JP2009143242A - Drive method of self-scan light-emitting element array, optical writing head and optical printer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a driving method which can reduce bias light in a non-separating type self-scan light-emitting element array. <P>SOLUTION: Two drivers 60 drive clock pulses 1, ϕ2, respectively, and the output terminals 63 of the respective drivers are respectively connected to a ϕ1 terminal 11 and a ϕ2 terminal 12. Each GND terminal 64 is connected to a GND. Further, in the drawing, 71 indicates a ϕ1 non-writing time signal input terminal, 72 indicates a ϕ1 writing time signal input terminal, 73 indicates a ϕ2 non-writing time signal input terminal, 74 indicates a ϕ2 writing time signal input terminal, 75 indicates a start signal terminal, and 76 indicates a power supply for giving a voltage V<SB>GA</SB>. To reduce the bias voltage, pulses are intermittently give ϕn as a ϕ1 non-writing time signal and a ϕ2 non-writing time signal. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for driving a self-scanning light-emitting element array, and more particularly to a method for driving a non-separable self-scanning light-emitting element array. The present invention further relates to an optical writing head comprising a driving circuit for executing such a driving method.

  A light emitting element array in which a large number of light emitting elements are integrated on the same substrate is used as an optical writing head such as an optical printer head in combination with a driving IC. The present inventors have already focused on a three-terminal light-emitting thyristor having a PNPN structure as a component of the light-emitting element array, and have already proposed that self-scanning of the light-emitting point can be realized (see Patent Documents 1, 2, 3, and 4). It has been shown that the optical printer head can be easily mounted, the light emitting element pitch can be made fine, and a compact self-scanning light emitting element array can be produced.

  Furthermore, the present inventors have proposed a self-scanning light-emitting element array having a structure separated from a light-emitting element (light-emitting thyristor) array, which is a light-emitting part, using a switch element (light-emitting thyristor) array as a shift part (Patent Document 5). reference).

  Hereinafter, the former self-scanning light emitting element array in which both the shift function and the light emitting function are assigned to the same element is referred to as a non-separable type, and the latter shift function (transfer function) and the light emitting function are assigned to separate thyristors. The self-scanning light emitting element array is referred to as a separation type.

JP-A-1-238996 Japanese Patent Laid-Open No. 2-14584 Japanese Patent Laid-Open No. 2-92650 JP-A-2-92651 JP-A-2-263668

In the non-separable self-scanning light emitting element array, since the writing position information is stored, the light is emitted slightly even when the image data is 0 (non-writing) (bias light). For this reason, when the non-separable self-scanning light emitting element array is used for the optical writing head, the image quality is deteriorated. Therefore, the separation type is such that a thyristor shielded so as not to emit light has a shift function so that no light is emitted when the image data is zero. The separation type can reduce the bias light, but has the following problems.
1. The circuit becomes complicated and the chip size increases. That is, it becomes expensive.
2. Separate power is required for driving the shift unit. The power of the shift unit is determined by the size of the current limiting resistor of the clock line, but in order to transfer at high speed, it is necessary to reduce the time constant of the clock line by reducing the current limiting resistor. For this reason, depending on the transfer speed, it is necessary to pass a current that is a fraction of the current that contributes to optical writing, raising the power consumption of the chip and increasing the temperature.

  An object of the present invention is to provide a driving method capable of reducing bias light in a non-separable self-scanning light-emitting element array rather than a separate self-scanning light-emitting element array having the above-described problems. is there.

  Another object of the present invention is to provide an optical writing head having a driving circuit that realizes such a driving method.

