. As described above, when the surface emitting laser is applied to the configuration according to Patent Document 1, the integrated light quantity (Cin characteristic) with respect to the drive pulse width passes through a place lower than the ideal straight line, and the laser does not emit light until the pulse width exceeds a certain value. Or since the integrated light quantity is not proportional to the pulse width in the vicinity of the minimum pulse width, there remains a problem that it is necessary to correct the driving pulse.

  In view of this, the present applicant disclosed in Patent Document 2 that light emission when a constant current drive is performed in a light emitting element driving apparatus having a voltage source for driving a voltage of the light emitting element and a current source for driving a current of the light emitting element. An invention is disclosed that corrects the voltage value of constant voltage driving applied at the start of lighting with reference to the terminal voltage of the element.

JP 2001-36186 A JP 2005-63997 A

  However, when a large number of light emitting elements such as a surface emitting laser are driven simultaneously, if the characteristics of the light emitting elements vary, the light emitting elements can be corrected even with a constant correction voltage (correction value) ΔV as shown in FIG. Variation occurs in the light emission amount ΔL (ΔL1, ΔL2). For this reason, there arises a problem that the correction amount of the Cin characteristic varies for each light emitting element.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a light emitting element driving device capable of accurately correcting Cin characteristics even if the characteristics and circuit characteristics of the light emitting elements vary. .

In order to achieve the above object, a light emitting element driving apparatus according to the present invention includes a plurality of light emitting elements, a voltage source for voltage driving the plurality of light emitting elements, and a current source for current driving the plurality of light emitting elements. And a detecting unit that detects the light amount of the light emitting element that is lit, and a control unit that generates a control voltage for causing the light amounts of the plurality of light emitting elements to match the target light amount based on the light amount detected by the detecting unit And at least one of the plurality of light emitting elements is driven by at least one of a current corresponding to the control voltage shared from the current source and a voltage corresponding to the control voltage supplied from the voltage source. Drive means, and the control unit controls the first control voltage when the light amount of each light emitting element is controlled to be the first light amount, and the light amount of each light emitting element is the second light amount. Second when controlled A control voltage is obtained for each of the plurality of light emitting elements, and a voltage-light quantity characteristic indicating a relationship between the control voltage and the light emission quantity of the light emitting element is obtained using the obtained first control voltage and the second control voltage. While calculating for each light emitting element, and calculating the correction control voltage when the light quantity of each light emitting element becomes a corrected light quantity obtained by adding a constant light quantity to the first light quantity by linear interpolation using the calculated voltage-light quantity characteristic. The selected light emitting element of the plurality of light emitting elements is driven by voltage driving with the correction control voltage, and after the voltage driving by the correction control voltage of the selected light emitting element, the selected light emitting element is The first light is turned on by at least one of a current supplied from a current source and a voltage supplied from the voltage source, and the constant light amount is a light emission amount of a light emitting element to be turned on. As setting a large amount of light, and sets small the constant amount of light.
According to the present invention, the correction voltage can be controlled so that the correction light quantity is constant based on the voltage-light quantity characteristic of each light emitting element. Therefore, the Cin characteristics can be accurately corrected even if the characteristics of the light emitting elements and the circuit characteristics vary.

  The image forming apparatus of the present invention has a configuration in which a laser element is driven by the light emitting element driving device according to any one of claims 1 to 4.

  The present invention can correct the Cin characteristic with high accuracy even when the characteristics and circuit characteristics of the light emitting element vary.

  Preferred embodiments of the present invention will be described with reference to the accompanying drawings.

  First, a first embodiment of the present invention will be described in detail with reference to the drawings. FIG. 3 is a diagram illustrating the overall configuration of the light emitting element driving apparatus according to the present embodiment. In FIG. 3, the light emitting element driving apparatus 10 drives a plurality of light emitting elements. In the configuration of FIG. 3, the light emitting element driving apparatus 10 drives 32 light emitting elements LD1 to LD32. In other words, the light emitting element driving device 10 has a 32 channel configuration. Each of the light emitting elements LD1 to LD32 is formed of a surface emitting diode (VCSEL: Vertical Cavity Surface Emitting Laser) and arranged in a matrix. The light emitting element driving device 10 is formed of, for example, an IC chip and includes a circuit described below.

