JP2007158022A - Semiconductor laser driver circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor laser driver circuit which stabilizes the PWM characteristic against the operating temperature variation of a semiconductor laser. <P>SOLUTION: The semiconductor laser driver circuit 1 controls a laser diode LD1 to make its output light quantity constant in an LD1_APC mode, resulting in that its output light quantity is P<SB>0</SB>with its current I<SB>0</SB>. It controls the laser diode LD1 to make its output light quantity constant in an LD1_bias APC mode, resulting in that its output light quantity is P<SB>1</SB>with its current I<SB>0</SB>-I<SB>1</SB>. In an OFF mode, a current I<SB>LD1</SB>inputted to the laser diode LD1 is expressed by I<SB>LD1</SB>=α×(P<SB>0</SB>I<SB>0</SB>-P<SB>0</SB>I<SB>1</SB>-P<SB>1</SB>I<SB>0</SB>)/(P<SB>0</SB>-P<SB>1</SB>). The semiconductor laser driver circuit 1 feeds the laser diode LD1 with the current I<SB>LD1</SB>as a bias current in a no-image forming region. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a semiconductor laser driving circuit for driving a semiconductor laser, and more particularly to a semiconductor laser driving circuit for controlling a semiconductor laser driving current when image formation is performed by blinking a semiconductor laser in an image forming apparatus.

  2. Description of the Related Art Conventionally, in some image forming apparatuses such as electrophotographic copying machines and printers, a semiconductor laser is used as a light emitting element for performing image exposure.

  FIG. 5 is a diagram showing a schematic configuration of an electrophotographic printer using a conventional semiconductor laser.

  As shown in FIG. 5, the conventional printer includes a rotary polygon mirror 1015, a laser scanner motor 1016 that rotates the rotary polygon mirror 1015, and a laser diode 1017 that is an exposure light source. The laser diode 1017 is turned on or off according to the image signal from the laser driver 1029, and the light-modulated laser light emitted from the laser diode 1017 is emitted toward the rotary polygon mirror 1015.

 The laser beam issued by the laser diode 1017 is reflected as a deflected beam by changing its angle continuously on its reflecting surface as the rotary polygon mirror 1015 rotates. The reflected light is subjected to correction of distortion and the like by a lens group (not shown), and is scanned in the main scanning direction of the photosensitive drum 1010 via the reflecting mirror 1018. One surface of the rotary polygon mirror 1015 corresponds to one line scanning, and the laser light emitted from the laser diode 1017 by the rotation of the rotary polygon mirror 1015 is scanned in the main scanning direction of the photosensitive drum 1010 line by line. The photosensitive drum 1010 is charged in advance by a charger 1011 and sequentially exposed by scanning with a laser beam to form an electrostatic latent image. The photosensitive drum 1010 rotates in the direction of the arrow in FIG. 5, the formed electrostatic latent image is developed by the developing device 1012, and the developed visible image is transferred to a transfer sheet (not shown) by the transfer charging device 1013. The The transfer paper onto which the visible image has been transferred is conveyed to a fixing device 1014, and after being fixed, is discharged outside the apparatus.

 Here, a BD sensor 1019 is disposed in the vicinity of the scanning start position in the main scanning direction on the side of the photosensitive drum 1010 or a position corresponding thereto. Each of the laser beams reflected by the reflecting surfaces of the rotary polygon mirror 1015 is detected by the BD sensor 1019 prior to scanning one line on the photosensitive drum 1010. The detected BD signal is input to the timing controller 1027 as a scanning start reference signal in the main scanning direction. The timing controller 1027 generates and controls the timing signals of the memory FIFO 1028 and the laser driver 1029 so that the writing start position in the main scanning direction of each scanning line is synchronized with the signal of the BD sensor 1019 as a reference. With the above configuration, the printer performs image formation.

  As described above, the semiconductor laser drive circuit performs image exposure on the photosensitive drum 1010 by generating an optical pulse signal of the semiconductor laser based on the electrical image signal, thereby forming an image.

  When image formation is performed appropriately, the light emission amount of the semiconductor laser when performing image formation needs to be a desired light amount.

  Here, if the temperature of the semiconductor laser is constant, the amount of light emission is determined by the current value. Therefore, by using a driving method that controls the driving current at the time of lighting constant, the light emission amount of the semiconductor laser is set to a desired light amount. It is possible to control.