The invention according to claim 1 is a self-scanning light-emitting element array in which light-emitting elements composed of light-emitting thyristors are linearly arranged and have a transfer function that uses light emission of the light-emitting elements for optical writing. In the transfer operation in which each of the elements is sequentially writable, when the light writing is not performed during the period in which the light emitting element is capable of optical writing, the current is intermittently turned on so that the turned on light emitting element is turned on again. The self-scanning light-emitting element array driving method is characterized by:

According to a second aspect of the invention, in the driving of the light emitting element, the light emitting element is driven by a constant current circuit capable of taking two or more output current values. This is a driving method of the element array.
According to the third aspect of the present invention, the absolute value of the voltage at which the transfer operation can be performed when the overlap time necessary for the transfer is sufficiently long is set to V ₀ , C is the capacity of the clock line, and I is the current value when optical writing is not performed. _n The overlap time t required for transfer _a The
      CV ₀ / I _n <T _a <5 x CV ₀ / I _n
The method for driving a self-scanning light emitting element array according to claim 2, wherein:
  According to a fourth aspect of the present invention, in the driving of the light emitting element, the light emitting element is driven by a resistance selection circuit capable of taking a resistance value of two or more and a constant voltage circuit. This is a method for driving a scanning light emitting element array.
According to the fifth aspect of the present invention, the absolute value of the voltage at which the transfer operation can be performed when the overlap time required for the transfer is sufficiently long is set to V ₀ , C is the capacity of the clock line, and R is the output resistance value when optical writing is not performed. _n , The clock line voltage to V _S The overlap time t required for transfer _a The
  -R _n Cln (1-V ₀ / V _S ) <T _a <-5xR _n Cln (1-V ₀ / V _S )
  The self-scanning light-emitting element array driving method according to claim 4, wherein:
  6. The self-scanning light-emitting element according to claim 2, wherein the current when optical writing is not performed is applied so as to overlap the current when optical writing is performed. An array driving method.

According to a seventh aspect of the present invention, there is provided a self-scanning light-emitting element array having a transfer function in which light-emitting elements including light-emitting thyristors are linearly arranged and using light emission of the light-emitting elements for optical writing, and the light-emitting elements In the transfer operation to sequentially enable optical writing, when optical writing is not performed in a period in which each of the light emitting elements can be optically written, current is intermittently applied so that the turned on light emitting elements are turned on again. An optical writing head comprising a constant current circuit.
The invention according to claim 8 is the optical writing head according to claim 7, wherein the constant current circuit is constituted by a current mirror.
The invention according to claim 9 is the optical write head according to claim 7, wherein the constant current circuit is constituted by a resistance selection circuit and a constant voltage circuit.

According to a tenth aspect of the present invention, there is provided an optical printer comprising the optical writing head according to the seventh, eighth or ninth aspect.

  According to the present invention, bias light can be reduced while taking advantage of the fact that the chip size, which is an advantage of the non-separable self-scanning light emitting element array, can be reduced.

It is a figure which shows the equivalent circuit of a non-separation type self-scanning light emitting element array chip. It is a figure which shows the electric current-light output characteristic of the light emitting element of FIG. It is a perspective view which shows the principal part of a typical optical writing head. It is a figure which shows the structure of an optical printer provided with an optical writing head. It is a figure which shows the result of having investigated the relationship between the light emission quantity at the time of non-writing, and image density. It is a figure which shows a constant current drive circuit. It is a figure which shows the connection diagram at the time of using a constant current drive circuit as a driver of a self-scanning light emitting element array. It is a figure which shows the drive waveform for demonstrating operation | movement of the circuit of FIG. It is a figure which shows an example of a resistance selection circuit. It is a figure which shows the other example of a resistance selection circuit.

FIG. 1 shows an equivalent circuit diagram of a non-separable self-scanning light emitting element array chip 10 to which the driving method of the present invention is applied. This self-scanning light-emitting element array chip includes light-emitting elements L ₁ , L ₂ , L ₃ ,... That are linearly arranged three-terminal light-emitting thyristors, and diodes D ₁ ,. D ₂ ,... Are connected. V _GA is a power source, and is connected to the gate electrode of each light emitting element through a V _GA line extending from the V _GA terminal 15 and a load resistance _RL . The gate electrode of the light-emitting element L ₁ is connected to a start pulse phi _S terminal 13. The cathode electrodes of the light emitting elements are alternately connected to clock lines extending from the transfer clock pulse φ1, φ2 terminals 11, 12. The anode of each light emitting element is connected to the substrate back surface common electrode. In the figure, reference numeral 14 denotes a back surface common electrode terminal. The resistor _RS is a current limiting resistor inserted in the start pulse line.

