JPH07106674A - Laser drive circuit - Google Patents

Abstract

PURPOSE:To obtain a laser driving current in which ringing is suppressed by lowering he level of noise superposed on the bias line at a constant current section. CONSTITUTION:One laser lighting signal 4A passed through an inversion/delay circuit INV&DLY has a delay of several nano seconds behind the other laser lighting signal 4B. Consequently, a difference appears in the level of pulse noise occurring on a bias line between two moments of time where the laser lighting signals 4A, 4B intersect each other. In other words, a low noise 1 is superposed on the bias line at the time of rising intersection of the signal 4B whereas a high noise 2 is superposed thereon at the time of rising intersection of the signal 4A. The moment of intersection when the level of pulse noise is lowered is included in the rising time (= laser ON time) of the signal 4B.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser drive circuit for controlling turning on and off of a semiconductor laser.

More specifically, the present invention uses, for example, a semiconductor laser to form a latent image on an electrophotographic photoreceptor,
The present invention relates to a laser drive circuit applicable to an electrophotographic image forming apparatus that visualizes the latent image with a developing device.

[0003]

2. Description of the Related Art Conventionally, a laser beam printer has been known as one of devices using a semiconductor laser. This laser beam printer feeds recording paper from a recording paper storage such as a paper cassette in accordance with a printing command from an external device such as a computer, and the timing of conveying the recording paper and image information from a video controller by a synchronous conveying means such as registration rollers. Images are recorded by transporting them in synchronization with the delivery timing.

FIG. 7 is a block diagram of a conventionally known laser beam printer 101, and FIG. 8 is a timing chart showing its operation. The operation of the laser beam printer 101 will be described with reference to these drawings as well.

First, the video controller 28 is
PRI is confirmed when the Y signal is TRUE.
Let NT signal be TRUE. The print control unit 26
When the RINT signal becomes TRUE, driving of the main motor 23 and the polygon motor 14 is started. When the main motor 23 is driven, the photosensitive drum 17 and the fixing roller 79 are
And the discharge roller 11 rotates. After that, the light amount control and high-voltage driving of the primary charger 19, the developing device 20, and the transfer charger 21 are sequentially performed.

The print control unit 26 includes a polygon mirror 1
When the rotation of the 4 is a steady state T ₁ seconds only paper feed clutch 2
4 is turned on to drive the paper feed roller 75, and the recording paper S is fed toward the registration roller pair 76. Then, the print control unit 26 sends a VSREQ signal to the video controller unit 28 at the timing when the leading edge of the recording paper S reaches the registration roller pair 6, and at the same time, the paper feed roller clutch 24.
Is turned off and the driving of the paper feed roller 75 is stopped.

When the video controller section 28 finishes developing the image information into a dot image and is ready to output the VDO signal, the video controller section 28 confirms that the VSREQ signal is TRUE, sets the VSYNC signal to TRUE, and synchronizes with this. Then, after T _V seconds, the output of the image data (VDO signal) for one page is started. At this time, the print control unit 26
After T ₃ seconds from the rise of the YNC signal, the registration roller clutch 25 is turned on and the registration roller pair 6 is driven. The registration roller pair 6 is driven for a time (T ₄ seconds) until the trailing edge of the recording sheet S passes through the registration roller pair 76.

During this time, the print control unit 26 keeps the HS
The YNC signal is sent to the video controller unit 28 at a predetermined timing synchronized with the laser scanning, and the laser light for scanning the photosensitive drum 17 is modulated based on the VDO signal.

Further, when printing the next page, the PRI is re-executed ₅ seconds after the rise of the VSYNC signal.
Let NT signal be TRUE. After that, the same operation as the first page is performed.

By such operations of the print control unit 26 and the video controller unit 28, the recording paper S is sequentially conveyed to the paper feed roller 75, the registration roller pair 76, the image recording unit 8, the fixing roller 79, and the paper discharge roller 11. The image is recorded.

Next, the drive circuit of the laser diode 13 will be described. As shown in FIG. 9, this laser drive circuit is composed of a constant current section 2 and a switching section 3. The constant current unit 2 outputs a constant current I ₁ corresponding to the DC voltage V _B applied to the bias line. The switching unit 3 is a circuit for switching the constant current I ₁ with the laser lighting signals 4A and 4B. The laser diode LD emits light in accordance with the operation of the switching unit 3.

The switching unit 2 comprises a differential amplifier circuit for inputting complementary laser lighting signals 4A and 4B. That is, it is composed of a first transistor Q1 that uses the laser diode LD as a load, and a second transistor Q2 whose emitter is commonly connected to the first transistor Q1. Then, the transistors Q1 and Q2 are complementarily turned ON / OFF according to the laser lighting signals 4A and 4B, and the laser diode LD emits light.