  However, the light emission characteristics of the semiconductor laser greatly depend on the operating temperature, and in order to obtain a desired light amount, it is necessary to control the driving current of the semiconductor laser with respect to changes in the operating temperature.

  In general, the current value at which the semiconductor laser has a desired light intensity is detected or calculated intermittently, and when forming an image, the semiconductor laser is driven based on this detection or the calculated current value. Thus, proper image formation is performed. For example, image formation is generally performed by a control method called APC (auto power control). Specifically, in the case of a system in which a polygon mirror is rotationally driven by a motor and a laser beam is scanned to perform raster scan on the photosensitive drum to form an image, the laser light passes through a non-image area other than the photosensitive drum. A lighting current having a desired light amount is detected or calculated at the scanning timing. Then, at the timing of scanning the image area on the photosensitive drum, image formation is performed by driving the semiconductor laser by the detection or the calculated lighting current.

  In addition, the semiconductor laser has a unique phenomenon called light emission delay. In order to improve the frequency characteristics during optical pulse modulation in the semiconductor laser, a bias current is supplied to the semiconductor laser when the semiconductor laser is turned off during image formation. A method of supplying a lighting current to the semiconductor laser when the semiconductor laser is turned on during formation is used.

  In low-speed printers and copiers, since the frequency for driving the semiconductor laser is relatively low, the bias current is often a fixed current. On the other hand, in a high-speed copying machine or printer, the frequency for driving the semiconductor laser needs to be relatively high.

  In order to effectively improve the frequency characteristics during optical pulse modulation in a semiconductor laser, it is desirable to set the bias current as close to the threshold current as possible.

  However, since the threshold current is highly dependent on the operating temperature and varies depending on the individual elements of the semiconductor laser, it is extremely difficult to actually set the bias current in the vicinity of the threshold current as a fixed constant current.

  Therefore, in order to improve the frequency characteristics at the time of optical pulse modulation in the semiconductor laser, the bias current characteristics with respect to the element and the temperature are detected as in the above-described APC, and the bias current is determined according to the detected characteristics. It is necessary to perform bias APC.

  An example of the bias APC will be described with reference to FIG.

In the bias APC, first, in the non-image forming region, performing constant light amount control for controlling the light intensity of the semiconductor laser at a constant light intensity P _0. Then, the current value I ₀ to the semiconductor laser at this time is stored by the storage means. Then, make certain quantity control for controlling the light intensity of the semiconductor laser at a constant light intensity P _1. Then, stored in the storage means a current value I ₁ to the semiconductor laser at this time.

  Here, the threshold current Ith of the semiconductor laser is a value calculated by the following equation (1).

_{Ith = (P 0 I 1 -P} 1 I 0) / (P 0 -P 1) ··· (1)
Therefore, a bias APC can be performed by providing an arithmetic circuit for performing the calculation of the above formula (1) and determining the bias current.

  However, in the state where the threshold current Ith is supplied to the semiconductor laser, the semiconductor laser does not oscillate, but the diode is lit. Since the amount of light for turning on the diode is not small, in some cases, the effect of turning on the diode occurs when the light is turned off during image formation, and the photosensitive drum may be slightly exposed, leading to deterioration in image quality.

  On the other hand, in the conventional semiconductor laser driving circuit, in order to prevent the influence of the diode light emission of the semiconductor laser, the bias current is set to the threshold current so that the light quantity of the diode light emission does not affect the image. It is controlled to be a value (current value Ip) calculated by the following equation (2), which is lower than Ith by the current value Ioffset. (For example, refer to Patent Document 1).

_{Ip = (P 0 I 1 -P} 1 I 0) / (P 0 -P 1) -Ioffset ··· (2)
JP-A-11-245444

  However, in recent years, image forming apparatuses such as printers and copiers have been improved in speed and image quality. For this reason, there has been a problem in conventional bias current control.

  For example, in a high image quality machine, there are many cases where pulse width modulation control is performed to match an optical pulse with image data. FIG. 7 shows the relationship PWM characteristics between the pulse width of the PWM signal and the light amount (output) in this case. In FIG. 7, the horizontal axis indicates the duty ratio of a PWM (pulse width modulation) signal input to the semiconductor laser driving circuit, and the vertical axis indicates the amount of laser light emitted from the semiconductor laser. In FIG. 7, when the semiconductor laser is fully lit, the light amount is adjusted to 20 mW. Also, as shown in FIG. 7, when the pulse width is 0%, the laser light is not emitted only by supplying a bias current, so the light quantity of the semiconductor laser is approximately 0 μW. If the pulse width does not increase to a certain extent, the semiconductor laser does not emit light due to the characteristics of the semiconductor laser driving circuit and the characteristics of the semiconductor laser. For this reason, the semiconductor laser starts laser emission when the pulse width is about 6%. The larger the amount of bias current, the smaller the amount of drive current for the semiconductor laser to emit laser light, so laser emission is started with a PWM signal having a small pulse width. As shown in FIG. 7, when the semiconductor laser starts laser emission, the amount of light increases as the pulse width increases.