FIG. 2 shows current-light output characteristics of the light emitting thyristor of FIG. The current-light output characteristics of the light-emitting thyristor have a small slope when the current is small, and the slope increases as the current increases, and the slope has a constant value. When current x, the light output and y, when the current is small, the characteristic is represented by y = ^{0.5x 2.4,} when the current is large, it is represented by y = 11x-44. This is because when the current is small, the current spread in the active layer of the light-emitting thyristor is small and the ratio of light emission just below the electrode is large, so that the light extraction efficiency decreases. By utilizing this characteristic, the bias light can be reduced.

  Actually, the non-writing current was 0.5 mA, and the writing current was 13 mA. Under this condition, the light output at the time of non-writing is about 0.1 μW, and the light output at the time of writing is 100 μW. The light quantity ratio between writing and non-writing was 1000 times.

  An optical write head is manufactured using this chip. FIG. 3 is a perspective view showing a main part of a typical optical writing head. The optical writing head 30 includes a self-scanning light-emitting element array 24 configured by arranging a plurality of self-scanning light-emitting element array chips 22 in a staggered arrangement on the mounting substrate 20 and a plurality of erecting equal-magnification lenses ( Rod lens) 26 and an erecting equal-magnification lens array 28. The light emitted from the light emitting element array 24 is collected by the lens array 28 and irradiated onto a photosensitive drum (not shown).

  FIG. 4 shows a configuration of an optical printer including such an optical writing head 30. A photoconductive material (photosensitive member) such as amorphous Si is made on the surface of the cylindrical photosensitive drum 32. This drum rotates at the speed of printing. The surface of the photosensitive drum of the rotating drum is uniformly charged by the charger 34. Then, the optical writing head 30 irradiates the photosensitive member with the light of the dot image to be printed, and neutralizes the charging where the light hits. Subsequently, toner is applied on the photosensitive member by the developing unit 38 in accordance with the charged state on the photosensitive member. Then, the toner is transferred onto the paper 44 fed from the cassette 42 by the transfer device 40. The sheet is heated and fixed by the fixing unit 46 and sent to the stacker 48. On the other hand, the drum after the transfer is neutralized by the erasing lamp 50 over the entire surface, and the remaining toner is removed by the cleaner 52.

  In the optical printer as described above, an image was formed under an optimum exposure condition (process speed) with a light output of 100 μW at the time of writing. As a result, it was confirmed that the bias light under the above conditions did not adversely affect the image. When the light output at the time of non-writing was increased, a decrease in the image quality of the image started to be observed around 0.5 μW, and the deterioration was clearly observed at 1 μW. In order to see this quantitatively, solid images of 3on-1off and 2on-2off patterns were printed out, and the relationship between the light emission amount during non-writing and the image density was examined. The results are shown in FIG.

  When a gray image is produced in a black and white two-tone image, the shade is expressed by the area ratio of white and black. Here, 3on-1off means a repeating pattern of 3 dots black and 1 dot white, and 2on-2off means a repeating pattern of 2 dots black and 2 dots white. In the figure, ● represents the concentration of 3on-1off, and ○ represents the concentration of 2on-2off. The image density was normalized by the density at 0.1 μW, which is the minimum light emission amount during non-writing. For the density, the printed paper was read again with a scanner, and the average value over the entire area was taken.

