JPS63280568A - Drive circuit for light emitting element - Google Patents

Abstract

PURPOSE:To eliminate the fluctuation of an optical output of a light emitting diode array by using a digital signal to control the output current of a drive circuit driving the light emitting element in multiple stages. CONSTITUTION:Constant current control utilizes a current mirror circuit comprising a bipolar transistor TR0 functioning as a reference component and a bipolar TRC functioning as an output component. Reference currents I0-In for controlling the output current are given by a digital current control signal, which is of n bits and the reference currents I0-In corresponding to each bit differ from each other and the output current is specified by the sum of the said reference currents I0-In. The n-bit current control signal gives 2n-way of reference currents I0-In and the output current is controlled in 2n stages of the same number. Thus, the drive current correcting the optical output dispersion of the light emitting diode array LED is found out easily from the control range. Thus, the unevenness in optical output is suppressed lower.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a light emitting element array device and a light emitting element drive circuit used as a recording light source of an electrophotographic printing device, and in particular to a light emitting element drive circuit that controls the amount of drive current in multiple stages. The present invention relates to a light-emitting element array device and a light-emitting element drive circuit suitable for.

[Conventional technology]

FIG. 6 shows an example of driving a light emitting diode array, which is an example of a conventional light emitting element array. In FIG. 6, 1 is a driver IC serving as a light emitting element drive circuit, LED is a light emitting diode array, and R is a limiting resistor. As can be seen from this figure, the light emitting diode was driven with a constant current by inserting a limiting resistor R in series. The conventional driver ICI used for this operates as described below.

DI of the driver ICI is an input terminal of the shift register SR, CLK is a shift clock terminal, and both data are sequentially taken into the shift register from DI by the shift clock. L A T CFi is a latch circuit that temporarily holds the image data taken into the shift register, and the terminal L
It is activated by the input signal launch strobe from ST. DST is an input terminal for enabling the drivers Dr to DN, and a driver strobe signal from here opens the gate 1''G, and when the output data of LA DIN'CH is It I It, the drivers are made conductive. Q!~QN are output terminals of the driver, which are connected to the energizing circuit of the light emitting diode arrays L, ED through the limiting resistor R.

The current flowing through the light emitting diode is the power supply voltage E.

The forward voltage drop of the light emitting diode (F) is determined by the serial output terminal of the star.Since the light emitting diode array head usually consists of several dozen elements, it requires a large number of driver ICs.
It is provided to transfer image data to the next driver IC.

In FIG. 6, 10 is a photosensitive drum, and a latent image of printed information can be created on the photosensitive drum 10 by emitting light from a light emitting diode based on image data.

Related circuits of this type include, for example, Japanese Patent Application Laid-open No. 6
1-185981 and the like.

[Problem to be solved by the invention 3 A light emitting diode array is fabricated on a single wafer using semiconductor technology. Normally, the light output of a two-light emitting diode array varies greatly over the entire wafer, so parts with small variations are cut out and used as chips. Although a light emitting diode array head with multiple such single semiconductor chips arranged in a row has a light output with small variations,
Unevenness in the initial output on a device-to-element basis and on a chip-by-chip basis remain. Therefore, there is a problem in that the electrophotographic image produced by the head, which is driven by a constant current, exhibits density unevenness and deteriorates the image quality.

Additionally, the light output of a light emitting diode array may change over time. As a result, electrophotographic images change over time.
It becomes darker or lighter.

Therefore, it becomes necessary to change the drive current over time. With conventional driver IC chips, it is not possible to control the amount of output current, so we have to change the limiting resistor.6 The limiting resistor is fabricated on the head board on which the chip is mounted, and it is necessary to reuse it. Another problem with the prior art is that it is difficult to reset the settings by adding processing, etc., and it is not possible to correct changes over time.

SUMMARY OF THE INVENTION In view of the above-mentioned conventional situation, it is an object of the present invention to provide a drive circuit and a light emitting element array device that can eliminate image quality deterioration caused by variations in light output of a light emitting diode array and changes over time.

[Means for solving problems]

The above object is achieved by controlling the amount of output current of a drive circuit that drives a light emitting element in multiple stages using digital signals.

The control circuit basically consists of a current mirror circuit, and the reference current for controlling the output current amount is configured to be given by a digital current control (No. 8).The current control signal consists of n bits, and each bit The reference currents corresponding to the output currents have different values, and the output current amount is defined by the sum of these reference currents.

