JPH05206550A - Light emitting element drive circuit - Google Patents

Abstract

PURPOSE:To enable a developed visible image to be improved in reproducibility to inputted image signals by a method wherein a delay means which delays that image signals transfer from an ON-state to an OFF-state is provided. CONSTITUTION:A drive circuit is composed of an input, image signal delay circuit 1 which corrects light pulses in width, a switching circuit 2 which drives a semiconductor light emitting element 4 in pulses, and a variable current source circuit 3 which determines a current which drives the semiconductor light emitting element 4. The waveforms of light signals inputted to or outputted from the delay circuit 1 are shown in figure. At this point, when an input signal changes from a Low level to a High level, an output signal is delayed by a delay time of an invertor. Therefore, an input image signal at a low level is elongated in time by DELTAt through the delay circuit 1 to elongate a semiconductor light emitting element in light pulse ON-time, whereby a light pulse can be compensated in width. At this point, the delay time DELTA t is required to be set to enable a printed developed image to be optimal to input image signals.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light emitting element driving circuit which is preferably implemented in an image forming apparatus which forms a latent image on a photosensitive member by using a light emitting element driven according to an image signal.

[0002]

2. Description of the Related Art A conventional semiconductor light emitting element drive circuit is constructed as shown in FIG. 2 in FIG.
Is a switching circuit unit that drives the light emitting element 4 according to the image signal, and 3 is a variable current source that determines a pulse drive current to the light emitting element 4. FIG. 5B shows drive waveforms of image signals and the like. The semiconductor light emitting element 4 is driven by the switching circuit unit 2 according to the image signal. In this case, the drive current I tends to have a smaller pulse width than the input image signal. It is known that when a semiconductor laser is used as the light emitting element, the width of the light emitting pulse L is further reduced due to the influence of the threshold current. FIG. 5C is a graph showing the relationship between the driving current I of the semiconductor laser and the intensity of the light emission pulse L. Further, when a latent image is formed on the photoconductor by the light emission pulse L, the portion that contributes to the development is a portion where the light intensity of the light emission pulse is high.

From the above, in the conventional example, when an image is formed while scanning a light emitting pulse from a semiconductor light emitting element on a photoconductor, the line width of a developed image is smaller than the line width expected from an input image signal. There was a problem that became.

Since the response of the switching circuit 2 varies depending on the current value I, the drive pulse width also depends on the current value I. It is generally known that the smaller the current value I, the narrower the pulse width. Further, since the drive current value I largely depends on the characteristics of the light emitting elements to be used, if light emitting elements having large characteristic variations are used, variations will occur among the image forming apparatuses.

If the drive current value I is changed to compensate for the deterioration of the light emitting element, the image quality will change.

[0006]

In the above conventional example, 1)
Thinning of the pulse width in the switching circuit unit 2) Thinning of the pulse width due to the influence of the threshold current when a semiconductor laser is used 3) Development when developing the latent image of the photoconductor by the light pulse Due to the thinning in the visible image, the line width of the actual developed image becomes smaller than the line width expected from the input image signal.

Further, since the narrowing of the driving light pulse width depends on the current value in the driving circuit, the semiconductor light emitting element may cause a difference between the devices or a change in the degree of the narrowing of the pulse width due to aging even in the same device. There was a problem that occurred.

[0008]

According to the present invention, 1) there is provided a delay means for generating a delay time when an image signal changes from ON to OFF.

2) A control means for controlling the delay time of 1) is provided according to the drive current of the semiconductor light emitting device. By doing so, the reproducibility of the developed visible image with respect to the input image signal is improved.

[0010]

1 is a block diagram of the first embodiment of the present invention.

In FIG. 1A, 1 is a delay circuit of an input image signal for correcting the light pulse width, 2 is a switching circuit for pulse driving the semiconductor light emitting element 4, and 3 is a current for driving the semiconductor light emitting element 4. It is a variable current source circuit that determines a value. Such a drive circuit is preferably composed of an IC.

FIG. 1B shows the inside of the delay circuit 1, and FIG. 1C shows the input and output waveforms to the delay circuit 1. At this time, when the input changes from Low to High, the output is delayed by the delay time of the inverter section.

Therefore, the optical pulse width is compensated by increasing the Low level time of the input image signal by Δt and increasing the optical pulse ON time of the semiconductor light emitting element by the delay circuit.

At this time, the delay time Δt is 2t ₁ × n (n: inverter 2), where t ₁ is the delay time in the inverter stage.
It is indicated by the number of sets when one stage is set, a natural number). This Δt must be set so that the developed image printed on the input image signal is in a suitable state.

