JP3710389B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus, and more specifically, light-modulated laser light from a laser light source is guided onto an image bearing surface such as a photosensitive member or an electrostatic recording medium, and on the surface, for example, The present invention relates to an image forming apparatus suitable for a copying machine, a laser beam printer, a facsimile, or the like that forms image information including an electrostatic latent image.
[0002]
[Prior art]
Conventionally, in a laser driving circuit of this type of image forming apparatus, a constant DC current is supplied to the laser (this current is referred to as a bias current), and in addition, a switch current that switches according to image data is supplied. Is common. The laser has a change point that is a threshold current, and if a current smaller than the threshold current is applied, the laser emits a diode that is a diffused light having a different phase without emitting a laser.
[0003]
The amount of light during the light emission of the diode is extremely small compared to the time of laser light emission. The laser emission is performed only when a current larger than the threshold value is supplied. This is because the laser needs to exceed a certain energy level in order to oscillate the laser. If a current exceeding this energy level is allowed to flow, the laser current can be increased or decreased slightly after that. The amount of emitted light can be controlled.
[0004]
Therefore, it is possible to exceed this energy level by applying a current in advance to increase the energy level inside the laser and adding the switch current necessary for lighting from there, rather than when no bias current is supplied. The time required for this is shortened, and the light emission characteristics of the laser can be improved. For this reason, a means for applying a bias current has been taken in the laser drive circuit of this type of apparatus.
[0005]
As can be seen from the above description, the laser element has a characteristic that the amount of light emission changes in proportion to the flowing current. Therefore, the amount of laser emission for image formation is controlled by controlling the current flowing through the laser. In general, APC (auto power control) control is performed to keep the laser power constant.
[0006]
This is because the amount of light emitted from the laser is detected by a photodiode built in the laser, and this detection signal is compared with a predetermined reference value that is the target of the laser power. If the value is larger, the current is reduced and the light quantity is reduced to keep the laser power constant.
[0007]
Since this control is actually performed by emitting a laser, it cannot be performed during image formation in which the light emission pattern is not constant, and is basically performed in a so-called blanking period outside the image area. Then, based on the result of the APC control performed during this blanking period, a constant current is passed through the laser within one horizontal synchronization period, and image formation is performed.
[0008]
Here, there is a temperature characteristic called droop as a characteristic of the laser. This is an element whose amount of light emission changes depending on the amount of current that the laser passes, but it also has a change factor with respect to temperature. That is, the amount of light emission varies depending on the temperature of the laser chip even when the current is the same when the laser is emitted. Therefore, APC control is performed during the blanking period described above to emit light for a 1H period with a constant current. Strictly speaking, the light emission amount changes at the beginning and end of the 1H light emission period. This phenomenon is due to the fact that when the laser emits light, the temperature of the laser chip itself rises, so that the amount of emitted light is affected by this temperature.
[0009]
In this type of conventional model, since the image forming speed was not so fast, the amount of light emitted by the laser was not so large, so the amount of droop was small, and there was no problem. Further, the laser chip is pressed into a metal barrel to dissipate heat as a heat sink, and the light amount control is performed only by conventional APC control.
[0010]
[Problems to be solved by the invention]
In recent years, digital copying has become faster and more precise, and laser writing methods have also changed. Even in a normal underfield scan (UFS) type laser scanning system, since the drum sensitivity does not change, it is necessary to power up the laser in order to obtain the same potential as the speed is increased. In addition, an overfield scan (OFS) type laser scanning system effective for high-speed writing requires a larger laser power than UFS.
[0011]
Therefore, it is expected that the laser power will increase further in the future due to the speed-up of the apparatus or the difference in the exposure method. When the power of the laser is increased, the droop amount described above increases, and the difference in the amount of laser light in one line becomes obvious, and there is a high possibility that the difference in density on the image appears clearly. As a droop characteristic, since the beginning of the scanning direction for scanning on the photosensitive member is always large and the end is small, the density of the image changes along the scanning direction of the laser.
[0012]
As a method for suppressing the heat generation amount of the laser chip, there are a method of effectively utilizing a heat sink and a method of controlling the temperature of the chip itself using a Peltier element or the like. However, no matter how large the heat sink is, it is impossible to absorb the temperature change in the chip that occurs instantaneously, and this is not an effective method for eliminating the droop characteristic. The temperature control using a Peltier element is the same, and is not very effective from the viewpoint of responsiveness and cost.