The light emitting element driving apparatus 10 has drivers 100 _{1 to} 100 ₃₂ for each channel, that is, for each of the light emitting elements LD1 to LD32. The light emitting element driving device 10 includes a common control potential setting circuit 200, a current amplifier 300, a light amount monitor 400, a forced lighting circuit 500, and an APC (Automatic Power Control) circuit 600 as a common control unit for each channel.

The drivers 100 _{1 to} 100 ₃₂ receive signals from the control unit common to the respective channels via the bus 150, and perform control for driving and controlling the light emitting elements LD1 to LD32, respectively. Specifically, the drivers 100 _{1 to} 100 ₃₂ perform APC control for controlling the light amount of the light emitting elements LD ₁ to LD ₃₂ and modulation control after the APC control. As will be described later, in APC control, the drivers 100 _{1 to} 100 ₃₂ control both the voltage and the current applied to the light emitting elements LD ₁ to LD ₃₂ . During voltage driving, the drivers 100 _{1 to} 100 ₃₂ control the capacitors Cd _{1 to} Cd ₃₂ respectively connected to the cathodes of the light emitting elements LD1 to LD32 via the terminals COUT. At the time of current driving, the drivers 100 _{1 to} 100 ₃₂ control the amount of current flowing through the light emitting elements LD ₁ to LD ₃₂ via the terminals LDOUT.

A plurality of drivers 100 _{1 to} 100 ₃₂ are connected in common via a terminal LDCOM and connected to a load 105. In the configuration of FIG. 3, the LDCOM terminals of the drivers 100 _{1 to} 100 ₄ are connected in common, and one end is connected to the other end of the load 105 connected to the ground. Each driver 100 _{1 to} 100 ₃₂ outputs a current (complementary output) corresponding to the drive current when the corresponding light emitting element is not driven. By flowing this current through the load 105, the operation is stabilized by constantly flowing a constant current to the light emitting element driving device 10 without depending on the number of lighting of the light emitting elements.

The light quantity control device 10 performs modulation control after setting the laser light quantity of each of the light emitting elements LD1 to LD32 to an appropriate value by APC control. The outline of APC control is as follows. First, the laser light amount of the light emitting element LD1 is adjusted. The driver 100 ₁ drives the light emitting element LD1. A current corresponding to the laser light amount of the light emitting element LD1 flows through a light receiver PD (for example, a photodiode, which corresponds to the above-described light receiver 11) provided in common to each of the light emitting elements LD1 to LD32. The current amplifier 300 turns on the switch SWSa with respect to the current flowing through the light receiver PD, and amplifies the current obtained by adding the addition current from the current source 450 with low impedance. In this case, when the switch SWSb is turned on, the added current is canceled by the reference current supplied from the current source 460, the remaining current is supplied to the resistor connected to the reference voltage Vref2, and the current output from the current amplifier 300 is supplied. The voltage is converted into a voltage, and this voltage (referred to as a detection voltage) is output to the APC circuit 600 via the switch SW19. The APC circuit 600 includes a plurality of operational amplifiers 61, a series circuit of one switch (any one of SWfb1 to SWfb32) and a capacitor (any one of Cfb1 to Cfb32). Each series circuit is connected between the output terminal and the inverting input terminal of the operational amplifier 61. Each series circuit constitutes a sample and hold circuit. One sample and hold circuit corresponds to one light emitting element. For example, a sample and hold circuit including the switch SWfb1 and the capacitor Cfb1 corresponds to the light emitting element LD1. Similarly, a sample and hold circuit including the switch SWfb32 and the capacitor Cfb32 corresponds to the light emitting element LD32.

The operational amplifier 61 amplifies the difference voltage when the light emitting element LD1 is driven and outputs the amplified voltage to the corresponding signal line of the bus 150. Driver 100 ₁ changes the drive current applied to the light emitting element LD1 so that this differential voltage becomes zero. As a result, the amount of laser light from the light emitting element LD1 changes, and the amount of current flowing through the light receiver PD changes. A detection voltage corresponding to the current flowing through the light receiver PD is output from the current amplifier 300 to the APC circuit 600. By such feedback control, the added current applied to the input output of the current amplifier 300 is canceled and disappears, and the driving state of the light emitting element LD1 is set so that the laser light quantity corresponds to the reference current generated by the APC reference voltage Vref. Set. The setting of the driving state means that both the driving current and the driving voltage applied to the light emitting element LD1 are adjusted to a value corresponding to the APC reference voltage Vref.

