JP2000162527A - Laser diode drive circuit and image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simplify circuit constitution for driving a laser diode without lowering performance related to a laser diode drive circuit and to reduce a cost of a product. SOLUTION: In the LD drive circuit of a device provided with an image forming area for forming an image and a position detective area for detecting a position of a laser beam in the main scan direction of the laser beam emitted from the LD 12, and requiring laser luminescent power different in respective areas, an APC circuit 44 controlling one laser luminescent power in the image forming area for one LD 12 and an image forming area laser drive circuit 51 driving the LD 12 in the image forming area by control of the APC circuit 44 are provided, and a position detective area laser drive circuit 50 driving the LD 12 in the position detective area and an OCP circuit 30 preventing the overcurrent of the LD 12 in the image forming area and the position detective area are provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser beam for forming an electrostatic image on an image carrier such as a photosensitive drum by scanning a laser beam emitted from a laser diode and forming the image using an electrophotographic process. The present invention relates to a laser diode driving circuit and an image forming apparatus used for a printer device, a digital copying machine, a facsimile device, and the like.

[0002]

2. Description of the Related Art A conventional example will be described below.

[0003] §1: Outline description of printer device ... see FIG. 8 FIG. 8 is a diagram showing a conventional printer device, FIG. 8A is a top view of the printer device, and FIG. (The internal configuration is partially illustrated). The printer shown in FIG. 8 is a laser beam printer (hereinafter, referred to as a “laser printer”) that performs printing by scanning a laser beam (hereinafter, also referred to as “laser light”) from a laser diode (hereinafter, also referred to as “LD”). Device "or simply" printer device ").

[0004] In this laser printer, an L
D unit (laser diode unit) 2, photosensitive drum 3, paper transport path 4, charging unit 5, exposure unit 6, developing unit 7,
A transfer unit 8, a fixing unit 9, a cleaning unit 18 and the like are provided and perform printing (or printing) by an electrophotographic method.
The printing process system includes steps such as charging, exposure, development, transfer, and cleaning using the photosensitive drum 3 as a medium, and a fixing step.

The photosensitive drum 3 uniformly charged in the charging step is turned on / off in an exposure step based on information from a control unit.
The laser beam is turned off. At this time, an electrostatic latent image is formed on the photosensitive drum 3, and in the next developing step,
The electrostatic latent image is changed into a toner image by bringing the charged toner into contact with the electrostatic latent image.

Next, the toner image is transferred onto a sheet in a transfer step, and in a fixing step, the toner is melted and fixed to be output as a permanent image. On the other hand, after the transfer process, the photosensitive drum 3 removes residual toner in a cleaning process, and then enters a charging process again. Printing by the electrophotographic method is performed as described above.

§2: Description of LD Unit—See FIG. 9 FIG. 9 is a diagram showing a conventional LD unit. As described above, the laser printer device is provided with the LD unit 2, and the LD unit 2 includes a light source unit including an LD 12, an aperture 13, a collimator lens 14, and a cylindrical lens (CylindricalLens) 15. The LD
A scanning motor unit 10 that performs scanning by reflecting a laser beam emitted from 12 on a polygon mirror (rotating polygon mirror) 11 rotated by a motor, and a focus / correction lens for performing focusing and optical correction. 16, 24
An aspherics lens (Aspherics Lens) 17 for narrowing a laser beam 27;
1, a half mirror 22, a protective optical unit glass 23, a position detection sensor (Beam Detect Sensor) 25, and the like.

[0008] Reference numeral 27 in the figure denotes a laser beam emitted from the LD 12. A photodiode is provided in the LD 12 for monitoring the power of the laser beam 27, but is not shown. This LD unit 2
Then, the laser light 2 emitted from the LD 12 of the light source section
7 is scanned to irradiate the photosensitive drum 3, and the optical path is as follows.

That is, the laser beam 27 emitted from the LD 12 passes through the LD 12, the aperture 13, the collimator lens 14, and the cylindrical lens 15, and irradiates the polygon mirror 11 which is rotationally driven by the motor of the scanning motor unit 10. The laser beam 27 reflected by the polygon mirror 11 is transmitted to the focus / correction lens 2.
4. The light passes through the half mirror 22, is reflected by the mirrors 19 and 20, passes through the focus / correction lens 16, and passes through the mirror 2
After being reflected at 1, the light passes through the protective optical unit glass 23 and irradiates the photosensitive drum 3.

The laser beam 27 reflected by the half mirror 22 passes through the aspherics lens 17 and is received by the position detection sensor 25. When the laser beam 27 irradiates the photosensitive drum 3 through the optical path as described above, the polygon mirror 11 is driven to rotate by the motor, so that the laser beam 27 is scanned in the main scanning direction on the photosensitive drum 3 by the rotation. You. At this time, the position of the laser beam 27 is detected by the position detection sensor 25.

§3: Description of Operation of LD Unit—See FIGS. 9 and 10 FIG. 10 is an illustration of the operation of a conventional LD unit, FIG. 10A is an explanatory diagram of laser beam scanning, and FIG. is there. The operation of the LD unit is as follows. The laser beam is emitted in the LD unit 2 as described above, and the laser beam scans the photosensitive drum 3 in the main scanning direction. This state is shown in FIG.

First, when a laser beam is emitted from the LD 12 to the polygon mirror 11 which is rotated and driven by a motor, the laser beam is reflected by the polygon mirror 11 and then scanned on the photosensitive drum 3 in the main scanning direction. At this time, a part of the laser light is received by the position detection sensor 25, and the start position of one cycle of the printing operation is detected.

At this time, the position of the sheet on the photosensitive drum 3 (the position of the maximum sheet size) is as shown, and this sheet position is within the scanning range (the maximum scanning range) of the laser beam. In this case, the scanning range of the laser beam on the photosensitive drum 3 is shown in FIG.
From position to b, the position detection point d by the position detection sensor 25 is within the range of a and b and outside the paper position. The position detection point d is located on either the left side or the right side of the sheet position, but the operation is the same on either side.

The paper position on the photosensitive drum 3 is from point e to point f. On both sides of the paper position, there are gaps G1 and G2 (parts that are not actually printed), and a region between them (a region G). (Between G1 and G2) is an area where printing is actually performed. When the position of the sheet is set in the range from point e to point f and scanning with laser light is performed, the elapsed time from point d to point e is T1, and the elapsed time from point e to point f is T2. , The elapsed time from point f to point b, and the elapsed time from point a to point d are T3.

[0015] In B of FIG 10, the horizontal axis represents time T, and the vertical axis shows a laser emission power P _L. Further, the laser emission power P _L in the position detection area and P _L = LV2, the laser emission power P _L in the image forming region and the P _L = LV1, LD
The laser emission power P _L for preventing destruction of
Let _L = LV3 (LV1 <LV2 <LV3). In addition,
T1 and T3 are elapsed times of the position detection area, and T2 is an elapsed time of the image forming area.

First, at T1 from point d to point e, P _L
= The laser emission power, which was LV2, is reduced to LV1. Next, at T2 from point e to point f, P _L = LV1
Image formation (printing). In this case, L at T2
The current flowing through D12 is changed (variable value) depending on the content of image formation. Next, T3 in the image formation from the point f to point b is completed, since the position detection area, again increasing the laser emission power P _L to P _L = LV2 (fixed value).

[0017] In this way, it is repeated cycles T1, T2, T3, when the laser emission power P _L in the process reaches the P _L = LV3, since the current flowing through the LD12 is over current, the Overcurrent is prevented (details will be described later). The necessity of changing the laser emission power between the image forming area and the position detection area is based on the following reason.

A laser beam detector having a low sensitivity is used. In the exposure apparatus, there are a polygon mirror, a collimator lens, and the like, at which diffraction, irregular reflection, and the like occur. If the sensitivity of the detection means is improved, light such as diffraction and irregular reflection is picked up, and accurate laser position detection cannot be performed. Therefore, it is necessary to irradiate light of a certain power to the detection unit.

However, there has recently been an increasing demand for printing high-resolution images. If the laser power irradiating the detection unit remains unchanged, one dot becomes large and a high-resolution image cannot be obtained. Therefore, in order to increase the resolution, it is necessary to reduce the laser power.