[0013]
As a method for correcting the inclination of the laser light amount, there are a method of changing the distribution of primary charge in a process and a method of giving an inclination to the sensitivity of the drum. Since the difference in image density is ultimately determined by the potential distribution on the drum, the potential distribution on the drum may be made constant by using the above-described method in the manufacturing process of the actual image forming apparatus. However, there is a limit to this current process-based correction method, and it is considered that a new method is necessary to correct the drum potential gradient due to the droop gradient that will occur in the future.
[0014]
The present invention has been made in view of such problems, and the object of the present invention is to increase the ambient temperature of the apparatus and the continuous temperature even if the light emission amount of the laser is increased for high speed and high definition of the apparatus. An object of the present invention is to provide an image forming apparatus capable of correcting a droop characteristic of a laser including a use environment such as use.
[0015]
[Means for Solving the Problems]
In order to achieve such an object, the invention according to claim 1 is directed to image formation in which light modulated light from a laser light source is guided onto a photoconductor to form image information. In an image forming apparatus provided with a laser drive circuit used in the apparatus, a constant current generating means for determining a switching current amount to be multiplied according to image information for causing the laser to emit light, and a constant current amount of the constant current generating means The first control means includes APC control means for determining the laser power for writing while monitoring the light quantity of the laser, and the second control means comprises: Current superimposing means for superimposing current in a pattern in the current determined by the first control means is provided. Detecting means for detecting the amount of light of the laser, holding means for holding the result of the APC control, the result of APC performed at the head of the line held by the holding means and the image creation area of the line And a comparison means for comparing the value detected by the detection means, and when the light quantity of the laser changes more than a certain value with respect to the result of the APC control, the second control means makes the constant When the current is superimposed on the current means and does not change beyond a certain level, the second control means does not superimpose the current on the constant current means. It is characterized by this.
[0019]
Claims 2 The invention described in claim 1 The first and second detection means for detecting the amount of light of the laser are used to perform APC control of the laser using the first detection means, and the detection result of the second detection means Is used as the comparison means, and the second detection means has a detection gain higher than that of the first detection means.
[0020]
According to the present invention having such a configuration, in the laser driving circuit in such an image forming apparatus, the constant current generating means that determines the amount of switching current to be multiplied according to image information in order to cause the laser to emit light. The first control means for controlling the constant current amount of the constant current generating means, the second control means for controlling the constant current amount of the constant current generating means, and the first control means for writing while monitoring the light quantity of the laser The APC control means for determining the laser power and the second control means are provided with a current superimposing means for superimposing the current in a pattern in the current determined by the first control means. The detecting means for detecting the amount of laser light, the holding means for holding the result of the APC control, the result of APC performed at the head of the line held by the holding means, and the detection means after the image creation area of the line And a comparison means for comparing the measured values with each other, and when the comparison result shows that the amount of laser light has changed by more than a certain value relative to the result of APC control, the second control means causes the current to be superimposed on the constant current means, and the constant When there is no more change, the second control means prevents the current from being superimposed on the constant current means. Therefore, by adding a correction current to compensate for the decrease in the light amount due to the droop characteristic to the current amount determined by the APC control, not only keeps the light amount of each line constant, but also stabilizes the light amount within the 1H line. It is also possible to plan.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[Example 1]
FIG. 1 is a cross-sectional view for explaining an embodiment of the image processing apparatus of the present invention. Documents stacked on one document feeder 1 are sequentially conveyed onto the surface of the document table glass 2 one by one. When the original is conveyed, the lamp 3 in the scanner portion is turned on, and the scanner unit 4 moves to irradiate the original. The reflected light of the document passes through the lens 8 via the mirrors 5, 6, and 7 and is then input to the image sensor 9. The image signal input to the image sensor 9 is directly or temporarily stored in an image memory (not shown), read again, and then input to the exposure control unit 10.
[0022]
The latent image formed on the photosensitive member by the irradiation light is developed by the developing device 12 or 13. The transfer paper is conveyed from the transfer paper stacking section 14 or 15 in synchronization with the latent image, and the developed toner image is transferred in the transfer section 16. The transferred toner image is fixed on the transfer paper by the fixing unit 17 and then discharged from the paper discharge unit 18 to the outside of the apparatus.
[0023]
FIG. 2 is a diagram showing a configuration of the exposure control unit, and reference numeral 31 in the drawing is a semiconductor laser. A photodiode for detecting a part of the laser beam is provided inside the semiconductor laser, and APC control of the laser diode is performed using a detection signal of the photodiode. The laser beam emitted from the laser 31 becomes substantially parallel light by the collimator lens 35 and the diaphragm 32 and enters the rotary polygon mirror 33 with a predetermined beam diameter.