  Here, since the pulse width of the PWM signal when the semiconductor laser starts laser emission differs depending on the value of the bias current, the amount of increase in the amount of light of the semiconductor laser corresponding to the pulse width of the PWM signal differs depending on the value of the bias current. . For this reason, the amount of increase in the amount of light of the semiconductor laser corresponding to the pulse width of the PWM signal has a characteristic that is offset according to the amount of bias current.

  When the pulse width is close to 100%, the semiconductor laser is difficult to be turned off due to the characteristics of the semiconductor laser driving circuit, the characteristics of the semiconductor laser, and the like, and is saturated. For this reason, when the pulse width is about 94%, the semiconductor laser is completely turned on, and the amount of light is saturated to 20 mW.

  Next, the relationship between the operating temperature of the semiconductor laser and the PWM characteristics will be described with reference to FIG. FIG. 7 shows PWM characteristics when the operating temperature of the semiconductor laser is changed to 50 ° C., 30 ° C., and 10 ° C.

As shown in FIG. 7, the PWM characteristics when the operating temperature of the semiconductor laser is changed to 50 ° C., 30 ° C., and 10 ° C. are different. Thus, the PWM characteristics vary according to the change in the operating temperature of the semiconductor laser. That is, even if the bias current Ip is controlled to a value calculated by Ip = (P ₀ I ₁ −P ₁ I ₀ ) / (P ₀ −P ₁ ) −Ioffset (formula (2)), There are cases where the PWM characteristics change with respect to the operating temperature, and the amount of light of the semiconductor laser differs for image data with the same duty ratio. For this reason, it has been difficult for the conventional semiconductor laser drive circuit to perform stable image formation.

  An object of the present invention is to provide a semiconductor laser driving circuit capable of stabilizing PWM characteristics with respect to operating temperature fluctuations of a semiconductor laser.

  In order to achieve the above object, a semiconductor laser driving circuit according to claim 1 is a semiconductor laser driving circuit for driving a semiconductor laser used in an image forming apparatus, wherein a first lighting current of the semiconductor laser is determined at the time of image formation. And a second determining means for determining a bias current to the semiconductor laser when the light is extinguished during image formation, wherein the first determining means and the second determining means are threshold currents of the semiconductor laser. The second determining means determines the bias current as α × Ith based on an arbitrary constant α and the threshold current value Ith of the semiconductor laser.

  According to a second aspect of the present invention, in the semiconductor laser driving circuit according to the first aspect, the arbitrary constant α is a value not less than 0.6 and less than 1.0.

  According to a third aspect of the present invention, there is provided the semiconductor laser driving circuit according to the first or second aspect, wherein the first determining means is configured to reduce the first light amount in the non-image forming region of the semiconductor laser. A first current value to be obtained is detected, and the lighting current is determined based on the first current value.

  According to a fourth aspect of the present invention, there is provided the semiconductor laser driving circuit according to the third aspect, wherein the second determining means is configured to obtain the second light amount in the non-image forming region of the semiconductor laser. The second current value is detected, and the bias current at the time of extinction is determined based on the first current value and the second current value.

  According to the semiconductor laser driving circuit of the first aspect, the first determining means for determining the lighting current of the semiconductor laser at the time of image formation, and the second for determining the bias current to the semiconductor laser at the time of turning off at the time of image formation. The correction means of the determination means corrects the lighting current and the bias current with respect to the fluctuation of the threshold current of the semiconductor laser. The second determining means determines the bias current as α × Ith based on the arbitrary constant α and the threshold current value of the semiconductor laser. In this way, the bias current is determined by setting the bias current of the semiconductor laser to α × Ith, which is α times the threshold current Ith, and the constant α as a constant that stabilizes the bias current even with respect to the operating temperature of the semiconductor laser. For this reason, the PWM characteristic can be stabilized with respect to the operating temperature fluctuation of the semiconductor laser. As a result, in a high-speed image forming apparatus, the image quality can be improved and the image formation can be highly stabilized.