In order to satisfy both of the above, it is necessary to change the laser power between the image forming area and the position detecting area.

§4: Description of LD Drive Circuit Example 1—See FIG. 11 FIG. 11 is a diagram showing a conventional LD drive circuit example 1. Hereinafter, an LD driving circuit example 1 will be described with reference to FIG. L
The D drive circuit example 1 is used to drive one LD 12.
An automatic laser light control circuit (hereinafter, referred to as an “APC circuit”; APC: Auto Power Control) for controlling the laser light emission power (laser light amount) in the position detection area while providing two laser drive circuits 31; This is an example in which the APC circuit for controlling the laser emission power in the formation region is configured by another circuit. That is, in this example, two APC circuits are provided for one laser drive circuit 31 for driving one LD 12.

As shown in FIG. 11, a laser drive circuit 31 for driving the LD 12 is provided in this LD drive circuit.
And an overcurrent protection circuit (hereinafter referred to as an “OCP circuit”) for preventing an overcurrent flowing through the LD 12. OCP: Over Curve
rent protection) 30, an image forming area APC circuit 35 for controlling the amount of laser light in the image forming area, a position detecting area APC circuit 36 for controlling the amount of laser light in the position detecting area, and a photodiode (PD). ) I / V conversion circuit 34 for converting current I flowing through 28 to voltage V
And amplifiers 37, 38 and the like.

The laser drive circuit 31 has a switching circuit 43 for switching the outputs of the image forming area APC circuit 35 and the position detection area APC circuit 36.
And a constant current circuit 32 for driving the LD 12 with a constant current specified by the MPU 40 (upper control unit). In this case, the laser drive circuit 31 is an IC (integrated circuit)
And the switching circuit 4 inside the IC.
3 and a constant current circuit 32 are provided.

The photodiode (PD) 28
Is an element for monitoring the laser light emitted from the LD 12, and the photodiode 28 outputs a current I proportional to the laser light.
The LD 12 and the photodiode 28 are each connected to a power supply (voltage = + V _CC ).

The LD drive circuit is connected to and controlled by the MPU 40 constituting a higher-level control unit. In this case, the MPU 40 has a digital-to-analog converter (hereinafter referred to as “DA
C ") 41 and 42 are provided.
By setting values (laser emission power reference values) in 41 and 42, the values are sent to the LD drive circuit.

An image forming area APC circuit 35 is connected to the DAC 41 via an amplifier 37.
The position detection area APC circuit 36
Is connected, and the image forming area APC circuit 35 is connected.
The output of the I / V conversion circuit 34 is connected to the APC circuit 36 and the position detection area APC circuit 36, respectively. Further, the outputs of the image forming area APC circuit 35 and the position detection area APC circuit 36 are connected to a laser drive circuit 31, and the LD 12 is driven by the laser drive circuit 31.

The image forming area laser emission power reference value LP1 (variable) from the MPU 40 is input to the image forming area APC circuit 35, and the position detecting area APC circuit 36
, A position detection area laser emission power reference value LP2 (variable) from the MPU 40 is input to the laser drive circuit 31, and a power switching signal PS from the MPU 40 to the laser drive circuit 31;
The laser drive signal LDS is input.

The operation of this circuit is as follows. In the position detection area, the value LP set by the MPU 40 in the DAC 42
2 is amplified by the amplifier 38 and then sent to the position detection area APC circuit 36. The position detection area APC circuit 36 controls the APC based on the input value LP2, and sends output data to the laser drive circuit 31.

In the image forming area, the MPU 40
After the value LP1 set in the AC 41 is amplified by the amplifier 37, it is sent to the image forming area APC circuit 35. The image forming area APC circuit 35 controls the APC based on the input value LP1 and sends output data to the laser drive circuit 31.

In response to a power switching signal PS from the MPU 40, the laser driving circuit 31 causes the switching circuit 43 to output the output of the position detection area APC circuit 36 and the image forming area AP.
The output of the C circuit 35 is switched. That is, the output is switched to the output of the position detection area APC circuit 36 in the position detection area, and is switched to the output of the image formation area APC circuit 35 in the image formation area.

Then, the switched output and the MPU4
The constant current circuit 32 is controlled using the laser drive signal LDS (ON / OFF signal) from 0 to drive the LD 12. If an overcurrent flows through the LD 12 while the LD 12 is being driven, the OCP circuit 30 prevents the overcurrent.

§5: Description of a specific example of LD drive circuit example 1
... See FIG. 12 FIG. 12 shows a specific example of a conventional LD drive circuit example 1. Hereinafter, a specific example of the LD drive circuit will be described with reference to FIG. In this example, a specific circuit example of the OCP circuit 30 and the laser drive circuit 31 is shown. OCP circuit 30
It includes a resistor R ₁ connected LD12 series, a comparator CMP for comparing the voltage of the resistor R ₁ reference value (reference voltage) V _S
And a resistor R ₄ connected to the output side of the comparator CMP,
And it includes a capacitor C _1, connected transistor Q ₂ to the resistor R _4.

[0033] In this case, the power supply (voltage = + V _a) and the resistor R _5, the series circuit of the R ₆ connected between ground and the resistor R _5,
The voltage V _S at the connection point of R ₆ is input as a reference value of the comparator CMP. That is, the connection point of the resistors R _5, R ₆ connected to one input terminal of the comparator CMP, is arranged to enter the voltage V _S as a reference value to the input terminal.

The laser drive circuit 31 includes a constant current circuit 32 including a transistor Q ₁ , resistors R ₂ and R ₃ , a capacitor C ₂ , and a transistor Q _3, and a switching circuit 43. In this case, the transistor Q ₁ is connected LD12 and resistor R ₁ in series is used to drive the LD12, resistors R _2, R _3, capacitor C ₂ is to supply a bias to the transistor Q _1. The transistor Q ₃ are turned on / off controlled by a laser drive signal LDS from MPU 40, is intended for short-circuiting / opening the condenser C ₂ accordingly.

In this circuit, the switching circuit 43 is switched by the signal PS, and the APC output in the position detecting area or the APC output in the image forming area is sent to the constant current circuit 32. Transistor Q ₃ is driven by the constant current circuit 32 in the signal LDS. At this time, the transistor Q ₃ is off, the capacitor C ₂ is charged with voltage corresponding to the output of the switching circuit 43.

Then, the transistor Q ₁ is driven by the voltage of the capacitor C ₂ , and a current flows through the LD 12. Further, when the transistor Q ₃ is turned on it is short-circuited capacitor C _2, to turn off the transistor Q _1. Thus, in the image forming region to form an image by driving the transistor Q ₃ by the signal LDS.

In the OCP circuit 30, the comparator CMP
As a result, the voltage V ₁ generated in the resistor R ₁ by the current flowing through the LD 12 is compared with the reference value V _S. Now, the resistor R ₁
If the voltage generated was V ₁ to, if V ₁ ≦ V _S, an output of the comparator CMP is at a low level L (= 0), the output is high of the comparator CMP to become V _1> V _S Level (= 1).

The output of the comparator CMP is L (= 0).
0 is the voltage of the capacitor C ₁ as long, the transistor Q ₂ is because it is off, the operation of the transistor Q ₁ is performed as described above. However, when the current flowing through the LD 12 becomes an overcurrent and V ₁ > V _S , the output of the comparator CMP becomes H (= 1), and the capacitor C ₁ is charged with a high-level output.

[0039] Therefore, the transistor Q ₂ is turned on, to the collector potential to the ground potential. Accordingly, the collector of the transistor Q ₂ to which has become the ground potential,
Across the capacitor C ₂ becomes the ground potential, rapidly reduces the base current of the transistor Q _1. With this operation, an overcurrent flowing to the LD 12 is prevented.

§6: Description of Laser Drive Circuit Example 2—See FIG. 13 FIG. 13 is a diagram showing a conventional LD drive circuit example 2. Hereinafter, a laser driving circuit example 2 will be described with reference to FIG.
In the LD drive circuit example 2, the APC circuit is one and the MPU
This is an example in which the reference value of APC from 40 is output to the APC circuit while being switched by the reference switching circuit 45 having an analog switch.