[0013]

However, the laser lighting signal 4 complementary to the switching unit 2 shown in FIG. 9 is used.
Differential amplification transistors Q1 and Q2 by applying A and 4B
In controlling ON / OFF of the pulse current, as shown in FIG. 10, pulse noise occurs in the bias line of the constant current portion 2 due to the crossing of the complementary laser lighting signals 4A and 4B at the rising and falling edges. . The output current I ₁ of this constant current unit 2
Has a value corresponding to the voltage V _B applied to the bias line, so that this pulse noise on the bias line causes ringing in the drive current of the laser diode LD, which adversely affects the life of the laser diode. Exert.

On the other hand, in the conventional switching speed, the influence of the pulse noise generated on the bias line was not so serious. But,
As the speed and definition of laser beam printers have increased, the speed of switching has increased, and the adverse effect of this pulse noise has become a problem.

Therefore, in view of the above points, an object of the present invention is to
An object of the present invention is to provide a simple laser drive circuit in which ringing that occurs when switching a laser diode is reduced.

[0016]

In order to achieve the above object, a laser drive circuit according to the present invention uses a base or gate of one transistor as a first input terminal and a base or gate of the other transistor as a second input terminal. And a differential amplification means for turning on one of the transistors to turn on the laser diode and supplying complementary laser lighting signals to the first and second input terminals. , And delay means for delaying the laser lighting signal supplied to one of the input terminals.

Further, in another laser driving circuit according to the present invention, the base or gate of one transistor is used as a first input terminal and the base or gate of the other transistor is used as a second input terminal, and either one of them is used. Differential amplifier means for turning on the laser diode by turning on one of the transistors, and supplying a complementary laser lighting signal to the first and second input terminals,
And a pulse width expansion means for expanding the pulse width of the laser lighting signal supplied to one of the input terminals.

[0018]

The pulse noise on the current value setting bias line caused by the rising and falling edges of the two complementary laser lighting signals is as follows when the laser diode is switched from on to off at the time of the intersection. When switching from off to on, both become larger (smaller) farther (closer) from the on state: in the present invention, one of the two laser lighting signals input to the differential amplifying means is delayed. By providing the delaying means or the pulse width expanding means for expanding the signal width of one of the laser lighting signals, the pulse noise generated on the bias line at the crossing of the rising and falling edges of the two laser lighting signals is reduced, The ringing of the laser diode drive current can be suppressed. This has the advantages that the life of the laser diode is extended and the time constant of the snubber circuit, which is a protection circuit for the laser diode, is reduced, and laser light emission with a fast rise is obtained.

[0019]

Embodiments of the present invention will now be described in detail with reference to the drawings.

Embodiment 1 Here, an embodiment in which the present invention is applied to a laser beam printer will be described.

FIG. 1 shows the basic circuit configuration of this embodiment and other embodiments described later. In the figure, A is a laser light source as an exposure means for forming an image using laser light. Reference numeral B is a laser light detecting means for monitoring the output of the laser light. C is a laser light amount control means for controlling the laser light source A so that a predetermined light output level can be obtained based on the output of the laser light detection means B. C1 is a light output level setting means, and C2 is a light output level determining means. D is a laser so that the timing of the laser light amount control means C is executed in an area excluding the maximum recordable area in the main scanning direction and in the image guarantee areas at the front and rear ends in the recording medium in the sub scanning direction. The timing control means controls the timing of the light amount control means C. E is a photoconductor.

FIG. 2 shows a laser drive circuit included in the laser light source A. FIG. 3 is a waveform diagram showing the operation of FIG.

In FIG. 2, Q1 and Q2 are switching transistors for turning on / off the laser drive current, and 2 is a constant current for setting the current value supplied to the laser diode LD based on the applied voltage V _B of the bias line. Parts 4A and 4B are laser lighting signals for controlling laser emission, and INV & DLY are inversion / delay circuits for inverting and delaying one laser lighting signal. Laser lighting signal 4 which has passed through this inversion / delay circuit INV & DLY
A has a delay of several nanoseconds with respect to the other laser lighting signal 4B.

With such a configuration, as is apparent from FIG. 3, there is a difference in the level of pulse noise generated in the bias line at two points where the laser lighting signal 4A and the laser lighting signal 4B intersect. That is, a small noise 1 is superimposed on the bias line at the rising crossing of the laser lighting signal 4B (crossing 1), and a large noise 2 is superimposed on the bias line at the rising crossing of the laser lighting signal 4A (crossing 2).