  According to the semiconductor laser driving circuit of the second aspect, the arbitrary constant α is a value of 0.6 or more and less than 1.0. Thus, the constant α is a constant that makes the bias current more stable with respect to the operating temperature of the semiconductor laser. Therefore, it is possible to further stabilize the PWM characteristic against the operating temperature fluctuation of the semiconductor laser.

  According to the semiconductor laser driving circuit of claim 3, the first determining means detects the first current value for obtaining the first light quantity in the non-image forming region of the semiconductor laser, and The lighting current is determined based on the current value of 1. For this reason, in a high-speed image forming apparatus, the image quality can be improved and the image formation can be further stabilized.

  According to the semiconductor laser driving circuit of the fourth aspect, the second determining means detects the second current value for obtaining the second light quantity in the non-image forming region of the semiconductor laser, and Based on the current value of 1 and the second current value, the bias current at the time of turn-off is determined. For this reason, in a high-speed image forming apparatus, the image quality can be further improved and the image formation can be further stabilized.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a circuit diagram showing a configuration of a semiconductor laser driving circuit according to an embodiment of the present invention.

  As shown in FIG. 1, the semiconductor laser drive circuit 1 according to the present embodiment includes a light emitting element 2, switches SW1, SW2, SW3, SW4, error amplifiers 12, 16, 22, 26, and resistors R1, R2. , R3, R4. The semiconductor laser drive circuit 1 is mounted on an image forming apparatus.

  The light emitting element 2 includes laser diodes LD1 and LD2 which are semiconductor lasers provided in a package, and a photodiode PD1 for monitoring the light emission amount of the laser diode. In the light emitting element 2, the cathode terminals of the laser diodes LD1 and LD2 and the anode terminal of the photodiode PD1 serve as a common terminal and are grounded. The cathode terminal of the photodiode PD1 is connected to the plus input terminals of the error amplifiers 12 and 22 via the switches SW1 and SW2, respectively, and to the minus input terminals of the error amplifiers 16 and 26 via the switches SW3 and SW4, respectively. It is connected.

  The positive input terminals of the error amplifiers 12 and 22 are connected to the power supply Vcc via resistors R1 and R2, respectively. The negative input terminals of the error amplifiers 16 and 26 are connected to the power supply Vcc via resistors R3 and R4, respectively. It is connected to the. Further, when any of the switches SW1, SW2, SW3, SW4 is turned on and a signal is input to any of the error amplifiers 12, 22, 16, 26, a DC bias is applied to the photodiode PD1. It is configured. Reference voltages Vref1 and Vref2 are applied to the negative input terminals of the error amplifiers 12 and 22, respectively, and reference voltages Vref3 and Vref4 are applied to the positive input terminals of the error amplifiers 16 and 26, respectively.

  As shown in FIG. 1, the semiconductor laser drive circuit 1 includes sample / hold circuits 11, 15, 21, 25, voltage-current conversion circuits 10, 14, 20, 24, function circuits 13, 23, and switching. Elements Tr1 and Tr2 are provided.

  The output terminals of the error amplifiers 12 and 22 are connected to the input terminals of the sample / hold circuits 11 and 21, respectively. The output terminals of the sample / hold circuits 11 and 21 are connected to the input terminals of the voltage / current converter circuits 10 and 20, respectively. The output voltages of the sample / hold circuits 11 and 21 are supplied from the voltage / current converter circuits 10 and 20, respectively. It becomes the current control voltage of the output current. The output terminals of the sample / hold circuits 11 and 21 are connected to the input terminals of the function circuits 13 and 23, respectively.

  The output terminals of the voltage / current conversion circuits 10 and 20 are connected to the anodes of the laser diodes LD1 and LD2, respectively, and to the collectors of the switching elements Tr1 and Tr2. The emitters of the switching elements Tr1 and Tr2 are connected to the input terminals of the voltage / current conversion circuits 14 and 24, respectively. A video signal (image data) is input from an image controller (not shown) to the bases of the switching elements Tr1 and Tr2.

  The output terminals of the error amplifiers 16 and 26 are connected to the input terminals of the sample / hold circuits 15 and 25, respectively. The output terminals of the sample / hold circuits 15 and 25 are connected to the input terminals of the function circuits 13 and 23, respectively. The output terminals of the function circuits 13 and 23 are connected to the voltage / current conversion circuits 14 and 24, respectively. The output voltages of the function circuits 13 and 23 are current control of currents output from the voltage / current conversion circuits 14 and 24, respectively. Voltage.