That is, the LD driving circuit example 2 uses the LD 12
One laser driving circuit 31 for driving the laser beam, one APC circuit 44 for controlling the laser light emission power (laser light amount) in the position detection area and the laser light emission power (laser light amount) in the image forming area. , Photodiode P
D to the APC circuit 4
4, an I / V conversion circuit 34, and amplifiers 37 and 38.
Reference switching circuit 4 having an analog switch 46
5 and so on.

The laser drive circuit 31 includes the LD 12
An OCP circuit 30 for preventing an overcurrent flowing through
A constant current circuit 32 for driving D12 at a constant current specified by the MPU 40 is provided. The reference switching circuit 45 is connected to the MPU 40 and the MPU 4
0 is controlled.

The MPU 40 includes DACs 41 and 42.
By setting values in these DACs 41 and 42, a laser emission power reference value (variable) LP1 in the image forming area and a laser emission power reference value L1 in the position detection area are set.
P2 (fixed) is output to the reference switching circuit 45. The operation of this circuit is as follows.

In the position detection area, the MPU 40
After the value (LP2) set to 2 is amplified by the amplifier 38,
Output to the reference switching circuit 45. In the image forming area, the MPU 40 amplifies the value (LP1) set in the DAC 41 by the amplifier 37, and outputs the amplified value to the reference switching circuit 45.

In the reference switching circuit 45, the analog switch 46 is switched by the signal PS from the MPU 40, and different laser emission power reference values LP1 and LP2 in the position detection area and the image forming area are output to the APC circuit 44. A
The PC circuit 44 takes in the output of the I / V conversion circuit 34 and the output of the reference switching circuit 45 to control the APC, and outputs the result to the laser drive circuit 31.

The laser drive circuit 31 receives the output signal from the APC circuit 44 and the signal LDS from the MPU 40 and drives the LD 12. In this case, the OCP circuit 30 is provided inside the laser drive circuit 31, and the operation is the same as that of the LD drive circuit example 1.

§7: Description of Specific Example of APC Circuit—See FIG. 14 FIG. 14 shows an example of the APC circuit. Hereinafter, the APC circuit 4
A specific example of No. 4 will be described with reference to FIG. APC circuit 4
4 includes a comparator 55, a transistor driving circuit 56,
Output side transistors 57 and 58 and a capacitor 59 for setting the reference of the constant current circuit are provided.

Then, the current I flowing through the photodiode 28 is converted into a voltage V by the I / V conversion circuit 34, and the voltage V is input to one input terminal of the comparator 55. The other input terminal of the comparator 55 has a reference value (LP
1 or LP2) and the transistor driving circuit 56
, An APC control signal (on / off control signal) APCD from the MPU 40 is input.

The operation of the APC circuit 44 is as follows.
When the APC control signal APCD is off, the transistors 57 and 58 are both off and the voltage of the capacitor 59 is maintained. The APC control signal APC
When D is APC control ON, the operation is as follows.

Now, the L output from the I / V conversion circuit 34
The D power monitor voltage V and V _M, and the voltage V _M, A
PC reference voltage (LP1 / LP2) and the result of comparison by the comparator CMP, and if _{V M <(LP1 / LP2)} , the transistor drive circuit 5 based on a signal from the comparator CMP
6 turns on the transistor 57 and turns off the transistor 58. Then, the capacitor 59 is charged via the transistor 57 by the power supply voltage + V _CC (for example, +5 V).

If V _M > (LP1 / LP2), the transistor driving circuit 56 turns off the transistor 57 based on the signal from the comparator CMP,
8 is turned on to discharge the capacitor 59. Then, the voltage of the capacitor 59 is output as a constant current circuit reference voltage. The specific example of the APC circuit can be realized by the same configuration in the image forming area APC circuit 35 and the position detection area APC circuit 36 of the LD driving circuit example 1.

[0052]

The above-mentioned prior art has the following problems.

In the conventional LD drive circuit example 1, two APC circuits are connected to one laser drive circuit for driving one LD, and the respective laser emission powers in the image forming area and the position detection area Was controlled.

However, the APC circuit requires a high-precision laser emission power control, and therefore has a large circuit scale and is expensive. Therefore, providing two such APC circuits causes an increase in product cost. Further, since two APC circuits are provided, the laser driving circuit needs a switching circuit for switching between the two APC circuits.

Since switching is performed between the image forming area and the position detecting area by this switching circuit, a time for the switching is required, and it is not possible to cope with an increase in the speed of image forming processing. Therefore, it is conceivable to shorten the switching time of the switching circuit. However, such a circuit capable of high-speed switching is expensive and causes an increase in product cost.

Further, the laser drive circuit requires the switching circuit and the constant current circuit, etc., and the circuit configuration becomes complicated and large-scale, resulting in an increase in cost.

In the conventional LD driving circuit example 2, it is necessary to switch the analog switch of the reference switching circuit between the image forming area and the position detecting area, and the switching time is required. I can not cope.

In order to shorten the switching time of the analog switch, a circuit capable of high-speed switching is required. However, such a high-speed switching switch is expensive and causes an increase in product cost.

An object of the present invention is to solve such a conventional problem, to simplify a circuit configuration for driving a laser diode without deteriorating performance, and to realize a cost reduction of a product.

[0060]

FIG. 1 is a diagram illustrating the principle of the present invention. The present invention is configured as follows to achieve the above object.

(1): In an LD drive circuit requiring different laser emission powers in the scanning direction within one scan of laser light emitted from an LD (laser diode) 12,
An automatic laser light amount control circuit (APC circuit 44) for controlling the first laser emission power for one LD 12 and a first laser drive circuit (image forming area laser drive circuit 51) for driving the LD 12 by the APC circuit 44 ), A second laser drive circuit (position detection area laser drive circuit 50) for driving the LD 12 with the second laser emission power, and the LD 12 driven with the first and second laser emission powers
Overcurrent protection circuit (OCP circuit 30) for preventing overcurrent
And

(2): In the LD drive circuit of (1),
The second laser drive circuit sets the overcurrent detection level of the OCP circuit 30 and the level of the drive current corresponding to the second laser emission power to the same level, so that the APC circuit is not used. And a function of controlling the second laser emission power.

(3): In the LD drive circuit of (1),
The OCP circuit 30 includes a level adjusting unit for adjusting an overcurrent detection level in accordance with a characteristic variation of the LD 12.

(4): In the LD drive circuit of (1),
The OCP circuit 30 has a function of preventing an overcurrent of the LD 12 even when a plurality of laser driving circuits are driven simultaneously by placing the OCP circuit 30 at a position where all the current flowing through the LD 12 can be detected.

(5): In one scan of laser light emitted from an LD, in an LD drive circuit requiring different laser light emission powers in the scanning direction, the first laser light emission power is controlled for one LD. An APC circuit, a first laser drive circuit for driving an LD under the control of the APC circuit, and a second laser drive circuit for driving a laser diode with the second laser emission power; Monitor the photocurrent of the photodiode that detects power, so that the laser diode is not destroyed,
A laser emission power limiting circuit is provided for setting a limit value of the laser diode drive current and limiting the laser emission power.

(6): In the laser diode drive circuit of (5), the second laser drive circuit includes a limit value of the laser emission power limiting circuit and a drive current corresponding to the second laser emission power. By setting the level to the same level, without using the automatic laser light amount control circuit,
It has a function of controlling the second laser emission power.

(7): A carrier on which a latent image is formed, an exposure unit for scanning the carrier with light for each line to form a latent image on the carrier, and a latent image on the carrier. A developing device for developing;
An image forming apparatus that has a detection unit that detects the position of light within one line of the exposure unit, and that corrects the position of a latent image on the carrier based on the position of light detected by the detection unit to form an image. The exposure unit includes an LD (laser diode) that emits light for a latent image, a scanning unit that scans the light emitted by the LD, and an APC circuit that controls the light emission power of the LD at a position where a latent image is formed on the carrier. Automatic laser light amount control circuit) and A
A first laser drive circuit (image forming area laser drive circuit) that drives the LD at a latent image forming position with respect to the carrier under control of the PC circuit, and a second laser drive circuit that drives the LD at a detection position of the detection unit (Position detection area laser drive circuit)
And a current protection circuit (O) for preventing an overcurrent of the LD driven by the latent image position with respect to the carrier and the laser power of the detection unit.
CP circuit).