Therefore, by setting the "crossing 1" at which the level of the pulse noise becomes small when the laser lighting signal 4B rises (= when the laser is ON), there is little ringing when the laser is ON and large ringing when the laser is OFF. Electric current is obtained. As a result, laser light with less ringing can be obtained.

Controlling the laser lighting signal to obtain laser light with less ringing in this way leads to simplification of the snubber circuit, reduction of the time constant, and obtainment of laser light with a fast rise. That is, it is possible to form a high-quality, high-definition, high-quality image.

Embodiment 2 FIG. 4 shows a laser drive circuit according to the second embodiment.
In the figure, DLY is a delay circuit, OR is an OR gate circuit, and INV is an inverting circuit for inverting the signal level.
The delay circuit DLY and the OR gate circuit OR form a pulse width expansion circuit for expanding the time width of the input pulse.

FIG. 5 is a waveform diagram showing the operation of FIG.

The laser lighting signal 4B which has passed through the pulse width expansion circuit shown in FIG. 4 is input to the differential transistor Q1 of the switching section. That is, by making the pulse width of the one laser lighting signal 4B larger than the pulse width of the other laser lighting signal 4A, the time point at which the two laser lighting signals 4A and 4B intersect, as shown in FIG. Can be raised to.

As a result, the level of noise superimposed on the bias line becomes small (see comparison with (A) and (B) in FIG. 3), and a laser drive current with little ringing is generated both when the laser drive current is ON and when it is OFF. can get. That is, laser light with less ringing can be obtained.

Controlling the laser lighting signal to obtain laser light with less ringing as described above leads to simplification of the snubber circuit, reduction of the time constant, and acquisition of laser light with a fast rise. That is, it is possible to form a high-quality, high-definition, high-quality image.

Other Embodiments FIG. 6A is a modification of the circuit shown in FIG. That is, in FIG. 2, one inversion / delay circuit INV &
While the laser lighting signal is delayed by using DLY, in FIG. 6A, a desired delay time is obtained by connecting an odd number of inverters in series. Since other operations are the same as those in FIG. 2, detailed description will be omitted.

FIG. 6B is a modification of the circuit shown in FIG. That is, while the delay circuit DLY is used in FIG. 4, in FIG. 6B, a desired delay time is obtained by connecting an even number of inverters in series. Since other operations are the same as those in FIG. 4, detailed description thereof will be omitted.

[0034]

As described above, according to the present invention, one of the laser lighting signals input to the differential amplifier circuit for driving the laser is delayed or the pulse width thereof is extended, so that the constant current section Since the level of noise superimposed on the bias line can be reduced, it is possible to obtain a laser drive current with less ringing. This leads to a decrease in the time constant of the snubber circuit, which is a protection circuit for the laser diode, enables quick laser emission, and provides high-quality and high-definition images in image forming devices such as laser beam printers. The effect of being able to be obtained.

[0035]

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of a laser beam printer to which the present invention is applied.

FIG. 2 is a diagram showing a laser drive circuit in the first embodiment.

FIG. 3 is a waveform diagram showing the operation of FIG.

FIG. 4 is a diagram showing a laser drive circuit according to a second embodiment.

5 is a waveform diagram showing the operation of FIG.

FIG. 6 is a diagram showing a laser drive circuit according to another embodiment.

FIG. 7 is an overall configuration diagram of a conventionally known laser beam printer.

FIG. 8 is a timing diagram showing the operation of FIG. 7.

FIG. 9 is a diagram showing a conventionally known laser drive circuit.

10 is a waveform chart showing the operation of FIG.

[Explanation of symbols]

A laser light source B laser light detection means C laser light amount control means D timing control means E photoconductor 2 constant current portion V _B bias voltage LD laser diode

Claims (2)

[Claims] 1. A laser is provided by using the base or gate of one transistor as a first input end, the base or gate of the other transistor as a second input end, and turning on one of the transistors. A differential amplification means for lighting the diode, and a delay means for delaying the laser lighting signal supplied to one of the input terminals when supplying the complementary laser lighting signal to the first and second input terminals. A laser drive circuit characterized by being provided. 2. A laser is provided by making the base or gate of one transistor the first input terminal, the base or gate of the other transistor the second input terminal, and turning on one of the transistors. Differential amplification means for lighting a diode, and a pulse width for expanding the pulse width of the laser lighting signal supplied to one of the first and second input terminals when supplying a complementary laser lighting signal to the first and second input terminals. A laser drive circuit comprising: an expanding unit.