  Next, the operation of the semiconductor laser drive circuit 1 having the above-described configuration will be described with reference to FIGS. FIG. 2 is a diagram showing the relationship between the current I and the light emission amount L of the laser diodes LD1 and LD2, and FIG. 3 is a sequence diagram showing the operation of the semiconductor laser drive circuit 1. As shown in FIG.

  As shown in FIG. 3, the operation mode of the semiconductor laser drive circuit 1 is first set to the LD1_APC mode. At this time, the switch SW1 is set to the ON state, and the switches SW2, SW3, and SW4 are set to the OFF state. The sample / hold circuit 11 is set to a sample mode in which the output voltage of the error amplifier 12 is sampled and output. On the other hand, the sample / hold circuits 15, 21, 25 are set in a hold mode in which their output voltages are held at the output voltages of the error amplifiers 16, 22, 26 sampled in the sample mode, respectively. The transistor Tr1 is set to the OFF state, and the transistor Tr2 is set to the ON state. As described above, each component is controlled by sending a signal from a controller (not shown).

  In the above-described state, when the laser diode LD1 is turned on according to the control of the controller (not shown), the light enters the photodiode PD1 as monitor light, and an electromotive current Imon is generated in the photodiode PD1. This electromotive current Imon flows through the resistor R1. When the resistance value of the resistor R1 is R1, a voltage of R1 × Imon is generated between the resistors R1, and the input voltage of the plus input terminal of the error amplifier 12 is decreased. When the input voltage at the plus input terminal of the error amplifier 12 becomes equal to or lower than the reference voltage Vref1, the output of the error amplifier 12 decreases and the control input (current control voltage) to the voltage-current conversion circuit 10 decreases. When the control input to the voltage / current converter circuit 10 decreases, the output current also decreases, and the input current to the laser diode LD1 decreases. For this reason, the light emission amount of the laser diode LD1 decreases.

  When the light emission amount of the laser diode LD1 decreases, the electromotive current Imon of the photodiode PD1 decreases, and the input voltage at the plus input terminal of the error amplifier 12 increases. When the input voltage of the positive input terminal of the error amplifier 12 becomes equal to or higher than the reference voltage Vref1, the output of the error amplifier 12 increases and the control input of the voltage / current conversion circuit 10 increases. When the control input of the voltage / current conversion circuit 10 increases, the output current also increases, and the input current of the laser diode LD1 increases. For this reason, the light emission amount of the laser diode LD1 increases.

As described above, in the semiconductor laser drive circuit 1, the error amplifier 12 and the like constitute a negative feedback circuit. For this reason, in the LD1_APC mode, the light emission amount of the laser diode LD1 is controlled so that the voltage due to the electromotive current of the photodiode PD1 becomes R1 × Imon = Vref1. Since the resistance value R1 of the resistor R1 and the reference voltage Vref1 are fixed values, the electromotive current Imon of the photodiode PD1 is controlled to be constant. Since the electromotive current Imon of the photodiode PD1 is proportional to the output light amount (light emission amount) of the laser diode LD1, the output light amount of the laser diode LD1 is controlled to be constant. Assuming that the output light amount of the laser diode LD1 when the electromotive current Imon of the photodiode PD1 is constant is P ₀ (first light amount), the current flowing through the laser diode LD1 becomes I ₀ (first current value). (See FIG. 2).

  Next, as shown in FIG. 3, the operation mode of the semiconductor laser driving circuit 1 is the LD1_bias APC mode. At this time, the switch SW3 is set to the ON state, and the switches SW1, SW2, and SW4 are set to the OFF state. The sample / hold circuits 11, 21, 25 are set in the hold mode, and the sample / hold circuit 15 is set in the sample mode. The transistors Tr1 and Tr2 are set to the ON state. As described above, each component is controlled by transmitting a signal from a controller (not shown).

Supplied in the above state, the sample / hold circuit 11 is in hold mode, because it holds the voltage sampled at LD1_APC mode (retention), the voltage-current converter 10 the same current _{I 0} and LD1_APC mode . When the laser diode LD1 is turned on, monitor light enters the photodiode PD1, and an electromotive current Imon is generated. This electromotive current Imon flows through the resistor R3. When the resistance value of the resistor R3 is R3, a voltage of R3 × Imon is generated between the resistors R3, and the input voltage of the negative input terminal of the error amplifier 16 is lowered. When the input voltage at the negative input terminal of the error amplifier 16 becomes equal to or lower than the reference voltage Vref3, the output of the error amplifier 16 increases and the control input to the voltage-current conversion circuit 14 increases. Also increased and the output current control input of the voltage-current conversion circuit 14 is increased, the current supplied to the laser diode LD1 becomes smaller than I _0. As a result, the light emission amount of the laser diode LD1 is reduced.