(8): In the image forming apparatus of (7),
The laser power (laser emission power) at the detection position by the detection unit is configured to be higher than the laser power (laser emission power) at the latent image position with respect to the carrier.

(Operation) The operation of the present invention based on the above configuration will be described.

(A): Operation of the above (1) The APC circuit 44 converts the image forming area laser emission power reference value LP1 from the host controller and the voltage proportional to the photocurrent of the photodiode for detecting the laser emission power. Based on
One laser emission power (first laser emission power) is controlled for one LD 12 in the image forming area. Then, in the image forming area laser driving circuit 51, the output of the APC circuit 44 (instruction value) and the image forming area laser driving signal LDS <b> 1 from the higher-level control unit cause the L in the image forming area.
D12 is driven.

In the position detection area, the position detection area laser drive circuit 50 drives the LD 12 (drives with the second laser emission power) in accordance with the position detection area laser drive signal LDS2 from the host controller. When the drive current flowing through the LD 12 when the LD 12 is driven in the image forming area and the position detection area becomes an overcurrent, the OCP circuit (overcurrent protection circuit) 30 suppresses the overcurrent and destroys the LD 12. To prevent it from being done.

In this case, it is necessary to perform high-precision laser emission power control in the image forming area, and therefore, an APC circuit is required. However, in the position detection area, high-precision laser emission power control is not required and a fixed value is sufficient, so that the configuration of the position detection area laser drive circuit can be simplified.

(B): Function of (2) The position detection area laser drive circuit 50 corresponds to the OCP detection value (overcurrent detection value) of the OCP circuit 30 and the laser emission power required in the position detection area. By setting the level of the LD drive current _{Id to} the same level, the APC circuit 4
The laser light emission power in the position detection area is controlled without using 4. For this reason, the configuration of the position detection area laser drive circuit can be simplified.

(C): Operation of the above (3) The LD 12 is controlled by the level adjusting means of the OCP circuit 30.
The OCP detection value (overcurrent detection level) is adjusted in accordance with the characteristic variation of. Therefore, even if there is a variation in the characteristics of the LD 12, it is possible to always adjust the OCP detection value to an appropriate value.

(D): Function of (4) The OCP circuit 30 is located at a position where all the current flowing through the LD 12 can be detected. Even when the drive circuits are driven simultaneously, the overcurrent of the LD 12 is reliably prevented. Therefore, it is possible to reliably prevent the LD 12 from being broken by an overcurrent.

(E): Operation of the above (5) The APC circuit controls one laser emission power in one image forming area for one LD. Then, under the control of the APC circuit, the image forming area laser driving circuit drives the LD in the image forming area. In the laser emission power limiting circuit, the photocurrent of the photodiode for detecting the laser emission power is monitored.
The laser drive power is limited by setting a limit value of the D drive current.

In this way, even if the characteristics of the laser emission power change, accurate laser emission power can always be obtained.

(F): Operation of the above (6) The position detection area laser drive circuit makes the limit value of the laser emission power limiting circuit equal to the level of the drive current corresponding to the laser emission power required in the position detection area. The laser emission power in the position detection area is controlled by setting to the level of. In this way, the laser emission power in the position detection area can be controlled without using an expensive APC circuit, and the LD can be prevented from being destroyed.

(G): Operation of the above (7) The exposure section of the image forming apparatus is an LD for emitting light for a latent image.
A scanning unit that scans the light emitted from the LD, an APC circuit that controls the light emission power of the LD at a position where the latent image is formed on the carrier, and a drive of the LD at the position where the latent image is formed on the carrier by control of the APC circuit. A first laser driving circuit, a second laser driving circuit for driving the LD at the detection position of the detection unit, and a latent image position with respect to the carrier and an overcurrent of the LD driven by the laser power of the detection unit. And a current protection circuit.

In this case, it is necessary to perform high-precision laser emission power control in the image forming area, and therefore, an APC circuit is required. However, in the position detection area, high-precision laser emission power control is not necessary and a fixed value is sufficient, so that the configuration of the position detection area laser drive circuit can be simplified and the configuration of the image forming apparatus is simplified. (H): Action of the above (8) The laser power (laser emission power) at the detection position by the detector is made larger than the laser power (laser emission power) at the latent image position on the carrier. ing. In this way, L is reduced without deteriorating the performance.
The circuit configuration for driving D can be simplified, and the cost of the product can be reduced.

[0081]

Embodiments of the present invention will be described below in detail with reference to the drawings. The configuration of the printer shown in FIG. 8, the configuration of the LD unit shown in FIG.
2 and the AP shown in FIG.
Since the configuration of the C circuit is the same in the present embodiment, the following description will be made with reference to these drawings.

§1: Overview of LD Drive Circuit The LD drive circuit according to the present embodiment has an image forming area for forming an image in the main scanning direction of the laser light emitted from the LD, and the position of the laser light. This is a circuit used for devices (laser printers, digital copiers, facsimile machines, etc.) that have a position detection area for detecting It is as follows.

(1) In the LD drive circuit, at least two or more laser drive circuits are provided for one LD. As described above, by arranging at least two or more laser drive circuits and driving the image forming area and the position detection area by different laser drive circuits, the circuit is simplified.

In this case, the LD is driven using one laser drive circuit in the position detection area, but a plurality of laser drive circuits are required in the image formation area (for example,
Printing with a large density difference). In such a case, two or more laser driving circuits may be used, and a laser driving circuit corresponding to each density may be used. In the following description, a case where there is one laser drive circuit in the image forming area and one laser drive circuit in the position detection area will be described.

(2): An APC circuit for controlling one laser emission power in one image forming area for one LD,
A position detection region laser drive circuit for driving the LD in the position detection region, including an image formation region laser drive circuit for driving the LD in the image formation region by control of the PC circuit;
An OCP circuit (overcurrent protection circuit) for preventing an overcurrent of the LD in the image forming area and the position detection area is provided.

In this case, the APC circuit in the position detection area is eliminated, and its function is replaced by the OCP circuit. And
By providing the OCP circuit at a position where all the current flowing through the LD can be detected, even if a plurality of laser drive circuits are driven at the same time, a circuit for reliably preventing the LD from being destroyed is provided.

(3): The laser beam emitted from the LD is L
Since the output power differs even if the same current flows due to the characteristic variation of D, the OCP circuit outputs an overcurrent detection level (O
(CP level) is provided.

(4): OC for detecting overcurrent flowing in LD
Instead of the P circuit, a laser light emission power limiting circuit that monitors the photocurrent of a photodiode (monitor element) that detects the light emission power of the laser light emitted from the LD and prevents the LD from being destroyed provides a position. The laser emission power is controlled also in the detection region, and the LD can be prevented from being broken.

In this case, by setting the limit value of the laser emission power limiting circuit to a level equivalent to the drive current for obtaining the laser emission power required in the position detection area,
It is possible to control the laser emission power in the position detection area without using a complicated APC circuit.

§2: Description of LD Drive Circuit Example 1—See FIG. 2 FIG. 2 is a diagram showing LD drive circuit example 1. This circuit example is an example in which two laser drive circuits and one APC circuit are provided to drive one LD. Normally, the laser emission power in the image forming area needs to be varied in order to control the density of printing, but the laser emission power in the position detection area does not need to be varied, and can be a fixed value and high-precision control. Do not need. Further, the OCP circuit for preventing the LD from being destroyed does not function as long as the APC circuit operates normally (it does not function unless an overcurrent flows).

Then, the drive current value of the LD is calculated as follows: (current value in image forming area) <(current value in position detection area) <(O
Paying attention to the characteristic having the relationship of “CP detection value”, the OC
Set OCP detection value of P circuit the level of the current value of the position detection area and (overcurrent detection level) (the value of the current I _d flowing in the LD 12) to the same level. In this case, the equivalent level includes not only a case where both levels are completely the same but also a case where there is a certain level difference.