[0024]
The rotating polygonal mirror 33 rotates at a constant angular velocity in the direction indicated by the arrow, and with this rotation, the incident light beam is reflected as a deflected beam that continuously changes its angle. The light that has become the deflected beam is subjected to a condensing action by the f-θ lens 34. On the other hand, since the f-θ lens simultaneously corrects distortion aberration so as to guarantee the temporal linearity of scanning, the light beam is directed on the photosensitive member 11 as an image carrier in the direction of the arrow in the figure. Combined scanning is performed at high speed. Reference numeral 36 denotes a BD (beam detect) sensor that detects reflected light from the rotary polygon mirror 33. The detection signal of the BD sensor 36 is a synchronization signal for synchronizing the rotation of the rotary polygon mirror 33 and the writing of data. Used.
[0025]
FIG. 3 is a block diagram showing the configuration of the laser control circuit of this embodiment. The image control unit 71 controls each part of the apparatus of FIG. 1 and processes the image signal read by the image sensor 9. It is. The image control unit 71 supplies an image signal from the image sensor 9 to a laser driving circuit line by line in synchronization with a BD signal described later, or a timing signal necessary for APC control of a semiconductor laser as described in detail later. Creating. Reference numeral 31 denotes a semiconductor laser chip shown in FIG. 2, which includes a semiconductor laser 51 and a photodiode 52 for detecting a part of the laser beam. Similarly, BD sensor 36 shown in FIG. 2 is used to detect a synchronization signal.
[0026]
The bias current source 61 and the pulse current source 60 are current sources used for controlling the semiconductor laser 51, and the switch 63 is a switch that is turned on / off according to an image signal. Here, the switch 63 is turned on when a high level signal is input, and the semiconductor laser 51 is lit when a current determined by the pulse current source 60 flows. Further, the switch 63 is turned off when a low level is inputted, and the semiconductor laser 51 is turned off. In this way, the semiconductor laser 51 is driven by controlling the switch 63 according to the image signal, and optical writing is performed on the photosensitive drum 11.
[0027]
In the apparatus shown in FIG. 1, the laser 51 is normally turned off when the printer is in a standby state where no printing operation is performed. During the printing operation, the laser 51 is turned on. At this time, APC (AUTO POWER CONTROLL) control is performed to maintain a constant light emission amount. This APC control is performed for each line in which the laser beam is scanned on the photosensitive member by the rotary polygon mirror 33, and this is called line APC control. Further, during the printing operation in this APC control, it is performed in the non-image area so as not to affect the image formation.
[0028]
Here, as described above, the semiconductor laser generally improves the light emission characteristics by flowing a bias current that is a constant current in addition to the switch current for turning on / off the laser in advance. It has been known. Conventionally, in such an image forming apparatus, the bias current is set to about half of the laser threshold current. However, in recent image forming apparatuses with higher definition, it is necessary to set the bias current in the vicinity of the threshold value.
[0029]
Therefore, also in the image forming apparatus of this embodiment, the bias current is set to about half of the threshold value in the normal image quality mode, and in the vicinity of the laser threshold value in the high image quality mode. In this way, even if the laser power at the time of writing is not changed, by changing the bias current amount with the same laser power, the laser emission characteristics are changed and the fine pulse emission reproducibility is improved, so that a high-definition image can be obtained. Data can be written to the photoreceptor. In order to set the bias current in the vicinity of the threshold value, in this embodiment, APC (hereinafter referred to as total APC) is not performed only for the power of the laser used for latent image formation, but also for the bias current. APC (hereinafter referred to as bias APC) is performed.
[0030]
That is, in the non-image area, first, the switch 63 is turned OFF to set the bias current, and only the bias current is supplied to the laser 51 so that the laser 51 is turned on. To do. At this time, a part of the laser beam of the semiconductor laser 51 is detected by the photodiode 52, and the detection signal is converted into a voltage signal by the current-voltage converter 54. The signal output of the current / voltage converter 54 is sampled by the sample hold circuit 65 and supplied to the operation amplifier 66.