[Effect]

The n-bit current control signal provides 2 degrees of reference current, and the output current is controlled in the same number of 2n stages. Therefore, a drive current that corrects the variation in light output of the light emitting diode array can be easily found within the control range, and by providing a current control signal corresponding to the variation, the variation in light output can be suppressed to a small level.

The current control signal is applied by connecting a jumper to the input terminal II I 11 or 110 IT or by writing to the built-in register circuit, so once set, it is necessary to compensate for changes over time in the output of the light emitting diode array. It is also easy to reset.

〔Example〕

An embodiment of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows a first embodiment of an electrophotographic apparatus using a driver IC according to an embodiment of the present invention. Driver IC 100 is preferably integrated and formed on a single semiconductor substrate.

Light emitting diode LED, limiting resistor R1 and photosensitive drum 1
Regarding 0, it is substantially the same as the conventional example. FIG. 2 is an operation timing chart of the driver IC 100. The image data is transferred from the outside of the driver IC 100 to terminal D.
The signal is taken in via I in synchronization with a shift clock into a shift register SR constituted by a MOS transistor serving as a first register means. CLK is the shift clock input terminal. Do is a plurality of driver ICs 1001,
100-2, --100-n (not shown) is a serial output terminal of a shift register for use in cascade connection, and when driving a large number of LEDs, adjacent driver ICs 100-1, -2 (not shown) are used. ,...Send image data to n. When the image data is stored in a predetermined position of the shift register SR, a latch strobe signal is input from the terminal LST, and the image data is transferred to the latch circuit LATCH, which is composed of a MOS transistor and serves as a second register means.
to hold. This holding continues until the next image data is taken into the shift register and the latch strobe is input again to latch it. During this time, the image data is in MOS
The image data is output to a gate G formed of a transistor, and when the gate G is opened by a driver strobe signal input from a terminal DST, image data is sent to the driver as a switching signal.

The driver consists of a switching transistor TRS that performs a switching operation upon receiving the image data and a constant current transistor TRC connected in series, and has output terminals Q1 to QN. Constant current control uses a bipolar transistor TRo as a reference element and a bipolar transistor TR as an output element.
This utilizes a current mirror circuit formed with C. T.R.

There is a constant ratio relationship between the currents in TRo and TRC, and the output current is limited by the reference current flowing through TRo. This T
Constant current control by Ro is in multi-output format, with a chip of 100
The circuit was configured to cover all outputs within the circuit. The reference current is an MO8 resistor Rr whose one end is connected to the transistor TRo and the other end is connected to the reference voltage EO. HRly R2+...
・Currents Io+It, Iz, . . . are defined by a reference current source consisting of a parallel circuit of Rn and flow through each resistor.
...I, is given as the sum of.

The gate terminal of MO8 resistor Ro is set to “1” level,
It is always on, and its current is Io as described above. The gate terminals of MO5 resistors Rt, Rz,...Rn are terminals IC1, Icx, =...=I
connected to cn, and the logic level ゛l 111,
Switching is controlled by a current control signal provided by ItO71. As a result, each resistor takes two values: an on-state resistance value and an off-state resistance value, that is, infinity. Therefore, when the current control signal has n bits, there are 2n types of regular current given as the current sum. As an example when n=3, as shown in Table 1,
23=8 different reference currents are created.

Table 1: Relationship table between reference current and current control signal Here, MO8 resistance value is I2 = 2 ・It, Ia = 22
・If you set it to be an exponential type of 2 as shown in step 1, it will become as shown in parentheses in the table, and the variable part will be in 8 steps with AE as a step, and the input current control signal will be easily controlled by the user. It can be a binary code that is easy to handle. By making the reference current multivalued as described above, the output current is also controlled to eight levels of current amount. Therefore,
By looking at the light output of the light emitting diode array and applying a current control signal to obtain an output current that compensates for variations in the light output, it is possible to drive the light emitting diode array without any unevenness in the output.

The current control signal connects input terminals Icl to ICn to V
Input is possible simply by connecting to the DD or ground terminal using jumper means such as wire bonding.
Resetting, which is required when the output of the light emitting diode array destination changes over time, can be easily performed.

In this embodiment, since the output current amount can be easily controlled in multiple stages as described above, it is possible to drive the light emitting element array without any uneven brightness at all times. Further, as an electrophotographic apparatus, there is an effect that an electrophotographic image with constant density and no density unevenness can be obtained at all times.