In the present invention, the cathode ground type is used as an example of the semiconductor light emitting element, but the same effect can be obtained by using the anode ground type (driving circuit is a suction type). Although the inverter is used as the delay gate, other logic gates may be used.

[Other Embodiments] FIG. 3 shows a second embodiment of the present invention.

The structure of this embodiment is the same as that of the first embodiment, and therefore its explanation is omitted.

FIG. 2A shows the structure of the delay circuit, FIG.
(B) shows a drive waveform.

In this embodiment, a resistor, a diode and a capacitor are used as the delay circuit. As shown in FIG. 2C, the input / output of the delay circuit is such that when the input image signal changes from High to Low, the diode turns on, so that the output changes to Low almost at the same time, but the input changes to Low.
When changing from to High, the output changes from Low to High according to the specific number of resistors and capacitors.

As a result, the input image signal changes from Low to Hi.
When it changes to gh, the output of the delay circuit (a) is L for the time Δt at which it reaches the slice level of the switching circuit 2.
It is possible to lengthen the ow level (light on time).

At this time, it is necessary to set the resistors, the capacitors, and the slice level of the switching circuit 2 so that the optical pulse width has an appropriate value for the input image signal.

FIG. 3 shows a block diagram of the third embodiment of the present invention.

In this embodiment, the description of the same parts as those in the first embodiment will be omitted.

Reference numeral 5 is a voltage-current conversion circuit that determines the set current of the variable current source 3 composed of, for example, a plurality of stages of current mirrors based on the laser current control signal. Reference numeral 6 is a laser light amount control signal converted into a digital signal. An analog / digital conversion circuit (hereinafter referred to as an A / D conversion circuit) 7 is a decoder that selects the delay circuits 1 to 3 according to a signal from the A / D conversion circuit 6.

With the above structure, the drive current of the semiconductor light emitting element follows the laser current control signal, so that the delay circuit can be selected by the laser current control signal. Then, when such a drive circuit is configured by an IC, an input terminal for a laser current control signal is provided. Therefore, by selecting the delay time of the delay circuit according to the laser drive current, stable control is possible over a wide current range.

At this time, the delay time of the delay circuit must be set to an appropriate value according to the corresponding drive current value.

At this time, the delay means in the delay circuit may be any means as shown in the first and second embodiments. Furthermore, in this embodiment, up to three delay circuits are used, but any number of delay circuits may be used depending on the situation.

With the above circuit configuration, it is possible to compensate for thinness due to variations in light emitting elements. Further, it is possible to prevent the degree of narrowing from changing even if the drive current is changed in order to correct the secular change of the light emitting element.

Further, in the case of application to an image forming apparatus capable of switching resolution, for example, the light amount of the light emitting element can be switched according to the resolution, and in such a case, an appropriate delay time can be set for each resolution.

FIG. 4 shows a block diagram of the fourth embodiment.

In this example, the selection means of the delay circuit is performed by a selection signal from the outside of the semiconductor light emitting element drive circuit. In this case, although the number of signal lines from the outside increases, the drive circuit can be simplified and the same effect as that of the third embodiment can be obtained.

[0032]

As described above, the input image signal is ON.
By changing the optical pulse width by providing a delay means when changing from OFF to 1) 1) An image to be printed can be accurately output with respect to the input image signal.

2) Stable image output can be obtained regardless of the drive current value of the semiconductor light emitting element.

3) As an image forming apparatus, it is possible to cope with high definition and high speed. And so on.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a second embodiment of the present invention.

FIG. 3 is a diagram showing a third embodiment of the present invention.

FIG. 4 is a diagram showing a fourth embodiment of the present invention.

FIG. 5 is a diagram showing a conventional technique.

[Explanation of symbols]

 1, 10, 11, 12 Delay circuit 2 Switching circuit 3 Variable current source 4 Semiconductor light emitting device 5 VI conversion circuit 6 A / D conversion circuit 7 Decoder

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H04N 1/23 103 A 9186-5C

Claims (5)

[Claims] 1. A drive circuit for pulse-driving a light-emitting element based on an image signal, comprising a delay means for delaying transition of the image signal from an ON state to an OFF state. 2. The light emitting element drive circuit according to claim 1, further comprising control means for controlling a delay time of the delay means according to a drive current of the light emitting element. 3. The light emitting element drive circuit according to claim 1, further comprising control means for determining a delay time by the delay means according to a control signal input to the light emitting element drive circuit. Element drive circuit. 4. A delay means for delaying at least one of rising and falling of a binary level modulated signal, and switching means for switching a drive current to a light emitting element based on an output of the delay means. A light-emitting element driver circuit having. 5. The light emitting element drive circuit according to claim 4, further comprising control means for controlling a delay time by the delay means according to a drive current to the light emitting element.