For example, if the current value in the position detection area is O
A value slightly lower than the CP detection value, that is, the condition of (current value in the position detection area) <(OCP detection value) is satisfied, and the difference value between the two is Δd = (OCP detection value) − (position detection area The current value may be set to a small value (a non-zero value within a controllable range). This is because the drive current in the position detection area does not require high-precision control, and such setting makes it possible to simplify the circuit configuration and realize an inexpensive circuit.

In a laser driving circuit in which the APC circuit functions, the current cannot be driven up to the OCP detection value. Therefore, two laser emission powers can be controlled only by adding a laser drive circuit that does not function the APC circuit. Hereinafter, a specific description will be given.

In the LD drive circuit, L in the position detection area
Position detection area laser drive circuit 5 for driving D12
0, an image forming area laser driving circuit 51 for driving the LD 12 in the image forming area, and an LD 12 in the image forming area.
Circuit 44 for controlling the laser emission power of the laser
An amplifier 37, an I / V conversion circuit 34 for converting a current I flowing through a photodiode (PD) 28 into a voltage V,
An OCP circuit 30 and the like for suppressing an overcurrent flowing through the LD 12 to prevent the LD 12 from being broken are provided.

In this case, the image forming area laser driving circuit 5
1 uses a laser drive circuit having a constant current circuit 32, and the position detection area laser drive circuit 50 uses a laser drive circuit without a constant current circuit. And the LD
The drive circuit is connected to the MPU 40 (upper control unit), and the MPU 40 is provided with a DAC 41.
, The image forming area laser emission power reference value LP1 is sent to the APC circuit 44.

In this case, the APC circuit 44 includes the amplifier 37
, And the output of the I / V conversion circuit 34. Further, the output of the APC circuit 44 is connected to the image forming area laser drive circuit 51. Further, the image forming area laser drive circuit 51 receives the image forming area laser drive signal LDS1 from the MPU 40, and the position detection area laser drive circuit 50 receives the position detection area laser drive signal LDS2 from the MPU 40.

The operation of the LD drive circuit is as follows. In the position detection area, the MPU 40 outputs a position detection area laser drive signal LDS2 to the position detection area laser drive circuit 50. Position detection area laser driving circuit by the signal 50 starts the operation of the position detection area to turn on the transistor Q _5. Then, with the transistor Q ₅ it is turned on to drive the LD 12, electric current I _d to the LD 12.

[0098] The current I _d LD 12 to emit laser light in laser emission power at the position detection area. Current I _d in this case is, are the same OCP detection value of the OCP circuit 30 (overcurrent detection value). Accordingly, the current I _d in the position detection area, the current I _d which suppressed the upper limit flows due OCP detection value of OCP circuit 30.

Next, in the image forming area, the value (LP1) of the DAC 41 set by the MPU 40 is amplified by the amplifier 37 and then sent to the APC circuit 44. The APC circuit 44 takes in the value (LP1) and the monitor voltage value from the I / V conversion circuit 34 to control the light emission power of the LD 12, and instructs the image formation area laser drive circuit 51 to indicate the laser light emission power ( (Indicated voltage).

The image forming area laser driving circuit 51
The instruction value from the C circuit 44 is input and the MPU 40
Image forming area laser drive signal LDS1 from
The LD 12 is driven. At this time, the constant current circuit 32
The LD 12 is driven with a constant current based on the indicated value specified by the APC circuit 44. While the LD 12 is driven as described above, the OCP circuit 30 detects an overcurrent,
When an overcurrent exceeding the OCP detection value flows, the overcurrent is suppressed to prevent the LD 12 from being broken.

§3: Description of Specific Example of LD Driving Circuit Example 1—See FIG. 3 FIG. 3 shows a specific example of LD driving circuit example 1. The LD drive circuit example 1 is configured, for example, as shown in FIG. The OCP circuit 30, a comparator C which compares the resistor R ₁ connected in series to the LD 12, the voltages V ₁ of the resistor R ₁ and the reference voltage V _S
MP and resistors R ₅ and R ₆ for generating the reference voltage V _S.
, A capacitor C ₁ and a resistor R ₄ connected to the output side of the comparator CMP, and transistors Q ₂ and Q ₄ connected to the resistor R ₄ .

The image forming area laser drive circuit 51
Is a transistor Q for driving the LD 12₁And the
Transistor Q₁For passing bias current through
R_Two, R _Three, Capacitor C_TwoAnd the image from MPU40
The transistor Q is formed by the formation region laser drive signal LDS1.
₁Transistor Q for controlling the_ThreeIt has. This
The resistance R_Two, R_Three, Capacitor C_Two, Tran
Sista Q₁Constitute the constant current circuit 32.

Further, the position detection area laser drive circuit 50
Is a transistor Q for driving the LD 12_FiveAnd the
Transistor Q_FiveResistance R which constitutes the bias circuit of FIG.₇,
R₈It has. And the transistor Q_TwoCollection of
To transistor Q₁Connect to the base of the transistor
Q_FourThe collector of transistor Q_FiveResistance R at the base of ₇
Connect through.

[0104] series circuit of the resistor R _5, R ₆ is connected between the power supply (voltage = + Va) and a ground (GND), a voltage V _S of the connection point of the resistors R _5, R _6, comparator CMP Is input as a reference value to one of the input terminals. The resistors R ₂ and R ₃ and the capacitor C ₂ are connected in series between a power supply (voltage = + Va) and ground (GND), and the transistor Q ₃ is connected in parallel with the capacitor C ₂ . In this case, the resistance R
Numeral ₆ denotes a variable resistor so that the reference voltage V _S can be changed according to the characteristics of the LD 12. And LD12
The overcurrent detection level can be adjusted by adjusting the variable resistor (resistor R ₆ ) in accordance with the characteristic variation or the like.

The operation of the above circuit is as follows. In the image forming area, the image forming area laser drive circuit 51
The LD 12 is driven by driving the transistor Q ₁ based on the output of the circuit 44. In this case, the capacitor C ₂
Is charged with APC indicated value (command voltage) from the APC circuit 44, the transistor Q ₁ is driven (ON / OFF) in accordance with the voltage. At this time, the image forming area laser driving signal LDS1 controls the transistor Q ₃ from the MPU 40, and drives the transistor Q ₁ by the control of the transistor Q _3. By this operation, image formation (printing) is performed in the image forming area.

[0106] In the position detection area, the position detection area laser driving signal LDS2 from MPU 40, the transistor Q ₅ of the position detecting area laser driving circuit 50 is turned on, current flows in the position detection area to LD 12. In this case, the voltage supplied by the position detection area laser driving signal LDS2, the bias current is supplied to the transistor Q ₅ through a resistor R _7, R _8, transistor Q ₅
Turns on.

[0107] When the LD12 as described above is driven, the said LD12 flows through the current I _d, the voltages V ₁ proportional to the current I _d from the resistor R ₁ is taken out, and inputs to the comparator CMP. Then, in the comparator CMP, to compare the voltages V ₁ taken out from the resistor R ₁ reference value (reference voltage V _S). As a result, if V ₁ <V _S , the output of the comparator CMP is at low level L (= 0), and V ₁
> V _S , the output of the comparator CMP becomes high level H (=
1).

Then, the output of the comparator CMP is L (= 0).
If the voltage of the capacitor C ₁ is zero, since the transistor Q ₂ is turned off, the transistor Q _2, Q ₄ is turned off. Therefore, the operations of the transistors Q ₁ and Q ₅ are performed as described above. However, when the current flowing through the LD 12 becomes an overcurrent and V ₁ > V _S , the output of the comparator CMP becomes H (= 1), and the capacitor C ₁ is charged with a high-level output.

Therefore, transistors Q ₂ and Q ₄ are turned on, and their collector potentials are set to the ground potential. Therefore,
The base potentials of the transistors Q ₁ and Q ₅ go low due to the collectors of the transistors Q ₂ and Q _{4 at} the ground potential. With this operation, an overcurrent flowing to the LD 12 is prevented.