  While controlling the light emitting element LD1 in this way, only the switch SWfb1 is turned on among the 32 sample hold circuits of the APC circuit 600, and the laser light quantity of the light emitting element LD1 corresponds to the APC reference voltage Vref. The voltage at the time of convergence to the value to be stored in the capacitor Cfb1. Similarly, the APC control is performed on the light emitting elements LD2 to LD32 one by one in order.

  As will be described later, the APC control is preferably performed twice. In the second APC control, the switch SWSa that was turned on in the first APC is turned off. Since the cancellation current supplied to the output side of the current amplifier 300 is the reference current + addition current, the received light current is controlled by a current corresponding to the reference current + addition current. The 32 sample and hold circuits in the APC circuit 600 can be commonly used in the first and second APC controls, but 32 sample and hold circuits may be newly provided for the second APC control.

  The light quantity monitor 400 outputs a light quantity monitor signal indicating the laser light quantity of each of the light emitting elements LD1 to LD32 from the current flowing through the current amplifier 300.

The forced lighting circuit 500 is a circuit that generates a synchronization signal required before performing APC control. In image processing apparatuses such as copiers, printers, and facsimiles in which the light emitting element driving device 10 is incorporated, an optical sensor is provided slightly before the drawing start position in order to accurately determine the image drawing position, and the light emitting element outputs light is determined drawing start position based on the timing of crossing the optical sensor.

  FIG. 5 shows a configuration example of a laser scanning system and output of each sensor in laser xerography, which is an aspect of an image forming apparatus including the light emitting element driving device of the present invention. The basic configuration of the laser beam scanning system in the laser xerography apparatus is as follows. The laser light emitted from the laser light source 10d is irradiated onto the photosensitive member surface 16 through the lens 15, the polygon mirror 12, and the lenses 13 and 14. As the polygon mirror 12 rotates, the laser beam repeatedly scans the photosensitive member surface 16. Further, part of the laser light emitted from the laser light source 10 d is input to the light receiver 11 via the semi-transmissive mirror 19. In FIG. 5, the output of the light receiver 11 at this time is shown as the light amount control sensor output, and the output of the optical sensor 17 provided slightly before the drawing start position is shown as the SOS (Start of Scan) sensor output. The area for APC is provided before and after the scanning area. Reference numeral 18 corresponds to the light emitting element driving apparatus 10 described above.

  As described above, since the individual laser light amounts of the light emitting elements LD1 to LD32 are smaller than those of the end face laser, a plurality of them are turned on simultaneously to scan the SOS sensor. In this case, it is particularly preferable to turn on only a plurality of light emitting elements located in the central portion among the light emitting elements arranged two-dimensionally. However, in APC control, the light emitting elements are turned on one by one and the conditions are set (gain of feedback loop). Therefore, if a predetermined number of light emitting elements are turned on at the same time, the feedback loop of APC control will oscillate. There is a possibility that. Therefore, in order to solve this problem, the forced lighting circuit 500 changes the load size of the current amplifier 300 according to the modulation signal (modulation data). That is, a load corresponding to the number of light emitting elements to be turned on is connected to the output of the current amplifier 300. In the configuration shown in the figure, a plurality of resistors are connected to the output of the current amplifier via a switch. Focusing on the operational amplifier 61, the forcible lighting circuit 500 reduces the current power conversion gain according to the number of light emitting elements to be turned on, so that the gain of negative feedback as a whole does not change. With such a configuration, a state equivalent to a state in which only one light emitting element is always turned on can be obtained. In other words, the gain of the feedback loop is a value in a state in which only one light emitting element is turned on. As a result, it is possible to prevent the feedback loop from oscillating.