[0031]
The operation amplifier 66 compares the output signal of the sample hold circuit 65 with the reference voltage of the bias reference voltage generator 67 and controls the current of the bias current source 61 according to the difference signal. That is, the APC control is performed so that the light amount of the semiconductor laser 51 becomes a desired light amount by controlling the current of the bias current source 61 so that the target light amount set as the reference voltage by the reference voltage generator 67 is obtained. Do. When this control is finished, the sample hold circuit 65 is in a hold state, and a bias current is supplied to the laser 51 while maintaining the controlled value.
[0032]
Following the bias APC, total APC is performed. Of course, this total APC is also performed in the non-image area. That is, after the bias APC is completed, the switch 63 is turned on, and the laser 51 is supplied with the current of the pulse current source 60 in addition to the set bias current to be lit. At this time, the sample hold circuit 68 is set to the sample state. To do. At this time, a part of the laser beam of the semiconductor laser 51 is detected by the photodiode 52, and the detection signal is converted into a voltage signal by the current-voltage converter 54. The output signal of the current / voltage converter 54 is sampled by the sample / hold circuit 68 and supplied to the operation amplifier 69.
[0033]
The operation amplifier 69 compares the output signal of the sample hold circuit 68 with the reference voltage of the total reference voltage generator 70, and controls the current of the pulse current source 60 according to the difference signal. That is, by controlling the current of the pulse current source 60 so that the target light amount set as the reference voltage by the total reference voltage generator 70 is obtained, the APC is performed so that the light amount of the semiconductor laser 51 becomes a desired light amount. Take control. When this control is finished, the sample and hold circuit 68 enters a hold state, and supplies a pulse current to the laser 51 while maintaining the controlled value.
[0034]
On the other hand, in the image area, as already described, the output signal of the current-voltage converter 54 in the non-image area is held by the sample hold circuits 65 and 68, and the operation amplifiers 66 and 69 A bias current and a pulse current are set by the reference voltages 67 and 70 to control the light quantity of the semiconductor laser 51. FIG. 4 shows the timing of the aforementioned line APC. The sample holds 65 and 68 are in a sample state at a high level and in a hold state at a low level. In FIG. 4, after the VIDEODATA signal is turned on in synchronization with S / HT in the non-image area, it is at the high level before the image area in order to obtain the synchronization signal by the BD sensor 36 described above. It is. The shaded portion in the VIDEO DATA signal is the image DATA.
[0035]
Next, the configuration of the droop correction unit will be described.
FIG. 5 is a diagram showing an internal configuration of the droop correction unit in FIG. 3. The PD52 output in the laser chip is input to the S / H circuit 81 and differentially amplified by a predetermined voltage 83 and an amplifier 82 that are set. The result is input to the current source 84, and the amount of current to be droop corrected is determined. This sequence will be described with reference to FIG. Since the APC control without droop correction is as described above, the principle and sequence for determining the amount of current for droop correction will be described.
[0036]
FIG. 8 is a diagram for explaining the droop correction, and shows a laser emission state in one horizontal synchronization section. First, the APC is controlled, then the light is turned on for BD detection, and then the light is turned on for image formation. Usually, this control is repeated. At this time, as shown in FIG. 8, in the lighting of the image area, the light amount is gradually decreased due to the droop characteristic of the laser. Therefore, it is necessary to correct the amount of light in the portion denoted by reference numeral 101. However, the laser emission state in the image region varies depending on the image. Therefore, in the case of a normal laser control method, it is considered that the droop amount is small because the lighting time is short and the heat generation amount is small in a line with little image data.
[0037]
However, in the case of a high-speed machine as exemplified in the embodiments of the present invention, in order to improve the laser emission characteristics, a bias current is allowed to flow up to the threshold of the laser. Accordingly, a considerable current always flows through the laser in the image area regardless of the presence or absence of image data. The amount of heat generated by the laser chip tends to depend more on the current that flows than the presence or absence of laser light emission. Therefore, even if the light emission section is cut off, it is considered that the amount of light decreases by drawing a similar collapse line.
[0038]
Therefore, first, based on the result of APC performed at the head of this line control, light is emitted again immediately after the image creation region, the correction SW is turned on (ON at HI), current is supplied from the droop correction circuit 62, and S The / H-H signal is set to HI to obtain a sample state, and the correction current amount is controlled. After a predetermined time, the correction SW and S / H-H are set to LO, and the correction amount for one line is held. As the timing in the sub-scanning direction, this control may be started from the time when the laser is turned on to shift to the line APC control.