FIG. 3 shows a driver ICl0 according to a second embodiment of the present invention.
It shows I. This embodiment is completely the same as the driver IC 100 of the first embodiment, except for the input form of the current control signal. Note that in FIG. 3, the light emitting diode LED, the photosensitive drum 10, etc. are not shown. The current control signal is configured with a built-in register so that it can be input online using software. In FIG. 3, 5RIC is a shift register serving as a storage means for storing n-bit current control data, and ICI is the same data input terminal, CLK
IC is the same shift clock input terminal, and ICO is the same data output terminal. ICO is for continuous connection when a large number of driver ICl0I are used. Shift register 5RIC
Parallel output of Pot, PO2, ...POo
are respectively MO3 resistance Rt + R2t...
It is connected to the gate terminal of Ro, and the shift register 5
A reference current is created based on the current control data input to the RIC, and thereby the amount of output current can be controlled in multiple stages as in the first embodiment.

The current control data is created based on the light output characteristics of the light emitting diode array to be connected, and is stored in advance in a storage section outside the ICl0I, such as a read-only memory, or inside the ICl0I (not shown). The current control data stored in the memory can be inputted by writing the current control data stored in the memory into the shift register 5RIC at power-on or prior to a series of image data scans, or in synchronization with the image data scans as the case may be.

As described above, according to the present embodiment, in addition to the same effects as in the first embodiment, the output current amount setting is not performed by hardware means such as jumper connection, but is performed by a writing operation to a register, so that it can be performed online. Therefore, there is no need to stop the generating diode array head using this driver or the main body system incorporating it (for example, an electrophotographic device).

FIG. 4 shows a driver IC 10 according to a third embodiment of the present invention.
2.

Shift register SR for both data scanning in this embodiment,
The configuration and operation of the latch circuit LATCH, gate G, and their input/output terminals DI, Do, CLK, LSI, and DST are completely the same as in the first embodiment described using FIGS. 1 and 2. Note that in FIG. 4 as well, the light emitting diode LED, resistor R9 photosensitive drum 10, etc. are omitted from illustration. Current control data register 5RIC, M
O5 resistor Ro, R1,...Rn, transistor TR.

A current amount control element consisting of an output Ql t O2,...
- The difference from the previous embodiments is that each QN is provided with all the components. The registers are connected in cascade, with a data input terminal ICI and a shift clock input terminal CLKI as a whole.
It has a shift register configuration with three terminals: C and data output terminal ICO.

A current control signal converted into digital data based on the light output characteristics of each element of the light emitting diode array is inputted from the terminal ICI to the shift register 5RIC in synchronization with the shift clock before image data scanning. Input current control data is applied to the MO5 resistor through serial-to-parallel conversion by a shift register, and a reference current for controlling the amount of current is prepared. Therefore, each output Qt j QZ
1... Since the driving VIJ current of the light emitting diode array connected to QN is controlled so that the light output of each element is constant, the resulting electrophotographic image has no density unevenness even when viewed microscopically. The images are of high quality. Furthermore, since the current amount setting operation can be performed online at any time, stable images are always obtained.

FIG. 5 shows a driver IC 1 according to a fourth embodiment of the present invention.
03 shows another configuration in which the amount of current is controlled for each output. In the same figure, the portion constituted by the shift register SR, latch circuit LATCH, and gates 1 to G is related to the scanning of image data and is the same as described in the first to second embodiments. 5, the light emitting diode LED, resistor R2 photosensitive drum 10, etc. shown in FIG. 1 are not shown.

5RIC is a shift register for current control data, O8 is a gate that controls the parallel output of the shift register, TRo and 'I' RC are transistors that constitute an output driver by a current mirror circuit, Ro, R
1, . . . Rrl is an MO3 resistor that provides a reference current for the current mirror circuit.

The current control data is input from the terminal ICI to the shift register 5RIC using a shift clock before scanning of image data starts. Its parallel output is connected to MO8O8 resistors -1 to Rn through gate GS. The opening/closing of the gate GS is determined by the output of the gate G which outputs the image data. The output of gate G is also
It is connected to the gate terminal of the MO3 resistor Ro and switches it directly. Therefore, when the image data output from the gate G is 11071, the MO8 resistance Ro, Rt~R
0 is all off state and infinite, that is, the reference current does not flow,
Transistor TRC is therefore non-conductive. Image data I
At t171, the MO3 resistor becomes conductive, and at the same time, the gate GS is opened, and the MO3O3 resistor -1 to R1 moves according to the data stored in the shift register 5RIC.
A reference current similar to that in the first embodiment flows through TRo. This causes the output transistor TRC to conduct and its current is controlled to a quantity proportional to the reference current. In this way, switching of the reference current is performed based on image data, and conduction/non-conduction of the output current is controlled.