  When the light emission amount of the laser diode LD1 decreases, the electromotive current Imon of the photodiode PD1 decreases, the voltage across the resistor R3 decreases, and the input voltage at the negative input terminal of the error amplifier 16 increases. When the input voltage at the negative input terminal of the error amplifier 16 becomes equal to or higher than the reference voltage Vref3, the output of the error amplifier 16 decreases and the control input to the voltage-current conversion circuit 14 decreases. When the control input to the voltage / current conversion circuit 14 decreases, the output current also decreases, and the input current to the laser diode LD1 increases. For this reason, the light emission amount of the laser diode LD1 increases.

As described above, in the semiconductor laser driving circuit 1, the error amplifier 16 and the like constitute a negative feedback circuit. For this reason, the light emission amount of the laser diode LD1 is controlled so that the voltage due to the electromotive current of the photodiode PD1 becomes R3 × Imon = Vref3 in the LD1_bias APC mode. Since the resistance value R3 of the resistor R3 and the reference voltage Vref3 are fixed values, the electromotive current Imon of the photodiode PD1 is controlled to be constant. Since the electromotive current Imon of the photodiode PD1 is proportional to the output light amount of the laser diode LD1, the output light amount of the laser diode LD1 is controlled to be constant. When the output light amount of the laser diode LD1 when the electromotive current Imon of the photodiode PD1 becomes constant is P ₁ (second light amount), the current flowing through the laser diode LD1 is I ₀ −I ₁ (second current value). (See FIG. 2).

  Next, as shown in FIG. 3, the operation modes of the semiconductor laser driving circuit 1 are an LD2_APC mode and an LD2_bias APC mode. Since the LD2_APC mode and the LD2_bias APC mode are the same as the operations in the LD1_APC mode and the LD1_bias APC mode, description thereof is omitted.

  Next, as shown in FIG. 3, the operation mode of the semiconductor laser drive circuit 1 becomes the OFF mode. At this time, the switches SW1, SW2, SW3, SW4 are set to the OFF state. The sample / hold circuits 11, 15, 21, and 25 are set to the hold mode, and the transistors Tr1 and Tr2 are set to the ON state. As described above, each component is controlled by sending a signal from a controller (not shown).

The operation of the laser diode LD1 in the above state will be described. The sample / hold circuit 11 is in the hold mode, and holds the sampled voltage in the LD1_APC mode. For this reason, the voltage-current conversion circuit 10 supplies the same current I ₀ as in the LD1_APC mode.

The sample / hold circuit 15 is in the hold mode, and holds the sampled voltage in the LD1_bias APC mode. Therefore, the output of the sample / hold circuits 11 and 15 is input to the function circuit 13. The output of the sample / hold circuit 11 is a voltage that determines the current I _0, and the output of the sample / hold circuit 15 is a voltage that determines the current I ₁ .

  The function circuit 13 performs an operation using the input voltage. Specifically, the function circuit 13 inputs a control voltage such that the voltage-current conversion circuit 14 outputs a current If expressed by the following formula (3) to the voltage-current conversion circuit 14.

If = (1-α) · I ₀ -αP ₀ I ₁ / (P ₀ -P ₁ ) (3)
Here, the coefficient α is a constant having a predetermined value, and P ₀ and P ₁ are also predetermined light amounts. Therefore, the function circuit 13 is configured by a simple addition / subtraction circuit. The coefficient α is an arbitrary value between 0.6 and 1.0. Since the transistor Tr1 is in the ON state, the current I _LD1 input to the laser diode _LD1 is
I _LD1 = α · (P ₀ I ₀ -P ₀ I ₁ -P ₁ I ₀ ) / (P ₀ -P ₁ ) (4)
Current, i.e.
I _LD1 = α × Ith (5)
Will be entered. Note that Ith of the laser diode LD1 is a threshold current. In the OFF mode, the laser diode LD2 operates in the same manner as the laser diode LD1, and a description thereof will be omitted.