[0039]
FIG. 7 is a diagram illustrating a circuit that supplies the correction amount determined by the control method described above in an appropriate one-line pattern. After the correction amount is determined by the control described above, only the correction SW 91 is turned on in the image area of the next line. As a result, a voltage corresponding to the correction amount determined by the control method described above is applied to the operational amplifier 94 in the circuit of FIG. Since the voltage applied at this time is charged in the capacitor 93, the voltage rises to some extent at first and gradually approaches a constant value (FIG. 9). This corresponds to the droop collapse line, and if current is supplied in this form, the droop will be canceled and light emission in one line will be stabilized.
[0040]
Reference numeral 92 denotes a discharge resistor that suppresses the operational amplifier output 94 so that the constant current circuit does not malfunction when no correction current is supplied, and 96 is a current amount detection resistor of the constant current circuit.
[0041]
As described above, by performing the correction current control for the droop as in the present embodiment, it becomes possible to make the light emission amount of one line of laser more constant.
[0042]
[Example 2]
Next, in addition to the first embodiment described above, a case where control is performed in consideration of the temperature characteristics of the laser will be described.
The light emission characteristics of the laser change with temperature. For example, when the laser is continuously turned on for a long time, the heat of other parts of the image forming apparatus is also applied, and the temperature of the laser chip is considerably increased. In such a state, the laser chip is kept at a fairly high temperature regardless of whether the laser emits light. In such a state, the droop amount is small as a characteristic of the laser. In such a case, there is no influence on the image without correcting the droop amount. In addition, the set value of the current amount of the circuit for correcting the droop amount becomes small, and there is a possibility that accurate control cannot be performed. Therefore, it is desirable not to perform droop amount correction control in such a state.
[0043]
In the present embodiment, a droop amount correction circuit having a configuration in which a droop amount is detected by monitoring a PD output built in a laser chip and whether or not correction is performed will be described.
[0044]
FIG. 10 is a diagram for explaining the configuration of the second embodiment of the present invention. In addition to the first embodiment, the I / V converter 101, the sample hold circuit 102, the buffer 104, and the output of the reference voltage 70 of Vref T are shown. Resistors 105 and 106 and a comparator 103 are added. The I / V converter 101 has a higher gain than the I / V converter 54, and can detect a finer change in the PD output. Resistors 105 and 106 divide the output of Vref T to generate a reference voltage for comparison with the PD output. At this time, the reference voltage is generally set at a level that does not affect image formation and is several percent lower than the APC target value. Therefore, since the voltage to be compared with the PD output is considerably small, a highly sensitive I / V converter 101 is required to prevent malfunction. At this time, a buffer 104 is added for impedance matching so as not to affect the Ver T output.
[0045]
FIG. 11 is an explanatory diagram of the control sequence of the present embodiment. By adding the above-described configuration, it is possible to compare the light emission amount reduced by the laser droop characteristic with a predetermined allowable level.
[0046]
FIG. 11 is obtained by adding the above-described operation sequence to FIG. In the non-image section shown in FIG. 11, first, bias / total APC is performed, then the light is turned on for BD detection, the light for image formation is turned on in the image area, and the control for droop correction is performed. The sequence of doing is drawn. On the other hand, FIG. 11 shows a sequence for comparing the light emission amount reduced by the droop and a predetermined value between the image formation and the droop correction control. First, after light emission for image formation is completed, laser light emission is performed for a certain period based on the value of the previous line APC. At this time, the droop correction current is not superimposed.
[0047]
Therefore, the correction SW 91 remains OFF, and the laser light amount slightly decreased due to the droop characteristic is detected from the PD 52, converted into a voltage signal by the I / V converter 101, and then input to the S / H circuit 102. Almost simultaneously with the lighting timing for comparison, the S / H signal of the S / H circuit 102 is set to HI (sampled at HI), and set to LO (held at LO) before turning off. Thus, the light emission amount at the time of light emission is held in the S / H circuit 102 as a voltage signal, and is compared by the comparator 103 with the reference voltage 70 created by dividing the voltage by Vref T. If the comparison result of the comparator 103 is LO (the amount of emitted light is lower than the reference voltage at LO), a control signal (correction SW, S / H H) for droop correction is output from the image control unit 71, and at HI If there is, it is not output and droop correction is not performed.
[0048]
As described above, the correction current control in consideration of the temperature characteristic of the laser with respect to the droop as in the present embodiment makes it possible to stabilize the light emission amount of one line of laser more uniformly.