  The image density tends to increase as the amount of light emitted at the time of non-writing increases. However, when it exceeds 1 μW, the state of density change also varies depending on the patterns such as 3on-1off and 2on-2off, which supports image quality degradation. . This is considered to be equivalent to an increase in the dark current of the drum due to the bias light and a decrease in the potential difference between writing and non-writing. As a result of this experiment, it was found that the ratio of light output between non-writing and writing (light quantity ratio) must be 1/100 or less. More desirably, it was found to be 1/200 or less.

  On the other hand, in the self-scanning light emitting element array, in order to memorize the ON state, it is necessary to pass a current that is at least equal to the holding current of the thyristor. This holding current value is mainly determined by the element structure, but usually takes a value of about 0.1 to 1 mA. For this reason, when the current at the time of writing is 13 mA, the current ratio can only be 26 times at most when the holding current is 0.5 mA. Therefore, to obtain a light quantity ratio of 100 times, the light emission efficiency at the time of non-writing needs to be ¼ or less of the light emission efficiency at the time of writing. Actually, as described above, when the non-writing current is 0.5 mA, the light output is 0.1 μW, so the luminous efficiency is 0.1 / 0.5 = 0.2 (μW / mA), writing Since the light output is 100 μW when the current at the time is 13 mA, the light emission efficiency is 100/13 = 7.7 (μW / mA), and the light emission efficiency at the time of non-writing is about 1/38 of the light emission efficiency at the time of writing. And satisfies the above conditions.

  In order to realize driving at the current ratio (26 times) of the first embodiment, the constant current driving circuit shown in FIG. 6 is used as a driver. The constant current drive circuit 60 includes a current mirror 67 that determines a non-write current, a current mirror 68 that determines a write current, and resistors 65 and 66. Current mirrors 67 and 68 are each composed of a plurality of bipolar transistors. That is, the current mirror 67 is composed of four transistors, and the current mirror 68 is composed of six transistors.

  In the figure, 61 is a non-write signal input terminal, 62 is a write signal input terminal, 63 is a driver output terminal, and 64 is a driver GND terminal.

  When an H level (for example, 3.3 V) is applied to the non-write input terminal 61, this voltage is converted into a current by the resistor 65, and ¼ of this current is sucked from the output terminal 63 as an output current. On the other hand, when the H level is applied to the write input terminal 62, it is converted into a current by the resistor 66, and a current six times this current is sucked from the output terminal 63 as an output current. The current value and current ratio drawn from the output terminal 63 can be determined by the resistors 65 and 66 and the number of transistors constituting each current mirror. The current value drawn from the output terminal 63 does not include 0.

  In this example, two current mirrors are combined, but other methods may be used.

  FIG. 7 shows a connection diagram when such a constant current driving circuit 60 is used as a driver of a self-scanning light emitting element array. Reference numeral 10 denotes the self-scanning light emitting element array chip shown in FIG.

The two drivers 60 drive clock pulses φ1 and φ2, and the output terminals 63 of the drivers are connected to the φ1 terminal 11 and the φ2 terminal 12, respectively. Each GND terminal 64 is connected to GND. In the figure, 71 is a signal input terminal when φ1 is not written, 72 is a signal input terminal when φ1 is written, 73 is a signal input terminal when φ2 is not written, 74 is a signal input terminal when φ2 is written, 75 is a start signal terminal, 76 is This is a power supply for applying the voltage _VGA .

  The operation of the circuit of FIG. 7 will be described with reference to the drive waveform shown in FIG. V (71), V (72), V (73), V (74), and V (75) in FIG. 8 indicate voltages at terminals 71, 72, 73, 74, and 75, respectively.

8, the overlap time t _a of the non-write pulse voltage V (71) and V (73), when taken overlapped sufficiently large time required for the transfer (e.g., 1 second), can transfer operation The absolute value (ignition voltage) of the absolute value of the L level voltage of the clock voltage waveform applied to the clock pulse terminals 11 and 12 is V ₀ , the parasitic capacitance of the clock line is C, and the non-write current value is I. _n ,
t _{a> CV} 0 / _{I n}
Choose to be. This is because, when charging the parasitic capacitance C with a constant current I _n, and after the time t _a, it is shown that to say that leads to ignition voltage V _0. Now, _assuming C = 15 pF, V ₀ = −2.8 V, and I _n = 0.5 mA, CV ₀ / I _n = about 84 ns.