The common control potential setting circuit 200 is a circuit that generates control potentials necessary for generating various currents required in the drivers 100 _{1 to} 100 ₃₂ . In the configuration of FIG. 3, the common control potential setting circuit 200 generates a common potential for setting a bias current flowing in each of the drivers 100 _{1 to} 100 _{32 and} a common potential for generating an offset current. Circuit. The bias current and the offset current are typical examples, and each driver 100 _{1 to} 100 ₃₂ can set a control potential necessary to generate other currents necessary for driving and control. The common control potential for setting the offset current is generated by a circuit including an operational amplifier (op-amp) 211, current sources 212 and 213, and loads 214 and 215. A common control potential for bias current setting and other current setting is also generated by the same circuit. In response to the offset current setting signal from the outside, the current source 212 supplies the instructed current to the load 214. The terminal voltage of the load 214 is applied to the plus side terminal of the operational amplifier 211. A constant current source 213 connected to the constant voltage source 216 causes a current corresponding to the output of the operational amplifier 211 to flow through the load 215. The terminal voltage of the load 215 is given to the negative terminal of the operational amplifier 211. The operational amplifier 211 controls the current source 213 so that the current source 213 flows the same current as the offset current set by the offset current setting signal. At this time, the output signal of the operational amplifier 211 is output to the corresponding bus line of the bus 150. On the other hand, the positive voltage of the constant voltage source 216 is output to the corresponding bus line of the bus 150. This bus line is common to the respective common control potentials, and is common to the drivers 100 _{1 to} 100 ₃₂ . Thus, the offset current value set from the outside is supplied to each of the drivers 100 _{1 to} 100 ₃₂ via the bus 150 in the form of a differential voltage. Each driver 100 _{1 to} 100 ₃₂ generates an offset current from the received differential voltage as described later. As a result, even if the power supply voltage of the constant voltage source 216 fluctuates, the potential difference is constant, and the influence of fluctuations in the power supply voltage can be avoided. Note that the output voltage of the operational amplifier 211 and the voltage of the constant voltage source 216 are preferably transmitted by parallel two wires.

Next, the internal configuration of the drivers 100 _{1 to} 100 ₃₂ will be described with reference to FIG. Since each of the drivers 100 _{1 to} 100 ₃₂ has the same configuration, the subscripts ₁ to ₃₂ will be omitted below and will be described simply as the driver 100.

  The driver 100 has two multipliers 21 and 22. Multiplier 21 is provided to control current source 30, and multiplier 22 is provided to control a corresponding one of capacitors Cd1 to Cd32 shown in FIG. Hereinafter, for convenience, one corresponding capacitor is denoted by Cd, and is indicated by a broken line in FIG. The capacitor Cd functions as a voltage source for a short time when the drive voltage to the laser rises. The current source 30 generates a current that flows through the corresponding light emitting element LD, and the capacitor Cd that functions as a voltage source supplies a driving voltage to the corresponding light emitting element LD.

  Here, the relationship between the driving current and the driving voltage (terminal voltage) of the surface emitting laser (voltage-current characteristics) is proportional (linear relationship) in a practical range because the internal resistance of the surface emitting laser is high. Further, the relationship between the drive current and the laser light quantity is also proportional (linear relationship) within a practical range. Based on such characteristics, the current amount of the current source 30 in the first APC control is determined so that the laser light amount of the light emitting element LD becomes the reference light amount (first light amount). In the second APC control, the laser amount is determined. The amount of light is determined to be the second amount of light. Similarly, the driving voltage accumulated in the capacitor Cd in the first APC control is determined so that the laser light amount of the light emitting element LD becomes the reference light amount (first light amount), and the laser light amount is the second in the second APC control. The amount of light is determined. By the interpolation or extrapolation process using these two values, the laser light quantity can be corrected to an arbitrary light quantity.

  Multipliers 21 and 22 can use 4-quadrant analog multipliers, and capacitors can be used as voltage sources to be connected to the multipliers. The inputs of the multipliers 21 and 22 have a differential configuration. Assuming that two differential inputs represented by + and − of each multiplier 21 and 22 are V1a, V1b and V2a and V2b, respectively, each multiplier 21 and 22 having a differential configuration has Iout = α (V1a−V1b). The current described by (V2a-V2b) is output. Where α is a constant.

  In such a laser driving device, a correction signal is input to one input terminal (multiplier terminal) of each multiplier 21 and 22, and a control voltage is input to the other input terminal (multiplier terminal). When the + side output of the complementary output of a multiplier that is normally configured as a differential is used, there is an offset current, but even if there is an offset in each of the multipliers 21 and 22, the capacitors C1 and C2 connected to the output are used. The offset is canceled during APC. The correction signal takes into consideration the situation where the laser light quantity varies depending on the scanning position of the laser beam, and has a control voltage corresponding to the scanning position of the laser beam.