[0049]
【The invention's effect】
As described above, according to the present invention, the constant current generation means for controlling the constant current amount of the constant current generation means is controlled with respect to the constant current generation means for determining the switching current amount to be multiplied according to the image information in order to emit the laser. 1 control means, second control means for controlling the constant current amount of the constant current generating means, first control means for determining the laser power for writing while monitoring the amount of laser light, and second control The means comprises current superimposing means for superimposing current in a pattern in the current determined by the first control means. The detecting means for detecting the amount of laser light, the holding means for holding the result of the APC control, the result of APC performed at the head of the line held by the holding means, and the detection means after the image creation area of the line And a comparison means for comparing the measured values with each other, and when the comparison result shows that the amount of laser light has changed by more than a certain value relative to the result of APC control, the second control means causes the current to be superimposed on the constant current means, and the constant When there is no more change, the second control means prevents the current from being superimposed on the constant current means. Therefore, even if the laser emission is increased to increase the speed / definition of the device, it is possible to correct the droop characteristics of the laser, including the operating environment (environment temperature, continuous use). Therefore, it is possible to obtain a more stable and high quality image. Further, even if the life of the laser is advanced and the operating current is increased, the correction is made, so that it can be used for a longer time than the life of the conventional laser. Therefore, it is possible to contribute to improvement of serviceability without using setting parts such as service parts in the scanner unit that is relatively difficult to replace.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view for explaining an embodiment of an image processing apparatus of the present invention.
FIG. 2 is a configuration diagram of an optical system in an exposure control unit.
FIG. 3 is a block diagram showing a configuration of a laser control circuit.
FIG. 4 is a diagram for explaining a control sequence of a conventional line APC.
5 is a configuration diagram of a droop correction circuit in FIG. 3. FIG.
FIG. 6 is a diagram for explaining a control sequence of the first embodiment.
7 is a configuration diagram of an internal circuit of the current source in FIG. 5. FIG.
FIG. 8 is a diagram for explaining the principle of droop correction;
9 is a diagram for explaining a voltage waveform applied to the operational amplifier in FIG. 7; FIG.
FIG. 10 is a diagram for explaining a configuration of a second embodiment of the present invention.
FIG. 11 is a diagram for explaining the sequence of the second embodiment;
[Explanation of symbols]
1 Document feeder
2 Platen glass
3 lamps
4 Scanner unit
5,6,7 mirror
8 Lens
9 Image sensor
10 Exposure control unit
11 Photosensitive drum
12,13 Developer
14,15 Transfer paper stacking section
16 Transfer section
17 Fixing part
18 Paper discharge unit
31 Semiconductor laser
32 aperture
33 Rotating polygon mirror
34 f-θ lens
35 Collimator lens
36 BD (beam detect) sensor
51 Semiconductor laser
52 photodiode
54 Current-voltage converter
60 pulse current source
61 Bias current source
63 switch
65,68 Sample hold circuit
66,69 Operation amplifier
67 Reference voltage generator
70 Total reference voltage generator
71 Image control unit
72 System Controller
73 Operation unit
81 S / H circuit
82 amplifiers
83 Predetermined voltage to be set
84 Current source
91 Correction switch
92,96 resistance
93 capacitors
94 operational amplifier
101 I / V converter
102 Sample hold circuit
103 Comparator
104 buffers
105,106 resistance

Claims (2)

In an image forming apparatus provided with a laser driving circuit used in an image forming apparatus that guides light-modulated laser light from a laser light source onto a photoconductor to form image information.
Constant current generating means for determining a switching current amount to be multiplied according to image information in order to emit the laser;
First and second control means for controlling a constant current amount of the constant current generating means,
The first control means includes APC control means for determining a laser power for writing while monitoring the light quantity of the laser,
The second control means, Bei give a current superposition means for superimposing a current pattern in the current determined by the first control means,
Detecting means for detecting the light quantity of the laser;
Holding means for holding the result of the APC control;
A comparison means for comparing the result of APC performed at the head of the line held by the holding means and the value detected by the detection means after the image creation area of the line;
When the comparison result shows that the light amount of the laser has changed more than a certain value with respect to the result of the APC control, the second control means causes a current to be superimposed on the constant current means, and if the change does not change more than a certain value, 2. An image forming apparatus , wherein no current is superimposed on the constant current means by the control means . The first detection means and the second detection means for detecting the light quantity of the laser are used to perform APC control of the laser using the first detection means, and the detection result of the second detection means is used as the comparison means. The image forming apparatus according to claim 1 , wherein the second detection unit has a detection gain higher than that of the first detection unit .

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