Meanwhile, to say that increasing the time t _a becomes a increasing the lighting time of the non-written state, an increase of the bias light. In view of this, the time _{t a} is _CV 0 / _{I n} 5 times or less is desirable.

In Example 3, pulses were applied intermittently as V (71) and V (73) in order to reduce the bias light. V (71), the length _{t H} of the pulse V (73) becomes the H similar to the _{t a,}
t _{H> CV} 0 / _{I n}
The length T _{L of the} pulse that becomes L and is the sum T with the time t _H required to turn it on again is necessary for the gate voltage of the thyristor to drop to V _s −V _D. Selected shorter than time. That is,

Here, R _g is the impedance expected from the gate of the thyristor, and C _g is the gate parasitic capacitance of the thyristor. Further, V _S is an L level voltage of the clock voltage waveform applied to the clock pulse terminals 11 and 12, V _D is a voltage drop per one stage of the diode connecting the gate of the thyristor, and the gate voltage is V _S −V. _Saying falling to _D is the same as the state in which the adjacent thyristor is on. Now, R _g = 30 kΩ, C _g = 10 pF, V _D = 1.4 V, and V _S = -3.3 V, which is about 166 ns. By selecting t _H = 100 ns and t _L = 40 ns, the substantial non-writing lighting time could be reduced by at least 40/140.

  Although the circuit of the second embodiment uses bipolar transistors and can be driven at a constant current, the constant current circuit can be constituted by a resistance selection circuit and a constant voltage circuit. The resistance selection circuit can be configured to switch the resistance using MOS or C-MOS. 9 and 10 show circuit examples.

  The resistance selection circuit shown in FIG. 9A is an open drain type inverter, and includes MOS-FETs 81 and 82 and resistors 83 and 84. The MOS-FETs 81 and 82 are circuits for selecting the two resistors 83 and 84.

  As in FIG. 6, 61 is a non-write signal input terminal, 62 is a write signal input terminal, 63 is a driver output terminal, and 64 is a driver GND terminal. The driver output terminal 63 is connected to GND via a resistor 83 or 84 in accordance with signals from the input terminals 61 and 62.

  In the resistance selection circuit shown in FIG. 9B, the drains of the MOS-FETs 81 and 82 in the circuit of FIG. By this pull-up, the off speed of the φ1 and φ2 lines can be increased.

  The resistance selection circuit shown in FIG. 10A uses the MOS-FET 86 instead of the resistance 85 in the circuit of FIG. 9B, and is connected to the power supply terminal 69 when both the input terminals 61 and 62 are L signals. . Reference numeral 87 denotes an off signal input terminal.

  The resistance selection circuit shown in FIG. 10B combines the off signal input in FIG. 10A by ORing the signals of the input terminals 61 and 62 with the OR circuit 88.

  A constant current drive circuit (driver) can be configured by combining the resistance selection circuit having the above configuration with a constant voltage circuit (not shown).

The driver including the resistance selection circuit of FIG. 9A can be driven in the same manner with the waveform of FIG. 8 with the same connection as the driver 60 shown in FIG. When using this driver, the choice of the time t _a overlaps the constant current driver of Figure 6 differs. That is, in FIG. 8, the overlap time t _a non write current, when taken sufficiently large overlap time required for the transfer (e.g., 1 second), the absolute value of the voltage that can transfer operations V ₀ , the capacity of the clock line C, and when the output resistance value when not performing optical writing and R _{n (resistor} 84), the voltage of the clock line and V _S, the overlap time t _a required for transfer,
_{-R n Cln (1-V 0} / V S) <t a
Choose to be. This is because, when charging the clock line as the integrating circuit of the time constant R _{n C,} and after the time t _a, it is shown that to say that leads to ignition voltage V _0. Now, _assuming that C = 15 pF, V ₀ = −2.8 V, V _S = −3.3 V, R _n = 5 kΩ, −R _n Cln (1−V ₀ / V _S ) = about 140 ns.