  First, by the first APC control, the first light amount (set as a reference value) is set as follows. Switch SWSa is on, SWSb is off, SW1 is off, SW2 is off, SW3 is off, SW5-1 is on, SW5-2 is off, SW5-3 is off, SW5-4 is on, SW6-1 is on SW6-2 is off, SW6-3 is off, SW6-4 is on, SW7 is off, SW8 is on, SW11 is on, SW11-1 is on, SW11-2 is off, SW12 is off, SW13 is on , SW15-1 is turned off, SW15-2 is turned on, SW16 is turned off, and the switch SWSa is turned on. Further, when setting the first light quantity, a 0 V correction signal is applied to the multiplier terminals of the multipliers 21 and 22. In this state, since the multiplier is 0, each multiplier 21 and 22 outputs an offset voltage regardless of what control voltage is input to the multiplicand terminal. Further, the first APC reference voltage Vref1 is applied to the operational amplifier 61 of the APC circuit 600 shown in FIG. The operational amplifier 61 outputs a control voltage so that the laser light amount of the light emitting element LD becomes the first APC reference voltage Vref1. This control voltage is supplied to the current source 30 through the switch SW8, the operational amplifier 26, the inverter 28, and the switch SW11 in FIG. The current source 30 supplies a current corresponding to the received control voltage to the light emitting element LD. The control voltage output from the operational amplifier 26 is stored in the capacitor C3-1 of the sample and hold circuit. Since the correction signal is set to 0V, the multiplier 21 outputs an offset voltage. Therefore, the capacitor C1 is charged with a difference voltage between the control voltage and the offset voltage output from the multiplier 21. On the other hand, the control voltage output from the operational amplifier 61 of FIG. 3 is supplied to the capacitor C2 and stored in the capacitor C4-1 of the sample and hold circuit. Since the correction signal is set to 0V, the multiplier 22 outputs an offset voltage. Therefore, the capacitor C2 is charged with a difference voltage between the control voltage and the offset voltage of the multiplier 22.

  Then, the second light quantity (this is called a correction light quantity) is set as follows by the second APC control. Switch SWSa off, SWSb off, SW1 off, SW2 off, SW3 off, SW5-1 off, SW5-2 on, SW5-3 on, SW5-4 off, SW6-1 off SW6-2 is on, SW6-3 is on, SW6-4 is off, SW7 is off, SW8 is off, SW11 is off, SW11-1 is on, SW11-2 is off, SW12 is off, SW13 is on , SW15-1 is off, SW15-2 is off, SW16 is off, and SWSa is off. Further, when setting the second light quantity, a correction signal having a predetermined voltage is applied to the multiplier terminals of the multipliers 21 and 22. Further, since the switch SWSa is off, the operational amplifier 61 outputs a control voltage for the first APC control so that the amount of light from the light receiver PD increases by the amount of current added by the current source 450. This control voltage is supplied to the current source 30 through the switch SW8, the operational amplifier 26, the inverter 28, the switches SW5-2 and SW5-3, the multiplier 21, the resistor R11, and the capacitor C1 in FIG. The current source 30 changes the current from the light receiver PD from the reference current to a current obtained by adding the addition current to the reference current in accordance with the received control voltage. The control voltage output from the operational amplifier 26 is stored in the capacitor C3-2 of the sample and hold circuit. The capacitor C1 is charged with a voltage difference between the control voltage and the output of the multiplier 21. The current applied to the light emitting element LD in the first APC control can be described as I + ΔI. On the other hand, the control voltage output from the operational amplifier 61 of FIG. 3 is supplied to the capacitor C2 and stored in the capacitor C4-2 of the sample and hold circuit. The capacitor C2 is charged with a voltage difference between the control voltage and the output of the multiplier 22. If the voltage stored in the capacitor C2 in the first APC control is V, the voltage stored in the capacitor C2 in the second APC control can be described as V + ΔV.

  Here, the switches SW6-1 and SW6-4 are turned on, and SW6-2 and SW6-3 are turned off. However, in the second and subsequent APCs, SW6-3 and SW6-1 are turned on, and SW6-2 and SW6-4 are turned off. Since this is the same condition as during modulation, an improvement in accuracy can be expected.

  At the time of modulating the light emitting element LD, a correction voltage corresponding to the light amount correction amount corresponding to the scanning position of the laser light is input to the multiplier terminals of the multipliers 21 and 22. Thereby, both the drive voltage applied to the surface emitting laser from the voltage source constituted by the multiplier 22, the capacitor C2, and the operational amplifier 26 and the drive current supplied from the current source 30 to the light emitting element LD are simultaneously controlled. The light emitting element LD emits light with a light amount corrected according to the scanning position of the laser light.