  Next, as shown in FIG. 3, the operation mode of the semiconductor laser driving circuit 1 becomes the DATA mode. At this time, the switches SW1, SW2, SW3, and SW4 are set to the OFF state, and the sample / hold circuits 11, 15, 21, and 25 are set to the hold mode by signals from a controller (not shown). Transistors Tr1 and Tr2 receive an image signal (DATA) from a controller (not shown).

In the above state, when the transistor Tr1 is turned on by an image signal from a controller (not shown), an α × Ith current I _LD1 is supplied as a bias current to the laser diode LD1 as in the OFF mode, and the image The laser diode LD1 is turned off in the formation region. Also, when the transistor Tr1 is turned OFF by the image signal from the controller, not shown, the laser diode LD1 current _{I 0} is supplied as a lighting current, the laser diode LD1 in amount of _{P 0} as with LD_APC mode Lights up in the image forming area. When the transistor Tr2 is turned on or off by an image signal from a controller (not shown), the laser diode LD2 operates in the same manner as the laser diode LD1.

The semiconductor laser drive circuit 1 executes the LD1_APC mode to DATA mode in the non-image forming area. Further, in the image forming region, the semiconductor laser driving circuit 1 supplies the current I _LD1 (bias current) to the laser diodes LD1 and LD2 to turn off the laser diodes LD1 and LD2, or supplies the current I ₀ (lighting current). lighting the laser diode LD1, LD2 in amount of _{P 0} and. Thus, the semiconductor laser drive circuit 1, each time the non-image forming region and image forming region is repeated, the operation of the LD1_APC mode ~DATA mode, Off or quantity _{P 0} of a current _{I LD1} laser diodes LD1, LD2 The light emission at is repeatedly performed.

  Here, the image forming area is an area irradiated with laser light emitted from the laser diodes LD1 and LD2 to form an image, and is, for example, a scanning area of the laser light on the photosensitive drum. The non-image forming region is a region irradiated with laser light emitted from the laser diodes LD1 and LD2 other than the image forming region, and is, for example, a region outside the photosensitive drum in the laser light scanning region.

  With the above operation, the semiconductor laser driving circuit 1 always uses the threshold value as the bias current supplied to the laser diode when the laser diode is turned off during the image forming operation, regardless of the fluctuation of the threshold current Ith due to the fluctuation of the operating temperature of the laser diode. It becomes possible to supply a bias current at a constant ratio of the current Ith, that is, a bias current represented by α × Ith.

  As described above, the semiconductor laser drive circuit 1 calculates the threshold current Ith of the laser diode by the APC mode and the bias APC mode every time it becomes a non-image forming region. Therefore, it is possible to detect an accurate threshold current corresponding to the operating temperature fluctuation of the laser diodes LD1 and LD2. In addition, during an image forming operation, the bias current supplied while the laser diode is extinguished can always be set to a constant ratio with respect to the detected threshold current, that is, α × Ith. Therefore, the semiconductor laser drive circuit 1 can obtain the PWM characteristics of the laser diodes LD1 and LD2 that are stable against fluctuations in the operating temperature of the laser diodes LD1 and LD2.

Here, in the formula (5), if the bias current I _LD1 = Ith−Ioffset, then Ioffset = (I−α) · Ith. Unlike the prior art, the semiconductor laser driving circuit 1 can set Ioffset to an optimum value by the APC mode, the bias APC mode, and the OFF mode every time it becomes a non-image forming region, and the operating temperature of the laser diodes LD1 and LD2 The LD1 and LD2PWM characteristics of the laser diode that are stable against fluctuations can be obtained.

  Next, the PWM characteristics of the laser diodes LD1 and LD2 in the semiconductor laser drive circuit 1 will be described.

  FIG. 4 is a diagram showing PWM characteristics of the laser diodes LD1 and LD2 in the semiconductor laser driving circuit 1. As shown in FIG. In FIG. 4, the horizontal axis indicates the duty ratio of the PWM signal input to the LD1 and LD2 of the laser diode. The vertical axis represents the amount of light (output) of the laser light emitted from the laser diodes LD1 and LD2. In the semiconductor laser drive circuit 1, the laser diodes LD1 and LD2 are adjusted so that the amount of light emission is 20 mW when all the lights are on. FIG. 4 shows PWM characteristics when the operating temperature of the laser diode is changed to 10 ° C., 30 ° C., and 50 ° C.