In the above operation, the OC of the OCP circuit 30
P detection value (overcurrent detection value) and LD12 in the position detection area
Current I flowing through_dTo the same level.
Thus, in the position detection area, the current I_dIncreases
When the OCP detection value reaches the OCP detection value,
Jista Q_FourTurns on and the transistor Q_FiveTurn off the power
Style I_dLower. Then, the resistance R₁Taken out of
Voltage V₁Drop, transistor Q_FourIs turned off again
And transistor Q_FiveTurns on. As a result, the voltage
I_dIncrease. Thereafter, the same operation is repeated, and the LD 12
Current I flowing through _dIs controlled by the value suppressed to the OPC detection value
Is done.

[0111] As described above, the position detection area laser driving circuit 50 includes transistors Q _5, can consist of three components comprising resistors R _7, R _8, can be simplified circuit. In other words, the position detection area laser drive circuit 50 has a very simple configuration by simply attaching the three components to the printed circuit board by means such as soldering.

§4: Description of LD Drive Circuit Example 2—See FIG. 4 FIG. 4 is a diagram showing LD drive circuit example 2. This circuit eliminates the OCP circuit connected in series with the LD 12 in the LD driving circuit example 1, and the laser emission power detection unit side including the photodiode 28 (monitor circuit side).
In this example, a laser emission power limiting circuit is connected.

In the LD 12, the characteristics of current versus laser emission power may change due to changes in characteristics over time or the like. In the LD drive circuit example 1, in the position detection area, the OCP circuit 3
Since the current value of the current flowing to the LD 12 is controlled at 0, a predetermined laser light emission power may not be obtained when the laser light emission power characteristic changes.

For this reason, by providing a laser emission power limiting circuit on the laser emission power detection unit (monitor circuit) side instead of the OCP circuit, a necessary laser emission power can be obtained even if the laser emission power characteristic changes. It is like that. The details will be described below.

This LD drive circuit operates the LD in the position detection area.
Detection area laser drive circuit 50 for driving the laser 12
An image forming area laser driving circuit 51 for driving the LD 12 in the image forming area, an APC circuit 44 for controlling the laser emission power of the LD 12 in the image forming area,
It comprises an amplifier 37, an I / V conversion circuit 34 for converting a current I flowing through a photodiode (PD) 28 into a voltage V, and a laser emission power limiting circuit 61 connected to the I / V conversion circuit 34.

In this case, the image forming area laser driving circuit 5
1 uses a laser drive circuit having a constant current circuit 32 and an OCP circuit 30;
Use a laser drive circuit without a constant current circuit or an OCP circuit. The MPU 40 (upper control unit) has a DA
By providing C41 and setting a value in the DAC41, A
The image forming area laser emission power reference value LP1 is sent to the PC circuit 44.

In this case, the APC circuit 44 includes the amplifier 37
, And the output of the I / V conversion circuit 34. Further, the output of the APC circuit 44 is connected to the image forming area laser drive circuit 51. Further, the image forming area laser drive signal LDS1 from the MPU 40 is input to the image forming area laser drive circuit 51, and the position detection area laser drive signal LDS2 from the MPU 40 and the laser emission power limit are input to the position detection area laser drive circuit 50. The output of the circuit 61 is input.

The operation of the above circuit is as follows. In the position detection area, the MPU 40 sends a position detection area laser drive signal LDS2 to the position detection area laser drive circuit 50.
Output. Position detection area laser driving circuit has received the signal 50 starts the operation of the position detection area to turn on the transistor Q _5.

[0119] Then, the transistor Q ₅ it is turned on to drive the LD 12, electric current I _d to the LD 12.
The current I _d LD 12 to emit laser light in laser emission power at the position detection area. The current I _{d in} this case
Is set to be equal to the limit value (overcurrent limit value) of the laser emission power limiting circuit 61. Accordingly, the current I _d in the position detection area, the current was suppressed to the upper limit by the limit value of the laser emission power limit circuit 61.

Next, in the image forming area, the value (LP1) of the DAC 41 set by the MPU 40 is amplified by the amplifier 37 and then sent to the APC circuit 44. The APC circuit 44 takes in the value (LP1) and the output voltage value of the I / V conversion circuit 34, controls the light emission power of the LD 12, and outputs an instruction value of the laser light emission power to the image forming area laser drive circuit 51. I do.

The image forming area laser driving circuit 51
The instruction value from the C circuit 44 is input and the MPU 40
Image forming area laser drive signal LDS1 from
The LD 12 is driven. At this time, the constant current circuit 32
The LD 12 is driven by the constant current specified by the APC circuit 44. While the LD 12 is driven as described above,
The OCP circuit 30 detects an overcurrent, and when an overcurrent exceeding the OCP detection value flows, the overcurrent is suppressed and the LD1
2 to prevent destruction.

The laser emission power limiting circuit 61 detects an overcurrent flowing through the LD 12 by comparing the output voltage of the I / V conversion circuit 34 with a reference value. by suppressing the current flowing through the transistor Q ₅ of the drive circuit 50 to prevent an overcurrent, LD1
2 to prevent destruction.

In the LD drive circuit example 2, the laser emission power limiting circuit 61 is used. If the output of this circuit is also used in the image forming area laser drive circuit 51, the
The P circuit 30 becomes unnecessary. However, this is not done for the following reasons.

That is, the image forming area laser driving circuit 5
1 uses a general-purpose IC (integrated circuit).
, An OCP circuit 30 and a constant current circuit 32 are incorporated. Therefore, it is advantageous to use the IC as it is (since the circuit configuration of this part can be the same as the conventional one).

On the other hand, the position detection area laser driving circuit 50 and the laser emission power limiting circuit 61 use discrete components mounted on a printed circuit board. In this case, the I
Although the laser emission power limiting circuit 61 can be used instead of the OCP circuit 30 in C, it is more advantageous to use the IC as it is and to change only the circuit composed of discrete components. Therefore, it is configured as in the above-described circuit example 2.

In the case where the IC is not used, but is constituted by discrete components, the OCP circuit 30 can be eliminated. In this case, the output of the laser emission power limiting circuit 61 is connected to the image forming area laser driving circuit 51.
, A circuit equivalent to the above circuit can be realized.

§5: Description of Specific Example of LD Drive Circuit Example 2—See FIG. 5 FIG. 5 is a specific example of LD drive circuit example 2. The laser emission power limiting circuit 61 includes a resistor R ₁₂ of the I / V conversion circuit 34.
A comparator CMP for comparing the voltage V (the voltage V and V = V _M) reference voltage and a series circuit of a resistor R _10, R ₁₁ for making the reference voltage, the output of the comparator CMP , A capacitor C ₇ , a resistor R _12, and a transistor Q ₆ connected to the resistor R ₁₂ .

[0128] Then, the series circuit of the resistor R _10, R ₁₁ is connected between ground and power supply (voltage = + Va), a comparator voltage V _S at the connection point of the resistors R _10, R ₁₁ as a reference voltage Input to CMP. In this case, the resistor R ₁₁ is composed of a variable resistor, it is so changed the reference voltage V _S according to the characteristics of the LD 12. The current limit value can be adjusted by adjusting the variable resistor (resistor R ₁₁ ) according to a change in the characteristic of the laser emission power or the like.

The position detection area laser drive circuit 50 includes a transistor Q ₅ for driving the LD 12, a resistor R ₇ constituting a bias circuit for the transistor Q ₅ ,
R ₈ is provided. Further, the image forming area laser drive circuit 51 includes a constant current circuit 32 and an OCP circuit 30. Then, connect the collector of the transistor Q ₆ in the base of the transistor Q ₅ through a resistor R _7.

The operation of the above circuit is as follows. Position detection
In the output area, the position detection area from the MPU 40 is driven by laser.
In response to the signal LDS2, the position detection area laser drive circuit 50
Transistor Q_FiveTurns on and the LD12 detects the position
Apply current in the area. In this case, the position detection area
The resistance given by the voltage given by the motion signal LDS2
R ₇, R₈Through the transistor Q_FiveBias current
Supplied and the transistor Q _FiveTurn on.

[0131] When the LD12 as described above is driven, the said LD12 flows current I _d is the laser emission power at that time is monitored by the photodiode 28. Therefore, the voltage from the resistor R ₁₂ of the I / V conversion circuit 34 V
_M is taken out and input to the comparator CMP. Then, in the comparator CMP, the voltage V taken out from the resistor R _12
M is compared with a reference value (reference voltage V _S ). As a result, V
₁ <If V _S, the comparator output is low for CMP L
(= 0), and when V ₁ > V _S , the output of the comparator CMP becomes high level H (= 1).