Here, the current control data is created based on the light output characteristics of each element of the light emitting diode array connected to each output.

Therefore, the effect of the current amount control according to this embodiment is that, as in the previous embodiment, a high-quality image that is always stable and free from uneven brightness and density can be obtained.

Furthermore, since the number of transistors for each output is reduced by one, the size of the driver IC can be reduced.

FIG. 7 shows a fifth embodiment of the present invention in which a D/A converter is used as the selection means.

In the figure, the DAC is a digital-to-analog converter, and the other parts are the same as in the previous embodiment.
Not shown. The DAC acts as a reference voltage source that provides a reference current (2) for the current mirror circuit. Furthermore, the DAC
is preferably formed on a single semiconductor substrate together with TRo, TRc, etc. Based on the light output-current characteristics of the light emitting diode array to be connected, digital data of the current that is produced so as to have a certain constant value of light output is given to the input of the DAC as a current control signal. This sets the output Out of the DAC, and the base current ■
flows. Transistors T Rc and T Rs operate as described in the text.

Ideally, a DAC should be added to each output Q of the driver IC, but it may also be added to each chip, as long as it is within the allowable size range for the driver IC.

Is the digital-to-analog converter DAC a highly accurate reference voltage? It is possible to select the Pi current in detail. Furthermore, since the absolute value can be adjusted using an external adjustment resistor Rt, the setting range of the reference current can be expanded. As described above, this embodiment employing a DAC eliminates variations in brightness by accurately setting the current flowing through the light emitting diode array so that the light output is constant. This results in an even electrophotographic image.

〔Effect of the invention〕

According to the present invention, it is possible to control the output current using a digital signal, so it is possible to drive the light emitting elements to eliminate variations in light output. Furthermore, since it is possible to easily correct temporal changes in the light output of the light emitting element, high quality electrophotographic images with constant density and no density unevenness can be obtained.

[Brief explanation of drawings]

1, 3, 4, 5, and 7 are driver IC configuration diagrams of an embodiment of the present invention, FIG. 2 is a timing chart explaining the present invention in detail, and FIG. 6 is a 1 is a diagram showing an example of a conventional light emitting diode array drive circuit. TRo, TRC, TR5-transistor, Ro, Rt~
R,...MO5 resistance, ICt~n, ICI...
Current controller 1 - roll signal input terminal, 5RIC...shift register for current control data, LED...
Light-emitting die Ka 1 Figure λ Figure 3 Figure Horse 4-Figure =-1--

Claims (1)