  A resistor R11 is connected in series with the capacitor C1. That is, in this embodiment, the sample and hold circuit 110 including the capacitor C1 is configured with a low-pass filter. Thereby, the high frequency noise which generate | occur | produces when switching on / off of switch SW11 can be suppressed. In addition, a capacitor C11 is connected in parallel to this low-pass filter. Thereby, it is possible to prevent the phase of the negative feedback loop from being delayed by the time constant of the low-pass filter. Similarly, by connecting a resistor R21 in series with the capacitor C2, the sample-and-hold circuit 120 including this is configured with a low-pass filter. Thereby, the high frequency noise which generate | occur | produces when switching on / off of switch SW8 can be suppressed. Further, a capacitor C22 for preventing the phase delay of the negative feedback loop is connected in parallel to the low-pass filter composed of the capacitor C2 and the resistor R21 to prevent oscillation in the negative feedback loop.

  The voltage application time adjustment circuit 800 controls the switch SW2 to adjust the time for applying a voltage to the light emitting element LD. This voltage is a voltage accumulated in the capacitor Cd. As described above, in this embodiment, the light emitting element LD is driven by controlling both the voltage and current applied to the light emitting element LD. When the light emitting element LD is driven, it is first driven with a voltage and then with a current. By making the voltage application time for voltage drive adjustable, the voltage application time according to the mounting state of the light emitting element LD can be appropriately set as in the case where the wiring from the LDOUT end to the laser in FIG. Can be set.

Voltage application time adjustment circuit 800, a delay circuit 81 and the exclusive OR circuit 82 and 2 Kumiyu, two delay circuits 81 are connected as shown by the inverter 83. The delay circuit 81 receives the voltage application time signal and the modulation signal, and delays the modulation signal according to the voltage application time signal. The exclusive OR of the output signal of one delay circuit 81 and the modulation signal is taken, and the switch SW2 is turned on by the output signal. As a result, the output signal has a first pulse rising at the rising edge of the modulation signal, a first pulse falling at the rising edge of the delayed modulation signal, and a second pulse falling at the falling edge of the delayed modulation signal. appear. That is, the voltage is applied at the rise and fall of the modulation signal with the same pulse width as the delay time of the delay circuit 81. In this way, it is possible to set an appropriate voltage application time. Similarly, the switch SW1 is controlled by the action of the other delay circuit 81 and the exclusive OR circuit 82 to supply an OFF bias, thereby controlling the operation of the light emitting element LD from on to off (speeding up).

  The current generation circuit 700 receives the differential voltage for each current output from the common control potential setting circuit 200 shown in FIG. 3, and outputs a current corresponding to the differential voltage. The operational amplifier 34 and the constant current source 32 of the current generation circuit 700 receive a differential voltage formed by the reference common potential and the reference offset potential, and generate an offset current corresponding to the differential voltage. The offset current flows to the load 24 via the switch SW16. The terminal potential of the capacitor C2 is determined in accordance with the offset current, whereby the driving voltage applied to the light emitting element LD by the capacitor C2 functioning as a voltage source can be adjusted. By adjusting the drive voltage, you can overshoot the drive pulse and follow the laser to a short pulse width to improve the reproducibility of highlights, and by setting the drive voltage slightly higher, the image outline can be It can also be used for image quality adjustment by appropriately setting these according to the image, such as emphasis. The operational amplifier 35 and the current source 31 receive the differential voltage formed by the reference common potential and the reference bias potential via the switch 750, and generate a bias current according to the differential voltage. Further, the current source 31 that has received the OFF bias voltage set by the voltage source in the drawing connected to the switch 750 generates a laser drive current corresponding to the OFF bias voltage.

  Note that the switch SW16, the load 24, and the current source 32 in FIG. The correction circuit 250 includes a common reference potential (denoted as a common reference in the figure) and a reference offset potential (denoted as a reference offset in the figure) generated by the operational amplifier 211, the constant current sources 212 and 213 and the loads 214 and 215 in FIG. ), The drive voltage output from the voltage source 230 is corrected.

  The light emitting element driving apparatus of the present embodiment having the above configuration corrects the output voltage of the voltage source based on the voltage-light quantity characteristic of the light emitting element LD so that the correction light quantity at the time of rising of the light emitting element LD becomes constant. The operation of this embodiment will be described with reference to FIG. FIG. 6 shows only the configuration of the essential part of the invention from the configuration of the present embodiment shown in FIGS. 3 and 4 for the sake of simplicity.