The output of the comparator CMP is L (= 0).
The voltage of the capacitor C ₇ if 0, the transistor Q ₆ is turned off. Thus, the operation of the transistor Q ₅ is carried out as described above. However, the current flowing through the LD 12 becomes an overcurrent, and when V _M > V _S , the output of the comparator CMP becomes high level H (= 1) and the capacitor C ₇ is charged with the high level H output.

[0133] Therefore, transistor Q ₆ is turned on, to the collector potential to the ground potential. Therefore, by the collector of the transistor Q _6, which has become the ground potential,
The base potential of the transistor Q ₅ becomes a low level. By this operation, the overcurrent flowing to the LD 12 is suppressed, and the LD 12
12 is prevented from being destroyed by overcurrent.

[0134] In the operation, the limit value of the laser emission power limit circuit 61 (the limit value of the overcurrent), equally setting the level of the current value I _d of the position detection area. In this way, the position detection region, the transistor Q ₆ of the current I _d soon laser emission when increasing power limiting circuit 61 is turned on, resulting in lowering the current I _d to turn off the transistor Q _5.

Then, the voltage V ₁ also drops, the transistor Q ₆ turns off, and the transistor Q ₅ turns on again. As a result, the voltage _Id increases. Repeating the same operation thereafter, the current I _d flowing through the LD12 is controlled by the value was suppressed to limit value of the laser emission power limit circuit 61.

[0136] As described above, the position detection area laser driving circuit 50 includes transistors Q _5, can consist of three components comprising resistors R _7, R _8, can be simplified circuit. In other words, the position detection area laser drive circuit 50 has a very simple configuration by simply attaching the three components to the printed circuit board by means such as soldering.

§6: Description of Operation of LD Unit—See FIGS. 6 and 7 FIG. 6 is an explanatory view of the operation of the LD unit. FIG. 6A is an explanatory diagram of laser beam scanning, and FIG. FIG. FIG. 7 is a timing chart. The operation will be described below with reference to FIGS. The LD unit will be described with reference to the conventional example shown in FIG.

In the LD unit 2, a laser beam is emitted as described above, and the laser beam scans the photosensitive drum 3 in the main scanning direction. This state is shown in FIG. First, when a laser beam is emitted from the LD 12 to the polygon mirror 11 that is rotationally driven by a motor, the laser beam is reflected by the polygon mirror 11 and then scanned on the photosensitive drum 3 in the main scanning direction. At this time, a part of the laser light is received by the position detection sensor 25, and the start position of one cycle of the printing operation is detected.

At this time, the paper position on the photosensitive drum 3 (the position of the maximum paper size) is as shown, and this paper position is within the scanning range (the maximum scanning range) of the laser beam. In this case, the scanning range of the laser beam on the photosensitive drum 3 is shown in FIG.
From position to b, the position detection point d (start position) by the position detection sensor 25 is within the range from a to b and outside the paper position. The position detection point d is located on either the left side or the right side of the sheet position, but the operation is the same on either side.

The position of the sheet on the photosensitive drum 3 is from point e to point f. On both sides of the sheet position, gaps G1 and G2 (parts that are not actually printed) are present, and an area therebetween (the area G1 and G2). (Between G1 and G2) is an area where printing is actually performed. When the position of the sheet is set in the range from point e to point f and scanning with laser light is performed, the elapsed time from point d to point e is T1, and the elapsed time from point e to point f is T2. , The elapsed time from point f to point b, and the elapsed time from point a to point d are T3.

[0141] B of FIG. 6, and 7, the horizontal axis represents time T, and the vertical axis shows a laser emission power P _L. The position detection laser in the area light emitting power P _L and P _L = LV2, the laser emission power P _L in the image forming area and P _L = LV1, the laser emission power P _L for preventing the destruction of the LD 12 P Let _L = LV3.

[0142] To set this case, OCP detection value of OCP circuit current value of the position detection area and (overcurrent detection value) (current flows through the LD 12 I _d) to the same level. That is,
The value of the current I _d in the position detection area is set to OCP detection value and a level approximately equal to (LV1 <LV2 ≒ LV3).

However, the laser emission power P _L = LV3
Is the OCP detection value when the OCP circuit is used, but is the laser emission power limit value level in the laser emission power limiting circuit 61 of the LD drive circuit example 2. T1 and T3 are position detection areas, and T2 is an image formation area.

As shown in FIG. 6B, in the position detection area, the LD 12 is driven such that LV2 ≒ LV3. In this case, the position detection area laser driving signal LDS2 from the position detecting area laser driving circuit 50 in the MPU40 drives the transistor Q _5. At this time, the current flowing through the LD 12 is at the level LV2, but when this current increases, the current is suppressed to the level LV3 by the OCP circuit 30 or the laser emission power limiting circuit 61. In this case, since LV2 ≒ LV3, the current flowing through the LD 12 is controlled to the above level.

Hereinafter, description will be made with reference to the timing chart of FIG. First, at T1 from point d to point e, P _L
= LV2 ≒ LV3, the laser emission power is LV
Lower to 1. Next, at T2 from point e to point f, P _L
= LV1, image formation (printing) is performed. In this case, T
In step 2, the current flowing through the LD 12 is changed (variable value) depending on the image forming content. Next, T3 in the image formation from the point f to point b is completed, since the position detection area, again increasing the laser emission power P _L to P _L = LV2 (fixed value).

[0146] In this way, it is performed by repeating the cycle of T1, T2, T3, when the laser emission power P _L in the process reaches the P _L = LV3, OCP circuit 30, or by laser emission power limit circuit 61 , LD12 becomes an overcurrent, and this overcurrent is prevented.

Thus, for example, in the case of the example shown in FIG. 3, in the position detection area, as soon as the current I _d increases, the transistor Q ₄ of the OCP circuit 30 is turned on, the transistor Q ₅ is turned off, and the current I _d is turned off. Lower _d . Then, the voltages V ₁ also drops, the transistor Q ₄ is a transistor Q ₅ again turned off is turned on. as a result,
The current _Id increases. Thereafter, the same operation is repeated,
Current I _d flowing through the LD12 is controlled by the value was suppressed to OCP detection value.

§7: Other explanations (1): In the LD driving circuit examples 1 and 2, there is one image forming area laser driving circuit for driving the LD 12 in the image forming area. The present invention is not limited to this example, and is also applicable to a circuit provided with a plurality of image forming area laser driving circuits.

That is, in the image forming area, when forming an image, a plurality of different levels of APC may be required. In such a case, a plurality of image forming area laser driving circuits may be provided, and a circuit necessary for image formation may be selected to form an image. As described above, the present invention can be similarly applied to an LD driving circuit provided with a plurality of image forming area laser driving circuits.

(2) The LD drive circuit is applicable not only to a laser printer but also to a digital copying machine, a facsimile machine and the like.

[0151]

As described above, the present invention has the following effects.

(1) In the first aspect, the APC circuit includes: a laser emission power reference value of the image forming area from the host controller;
The first laser emission power is controlled in the image forming area for one LD (laser diode) based on a voltage proportional to the photocurrent of the photodiode for detecting the laser emission power. In the first laser driving circuit (image forming area laser driving circuit), the output (instruction value) of the APC circuit
Then, the LD is driven in the image forming area based on the image forming area laser drive signal from the host controller.

In the position detection area, the second laser drive circuit (position detection area laser drive circuit) drives the LD according to the position detection area laser drive signal from the host control unit (drives with the second laser emission power). ). And OC
The P circuit (overcurrent protection circuit) suppresses the overcurrent when the LD is driven by the first and second laser emission powers and the drive current flowing through the LD becomes overcurrent, and the LD is destroyed. To prevent

In this case, it is necessary to perform high-precision laser emission power control in the image forming area, and therefore, an APC circuit is required. However, in the position detection area, high-precision laser emission power control is not required and a fixed value is sufficient, so that the configuration of the position detection area laser drive circuit can be simplified.