  When the bias current supplied to the laser diodes LD1 and LD2 is the threshold current Ith, the laser diodes LD1 and LD2 generate laser light having a light amount corresponding to the threshold current even if the duty ratio is 0%. As the bias current is lowered below the threshold current Ith, the PWM characteristics are offset downward with a constant slope as shown in FIG.

  As shown in FIG. 4, the lines indicating the PWM characteristics of the laser diodes LD1 and LD2 in the semiconductor laser driving circuit 1 are obtained when the operating temperatures of the laser diodes LD1 and LD2 are different from 10 ° C., 30 ° C., and 50 ° C. Are almost identical. Thus, in the semiconductor laser drive circuit 1, it can be seen that the laser diodes LD1 and LD2 have stable PWM characteristics against changes in operating temperature.

That is, the bias current supplied to the laser diodes LD1 and LD2 is expressed by a numerical value (I _LD1 = α · (P ₀ · I ₀ −P ₀ · I ₁ −P ₁ · I ₀ ) / (P _When controlled to ₀₋ P ₁ ) = α · Ith), the PWM characteristics of the laser diodes LD1 and LD2 are stabilized with respect to the operating temperature. For this reason, the light emission amounts of the laser diodes LD1 and LD2 are substantially the same for the image data having the same duty ratio, and the reproducibility of the light emission amounts of the laser diodes LD1 and LD2 for the image data having the same duty ratio is improved. Therefore, stable image formation can be performed in the image forming apparatus.

  Further, the coefficient α is set to 0.6 or more and less than 1.0 because there is a problem that when the coefficient α is decreased, the rise of the laser diodes LD1 and LD2 is delayed and the dynamic range is narrowed when PWM control is performed. If α is 1.0, the laser diodes LD1 and LD2 are turned on, and when the laser diode is turned off at the time of image formation, the image quality may be deteriorated due to the effect of turning on the diode. Thus, the coefficient α is practically a value of 0.6 or more and less than 1.0.

  As described above, the semiconductor laser drive circuit 1 according to the present embodiment can stabilize the PWM characteristic against the operating temperature fluctuation of the laser diode. Thereby, stable image formation can be performed. Therefore, in a high-speed image forming apparatus, the image quality can be improved and the image formation can be highly stabilized.

It is a circuit diagram which shows the structure of the semiconductor laser drive circuit based on embodiment of this invention. It is a figure which shows the relationship between the electric current of a laser diode and the light emission amount in the semiconductor laser drive circuit which concerns on this Embodiment. It is a sequence diagram showing operation | movement of the semiconductor laser drive circuit which concerns on this Embodiment. It is a figure which shows the PWM characteristic of the laser diode in the semiconductor laser drive circuit which concerns on this Embodiment. It is a figure which shows schematic structure of the printer of the electrophotographic system using the conventional semiconductor laser. It is a figure which shows the relationship between the electric current of a semiconductor laser used for description of operation | movement of bias APC performed in the conventional semiconductor laser drive circuit, and light emission amount. It is a figure which shows the PWM characteristic for every operating temperature of the semiconductor laser in the conventional semiconductor laser drive circuit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor laser drive circuit 2 Light emitting element 10, 14, 20, 24 Voltage-current conversion circuit 11, 15, 21, 25 Sample / hold circuit 12, 16, 22, 26 Error amplifier 13, 23 Function circuit

Claims (4)

In a semiconductor laser driving circuit for driving a semiconductor laser used in an image forming apparatus,
First determining means for determining a lighting current of the semiconductor laser during image formation;
Second determining means for determining a bias current to the semiconductor laser at the time of turning off during image formation;
The first determination means and the second determination means each have a correction means for fluctuations in the threshold current of the semiconductor laser, and the second determination means sets an arbitrary constant α and a threshold current value Ith of the semiconductor laser. The semiconductor laser drive circuit according to claim 1, wherein the bias current is determined to be α × Ith.   2. The semiconductor laser driving circuit according to claim 1, wherein the arbitrary constant [alpha] is a value not less than 0.6 and less than 1.0.   The first determining means detects a first current value for the semiconductor laser to obtain a first light amount in a non-image forming region of the semiconductor laser, and the lighting current is based on the first current value. The semiconductor laser driving circuit according to claim 1 or 2, wherein:   The second determining means detects a second current value for the semiconductor laser to obtain a second light quantity in the non-image forming region of the semiconductor laser, and the first current value and the second current value are detected. 4. The semiconductor laser driving circuit according to claim 3, wherein the bias current at the time of extinction is determined based on the above.

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