  The correction circuit 250 shown in FIG. 6 linearly interpolates output voltages corresponding to the first light amount L1 (reference light amount) and the second light amount L2 (corrected light amount) obtained during the light amount control, and corrects each light emitting element LD. Set the light intensity (correction voltage). FIG. 7A shows voltages V1 and V2 when the light emitting element LD1 takes the first light amount L1 (reference light amount) and the second light amount L2 (corrected light amount). In addition, voltages V3 and V4 when the light emitting element LD2 takes the first light amount L1 (reference light amount) and the second light amount L2 (correction light amount) are also shown. The correction circuit 250 obtains a characteristic curve shown in FIG. 7B by linear interpolation for each light emitting element, and obtains a correction value ΔV when the light emitting element takes the correction light amount ΔL from the characteristic curve. Since the light emission amounts of the light emitting elements LD1 and LD2 when the same voltage is applied vary, the correction value ΔV1 of the light emitting element LD1 and the correction value ΔV2 of the light emitting element LD2 take different values.

  This will be described in more detail with reference to FIG. First, the switch SW11-1 and the switch SW2 are turned on at the timing of APC1 for obtaining the first light quantity L1 (reference light quantity (without correction)) shown in FIG. 8, and the voltage source 230 and the current source 30 are controlled by the control voltage. Is done. The correction circuit 250 is in a state in which the voltage source 230 is not corrected, but the light / voltage converted signal at this time, that is, a signal corresponding to the voltage applied to the light emitting element when the reference light amount is set is held in the correction circuit 250. Is done.

Next, the switch SW11-1 and the switch SW2 are turned on at the timing of APC2 for obtaining the second light quantity at the time of maximum correction (correction signal X = F: 4 bits), and the voltage source 230 and the current source 30 are controlled by the control voltage. Is done. Signal corresponding to the applied voltage of the light emitting element when set to the correction circuit 250 optical / voltage conversion when the signal or second light amount is held in the correction circuit 250. The correction circuit 250 obtains a characteristic curve by performing linear interpolation from the voltage at the first light amount L1 (reference light amount) and the voltage at the second light amount L2 (corrected light amount) obtained as described above. . Then, a correction voltage ΔV when the light emitting element takes a correction light amount ΔL (in this embodiment, a correction signal X = 8: 4 bits) is obtained from this characteristic curve. The obtained correction voltage ΔV is superimposed on the drive voltage by the voltage source 230. At the time of image formation, since the correction voltage ΔV set at the time of controlling the light amount is applied only at the rising edge of the lighting pulse of the light emitting element LD, the switches SW11-1 and SW2 are turned on only at the rising edge of the light emitting element LD. , SW2 is turned off, and current driving is performed with a light amount corresponding to the first light amount L1 (reference light amount).

  In this way, when driving the laser with the voltage current drive method, the output voltage of the voltage source is corrected so that the correction light quantity at the rising edge of the light emission pulse is constant based on the voltage-light quantity characteristic of the light emitting element. Even if the laser characteristics and circuit characteristics vary, the Cin characteristics (integrated light quantity with respect to the pulse width) can be accurately corrected.

As shown in FIG. 9, the Cin characteristic gradually approaches an ideal straight line because the areas of areas a and d affected by waveform rounding become equal as the amount of light increases. On the other hand, if the correction amount A is constant , the influence of the correction becomes smaller as the amount of light increases. Therefore, the correction amount A is set to be optimum when the light amount is the minimum (that is, when the Cin characteristic deviates most from the ideal line). For example, these two effects cancel each other, and the Cin characteristic remains almost optimal even when the amount of light is increased. Alternatively, when it is necessary to adjust with high accuracy in order to keep the correction light amount constant even if the light amount increases, the correction ratio is increased in accordance with the increase in the set light amount as shown in FIG. The correction signal to be determined may be set small, or the second light quantity fluctuation ratio may be reduced during the APC as shown in FIG.

  The above-described embodiment is a preferred embodiment of the present invention. However, the present invention is not limited to this, and various modifications can be made without departing from the scope of the present invention.