(2): In the second aspect, the second laser drive circuit (position detection area laser drive circuit) is an OCP circuit.
By setting the CP detection value (overcurrent detection value) and the level of the LD drive current corresponding to the laser emission power required in the position detection area to the same level, the position detection area can be used without using the APC circuit. To control the laser emission power at.
For this reason, the configuration of the position detection area laser drive circuit can be simplified.

(3): According to the third aspect, the OCP circuit adjusts the OCP in accordance with the characteristic variation of the LD by the level adjusting means of the OCP circuit.
Since the detection value (overcurrent detection level) is adjusted, it is possible to always adjust to an appropriate OCP detection value even if the LD characteristics vary.

(4): In the fourth aspect, the OCP circuit comprises an LD
Is located at a position where all the current flowing through the laser can be detected, so that the overcurrent of the LD is reliably prevented not only when one LD is driven but also when a plurality of laser drive circuits are driven simultaneously. Therefore, it is possible to reliably prevent the LD from being destroyed by the overcurrent.

(5): In claim 5, the APC circuit controls the first laser emission power in one image forming area for one LD. The first laser driving circuit (image forming area laser driving circuit) is controlled by the APC circuit.
Drives the LD in the image forming area.

The laser emission power limiting circuit monitors the photocurrent of the photodiode for detecting the laser emission power, and sets the limit value of the LD drive current to limit the laser emission power so that the LD is not destroyed. . In this way, even if the characteristics of the laser emission power change, accurate laser emission power can always be obtained.

(6): In claim 6, the second laser drive circuit (position detection area laser drive circuit) corresponds to the limit value of the laser emission power limiting circuit and the laser emission power required in the position detection area. The laser emission power in the position detection area is controlled by setting the level of the drive current thus obtained to the same level. In this way, the laser emission power in the position detection area can be controlled without using an expensive APC circuit, and the LD can be prevented from being destroyed.

(7): In claim 7, the exposure section of the image forming apparatus comprises: an LD for emitting light for a latent image; a scanning section for scanning the light emitted from the LD; An APC circuit that controls the light emission power of the LD at a position, a first laser drive circuit that drives the LD at a position where a latent image is formed on the carrier under the control of the APC circuit, and a Laser drive circuit and an LD driven by the laser power of the latent image position with respect to the carrier and the detection unit
And a current protection circuit for preventing an overcurrent.

In this case, it is necessary to perform high-precision laser emission power control in the image forming area, and therefore, an APC circuit is required. However, in the position detection area, high-precision laser emission power control is not necessary and a fixed value is sufficient, so that the configuration of the position detection area laser drive circuit can be simplified and the configuration of the image forming apparatus is simplified. (8): In claim 8, the laser power (laser emission power) at the detection position by the detection unit is larger than the laser power (laser emission power) at the latent image position with respect to the carrier. . With this configuration, the circuit configuration for driving the LD can be simplified without lowering the performance, and the cost of the product can be reduced.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 shows an LD driving circuit example 1 according to the embodiment of the present invention.
It is.

FIG. 3 is an LD driving circuit example 1 according to the embodiment of the present invention;
Is a specific example.

FIG. 4 is an LD drive circuit example 2 according to the embodiment of the present invention;
It is.

FIG. 5 shows an LD drive circuit example 2 according to the embodiment of the present invention.
Is a specific example.

FIG. 6 is an explanatory diagram of an operation of the LD unit according to the embodiment of the present invention.

FIG. 7 is a timing chart in the embodiment of the present invention.

FIG. 8 is a diagram illustrating a conventional printer device.

FIG. 9 is a diagram showing a conventional LD unit.

FIG. 10 is a diagram illustrating the operation of a conventional LD unit.

FIG. 11 is a first example of a conventional LD drive circuit.

FIG. 12 is a specific example of a conventional LD drive circuit example 1.

FIG. 13 is a second example of the conventional LD driving circuit.

FIG. 14 is an example of an APC circuit.

[Explanation of symbols]

Reference Signs List 1 housing 2 laser diode unit (LD unit) 3 photosensitive drum 4 paper transport path 5 charging unit 6 exposure unit 7 developing unit 8 transfer unit 9 fixing unit 10 scanning motor unit 11 polygon mirror 12 laser diode (LD) 13 aperture 14 collimator Lens 15 Cylindrical lens 16, 24 Focusing / correcting lens 17 Aspherics lens 18 Cleaning unit 19, 20, 21 Mirror 22 Half mirror 23 Protective optical unit glass 25 Position detection sensor 27 Laser light 28 Photodiode (PD) 30 OCP Circuit 31 Laser drive circuit 32 Constant current circuit 34 I / V conversion circuit 35 Image formation area APC circuit 36 Position detection area APC circuit 37, 38 Amplifier (amplifier) 40 MPU (Microprocessor unit) 41, 42 Digital-to-analog converter (DA
C) 43 switching circuit 44 APC circuit 45 reference switching circuit 46 analog switch 50 position detection area laser driving circuit 51 image forming area laser driving circuit 55 comparator 57, 58 transistor 59 capacitor 59 laser emission power limiting circuit

Claims (8)

[Claims] A laser diode driving circuit which requires different laser emission powers in a scanning direction within one scan of laser light emitted from a laser diode, wherein the first laser emission power is supplied to one laser diode. An automatic laser light amount control circuit for controlling, a first laser drive circuit for driving a laser diode under the control of the automatic laser light amount control circuit, and a second laser drive circuit for driving the laser diode with a second laser emission power And an overcurrent protection circuit for preventing an overcurrent of the laser diode driven by the first and second laser emission powers. 2. The second laser driving circuit sets an overcurrent detection level of the overcurrent protection circuit and a level of a driving current corresponding to the second laser emission power to the same level. Without using the automatic laser light amount control circuit,
2. The laser diode drive circuit according to claim 1, further comprising a function of controlling the second laser emission power. 3. The laser diode driving device according to claim 1, wherein said overcurrent protection circuit includes a level adjusting means for adjusting an overcurrent detection level according to a variation in characteristics of the laser diode. circuit. 4. The overcurrent protection circuit according to claim 1, wherein the overcurrent protection circuit is located at a position where all the current flowing through the laser diode can be detected.
2. The laser diode driving circuit according to claim 1, further comprising a function of preventing an overcurrent of the laser diode. 5. A laser diode driving circuit which requires different laser emission powers in a scanning direction within one scan of laser light emitted from a laser diode, wherein the first laser emission power is supplied to one laser diode. An automatic laser light amount control circuit for controlling, and a first laser driving circuit for driving a laser diode under the control of the automatic laser light amount control circuit, and a second laser driving the laser diode with the second laser emission power. A laser driving circuit, a laser emission power for monitoring a photocurrent of a photodiode for detecting the laser emission power, and setting a limit value of the laser diode drive current to limit the laser emission power so that the laser diode is not destroyed. A laser diode drive circuit comprising a limiting circuit. 6. The second laser driving circuit automatically sets a limit value of the laser emission power limiting circuit and a level of a driving current corresponding to the second laser emission power to an equivalent level. Without using the laser light amount control circuit,
6. The laser diode drive circuit according to claim 5, further comprising a function of controlling the second laser emission power. 7. A carrier on which a latent image is formed, light is scanned on the carrier for each line to form a latent image on the carrier, and a latent image on the carrier is developed. A developing device, and a detecting unit for detecting a position of light in one line of the exposing unit. Based on the position of the light detected by the detecting unit, the position is corrected to a latent image on the carrier to form an image. In the image forming apparatus, the exposing unit controls a laser diode that emits light for a latent image, a scanning unit that scans the light emitted by the laser diode, and a light emitting power of the laser diode at a position where a latent image is formed on the carrier. An automatic laser light amount control circuit, a first laser drive circuit for driving a laser diode at a latent image forming position with respect to the carrier under the control of the automatic laser light amount control circuit, and a second laser driving circuit for driving the laser diode at a detection position of the detection unit The leh An image forming apparatus, comprising: a driving circuit; and a current protection circuit for preventing an overcurrent of a laser diode driven by a latent image position with respect to a carrier and a laser power of a detection unit. 8. The image forming apparatus according to claim 7, wherein the laser power at the position detected by the detector is higher than the laser power at the latent image position with respect to the carrier.