[Claims] 1. A current mirror circuit having one reference element and at least one output element, a reference current source capable of outputting a plurality of reference current values, and a plurality of references outputted from the reference current source. The output control means includes a selection means for specifying and selecting one of the current values, and an output control means for controlling on or off of the output current flowing to the output element of the current mirror circuit according to an external light emission control signal. 1. A light-emitting element drive circuit, characterized in that an output current controlled by a light-emitting element can be caused to flow through the light-emitting element. 2. A current mirror circuit having one reference element and multiple output elements, a reference current source capable of outputting multiple reference current values, and specifying one of the multiple reference current values output by the reference current source. and output control means for separately controlling on or off of the output currents flowing through the plurality of output elements of the current mirror circuit according to an external light emission control signal, 1. A light-emitting element drive circuit, characterized in that each output current controlled by the light-emitting element can be caused to flow separately to a plurality of light-emitting elements. 3. In claim 1 or 2, the reference current source includes a parallel circuit of a resistance element and at least one semiconductor switching element whose ON or OFF state is controlled by the selection means. A light emitting element drive circuit characterized by: 4. The light emitting device drive circuit according to claim 3, wherein the semiconductor switching device is a MOS transistor. 5. The light emitting element drive circuit according to claim 1 or 2, wherein the selection means is a digital signal input from the outside. 6. The light emitting element drive circuit according to claim 5, wherein the selection means includes storage means for holding the digital signal inputted from the outside. 7. The light emitting element drive circuit according to claim 6, wherein the storage means is a shift register. 8. The light emitting element drive circuit according to claim 1 or 2, wherein the reference element and the output element are comprised of bipolar transistors. 9. In claim 1 or 2, the output control means includes a first register means that sequentially converts a serial light emission control signal from the outside into a parallel light emission control signal; second register means for temporarily holding the output of the register means; connected in series with the output element of the current mirror circuit;
A light emitting element drive circuit comprising: a switching transistor that is controlled to be turned on or off based on the above output. 10. The light emitting device driving circuit according to claim 9, wherein the first register means and the second register means are composed of MOS transistors, and the switching transistor is composed of a bipolar transistor. . 11. A current mirror circuit having one reference element and a plurality of output elements, a reference current source capable of outputting a plurality of reference current values, and one of the plurality of reference current values output by the reference current source. A selection means for making a selection, and an output control means for individually controlling on or off of output currents flowing through a plurality of output elements of the current mirror circuit according to an external light emission control signal are formed on a single semiconductor substrate. at least one semiconductor integrated circuit, and at least one semiconductor chip on which a plurality of light emitting elements and resistors are formed that emit light by inputting respective output currents controlled by the output control means. A light emitting element array device characterized by: 12. The light emitting device according to claim 11, wherein the reference current source includes a parallel circuit of a resistive element and at least one semiconductor switching element whose ON or OFF is controlled by the selection means. Array device. 13. The light emitting element array device according to claim 12, wherein the semiconductor switch element is a MOS transistor. 14. The light emitting element array device according to claim 11, wherein the selection means is a digital signal input from the outside. 15. The light emitting element array device according to claim 14, wherein the selection means includes storage means for holding the digital signal inputted from the outside. 16. The light emitting element array device according to claim 15, wherein the storage means is a shift register. 17. The light emitting element array device according to claim 11, wherein the reference element and the output element are comprised of bipolar transistors. 18. In claim 11, the output control means includes a first register means that sequentially converts a serial light emission control signal from the outside into a parallel light emission control signal, and an output of the first register means. second register means for temporarily holding the current mirror circuit;
A light emitting element array device comprising: a switching transistor that is controlled to be turned on or off based on the above output. 19. The light emitting element array device according to claim 18, wherein the first register means and the second register means are composed of MOS transistors, and the switching transistor is composed of a bipolar transistor. . 20. A current mirror circuit having one reference element and a plurality of output elements, a reference current source that can output a plurality of reference current values, and one of the plurality of reference current values output by the reference current source. A selection means for making a selection, and an output control means for individually controlling on or off of output currents flowing through a plurality of output elements of the current mirror circuit according to an external light emission control signal are formed on a single semiconductor substrate. at least one semiconductor integrated circuit formed with a plurality of light emitting elements that emit light by inputting respective output currents controlled by the output control means; An electrophotographic printing device comprising: a photosensitive drum that is exposed to light; 21. Claim 20, characterized in that the reference current source includes a parallel circuit of a resistance element and at least one semiconductor switching element whose ON or OFF is controlled by the selection means. Electrophotographic printing equipment. 22. The electrophotographic printing apparatus according to claim 21, wherein the semiconductor switch element is a MOS transistor. 23. The electrophotographic printing apparatus according to claim 20, wherein the selection means includes a digital signal input from the outside. 24. The electrophotographic printing apparatus according to claim 23, wherein the selection means includes a storage means for holding a digital signal input from the outside. 25. The electrophotographic printing apparatus according to claim 24, wherein the storage means is a shift register. 26. The electrophotographic printing apparatus according to claim 20, wherein the reference element and the output element are comprised of bipolar transistors. 27. In claim 20, the output control means includes a first register means that sequentially converts a serial light emission control signal from the outside into a parallel light emission control signal, and an output of the first register means. second register means for temporarily holding the current mirror circuit;
An electrophotographic printing device comprising: a switching transistor that is controlled to be turned on or off based on the above output. 28. Electrophotographic printing according to claim 27, wherein the first register means and the second register means are composed of MOS transistors, and the switching transistor is composed of a bipolar transistor. Device. 29. Claim 12, wherein the selection means increases the reference current corresponding to each bit of the n-bit current control signal by zero when it is not selected, and by at least 2^0·I, 2^1 when it is selected.・I, 2^2・I,
・・・・・・2^n^-^1・I is arranged in an exponential form of 2, and the selected sum also defines the variable control amount of the output current. Light emitting element drive circuit. 30. The light emitting device according to claim 1, wherein the selection means controls the amount of current of all the output control means in the chip by a current control signal consisting of a set of n bits. Element drive circuit. 31. Claim 30 is characterized in that an N-bit register is built-in for a current control signal consisting of a set of n bits, and current control data is input to the register. Light emitting element drive circuit. 32. In claim 1, the selection means has a current mirror circuit controlled by a current control signal consisting of n bits for each output of the output control means, and a register for storing the current control signal. A light emitting element drive circuit characterized in that the amount of current is controlled for each output.