(A) is a figure which shows the structure of the prior art (patent document 1) at the time of using a voltage drive as a method of modulating the emitted light of a laser with high internal resistance at high speed, (b) is the drive shown to (a). It is a graph which shows the light emission waveform of the semiconductor laser 703 in a circuit. It is a figure which shows a voltage-light quantity characteristic, and is a figure which shows the calculation method of the conventional correction voltage. It is a circuit diagram which shows the whole structure of a light emitting element drive device. It is a circuit diagram which shows the structure of the driver 100 of a light emitting element drive device. It is a figure which shows the structural example of the laser scanning system in the laser xerography which is an aspect of an image forming apparatus provided with a light emitting element drive device. It is the figure which simplified and showed the light emitting element drive device, and is a figure which shows the principal part of invention. It is a figure for demonstrating the calculation method of the correction voltage of this invention. It is a figure for demonstrating the calculation method of the correction voltage of this invention. It is a figure which shows the correction amount when the light quantity of a light emitting element is large and small. It is a figure which shows the relationship between the increase in light quantity, and corrected light quantity.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10, 18 Light emitting element drive device 105 Load 10d Laser light source 11 Light receiver 12 Polygon mirror 13, 14, 15 Lens 16 Photoconductor surface 17 SOS sensor 21, 22 Multiplier 28 Inverter 23, 24, 214, 215 Load 81 Delay circuit 82 Exclusive OR circuit 150 Bus 100 _{1 to} 100 ₃₂ , 100 Driver PD Receiver 200 Common control potential setting circuit 216 Constant voltage source 230 Voltage source 250 Correction circuit 252 253 Current mirror circuit 212 213 Constant current source 300 Current amplifier 400 Light intensity monitor 500 Forced lighting circuit 600 APC circuit 700 Current generation circuit 800 Voltage application time adjustment circuit 26, 34, 35, 61, 211, 251 Operational amplifier (op amp)
30, 31, 32, 32 ', 450, 460 Current source 110, 220 Sample hold circuit LD1-LD32 Light emitting element Vref, Vref1, Vref2 APC reference voltage COUT, LDOUT, LDCOM terminals R2, R3, R11, R21, R31, R32 , R33, R34 resistor C1, C2, C3-1, C4-1, C3-2, C4-2, C11, C22, Cd, Cd 1 ~Cd 32, Cfb32~Cfb32 capacitor SW1, SW2, SW3, SW5-1 , SW5-2, SW5-3, SW5-4, SW6-1, SW6-2, SW6-3, SW6-4, SW7, SW8, SW11, SW11-1, SW11-2, SW12, SW13, SW15-1 , SW15-2, SW16, SW19, SWfb1 to SWfb32 switches Tr1, Tr3, Tr31, Tr33, Tr36 P MOS transistor Tr32, Tr34, Tr35 N-MOS transistor

Claims (1)

A plurality of light emitting elements ;
A voltage source for the voltage driving the plurality of light emitting elements,
A current source for current- driving the plurality of light emitting elements;
Detecting means for detecting the light amount of the light-emitting element that is lit;
Based on the light amount detected by the detection means, a control unit that generates a control voltage for causing the light amounts of the plurality of light emitting elements to match the target light amount, and
At least one of the plurality of light emitting elements is adjusted such that a current supplied from the current source is adjusted to a current corresponding to a control voltage, or a voltage supplied from the voltage source is adjusted to a voltage corresponding to a control voltage. Driving means for driving at least one of the generated voltage and
The controller controls a first control voltage when the light amount of each light emitting element is controlled to be the first light amount, and a second when the light amount of each light emitting element is controlled to be the second light amount. The voltage-light quantity characteristic indicating the relationship between the control voltage and the amount of emitted light of the light emitting element using the first control voltage and the second control voltage obtained for each of the plurality of light emitting elements. Is calculated for each light emitting element, and a correction control voltage is calculated when the light quantity of each light emitting element becomes a corrected light quantity obtained by adding a constant light quantity to the first light quantity by linear interpolation using the calculated voltage-light quantity characteristic. In addition, the selected light emitting element of the plurality of light emitting elements is driven by voltage driving with the correction control voltage, and after the voltage driving by the correction control voltage of the selected light emitting element, the selected light emitting element is The current A current supplied from, is lit by at least one of the voltage supplied from the voltage source,
The light- emitting element driving apparatus according to claim 1, wherein the constant light amount is set to be smaller as the first light amount, which is the light emission amount of the light-emitting element to be lit, is set larger .

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