On the other hand, to say that increasing the overlap time t _a, becomes possible to increase the lighting time of the non-written state, an increase of the bias light. With this in mind, the overlap time _{t a} is _{-R n Cln (1-V 0} / V S) 5 times or less is desirable.

In driver shown in Examples 2 and 4, setting the ratio of the non-write current I _n and the write current I _w to about 20 to 50 times, by I _n on / off, it is possible to change the writing light output . Further, with respect to the timing of flowing the I _n is I _w, by turning on the appropriate time, it is possible to perform light amount correction for each light emitting point. As shown in Example 1, but hardly emission when passing by I _n alone, by superimposing the I _w which is higher in luminous efficiency, use efficiently current.

DESCRIPTION OF SYMBOLS 11, 12 ... Clock pulse (phi) 1, (phi) 2 terminal, 13 ... Start pulse (phi) _S terminal, 14 ... Back surface common electrode terminal, 15 ... _VGA terminal, 30 ... Optical writing head, 60 ... Constant current drive circuit, 61 ... At the time of non-writing Signal input terminal, 62 ... Signal input terminal at the time of writing, 63 ... Driver output terminal, 64 ... Driver GND terminal, 65, 66 ... Resistor, 67 ... Current mirror, 81, 82 ... MOS-FET

Claims (10)

A self-scanning light-emitting element array having a transfer function in which light-emitting elements composed of light-emitting thyristors are linearly arranged and the light emission of the light-emitting elements is used for optical writing ,
In the transfer operation in which each of the light emitting elements is sequentially optically writable, when optical writing is not performed during the period in which the light emitting element can be optically written, a current is supplied so that the turned on light emitting element is turned on again. A method of driving a self-scanning light-emitting element array, characterized by being intermittently applied. 2. The method of driving a self-scanning light emitting element array according to claim 1 , wherein the light emitting element is driven by a constant current circuit capable of taking an output current value of two or more values. When taken sufficiently large overlap time required for the transfer, the absolute value of the transfer operation which can voltage V _0, the capacitance of the clock line C, and the current value when not performing optical writing was I _n when to, the overlap time t _a required for the transfer,
_{CV 0 / I n <t a} <5 × CV 0 / I n
The method of driving a self-scanning light emitting element array according to claim 2 , wherein:   2. The method of driving a self-scanning light-emitting element array according to claim 1, wherein the light-emitting element is driven by a resistance selection circuit capable of taking a resistance value of 2 or more and a constant voltage circuit. When the overlap time required for transfer is sufficiently long, the absolute value of the voltage at which the transfer operation can be performed is V ₀ , the capacity of the clock line is C, the output resistance value when optical writing is not performed is R _n , the voltage of the clock line is taken as V _S, the overlap time t _a required for transfer,
_{-R n Cln (1-V 0} / V S) <t a <-5 × R n Cln (1-V 0 / V S)
The method of driving a self-scanning light emitting element array according to claim 4 . The current when not performing optical writing method for driving a self-scanning light-emitting element array according to claim 2 or 4, wherein the applying superimposed on current when performing optical writing. A self-scanning light-emitting element array having a transfer function in which light-emitting elements made of light-emitting thyristors are linearly arranged and light emission of the light-emitting elements is used for optical writing;
In the transfer operation in which the light emitting elements are sequentially in a state where optical writing can be performed, when optical writing is not performed during a period in which each of the light emitting elements can be optically written, a current is supplied so that the turned on light emitting elements are turned on again. a constant current circuit for supplying intermittently
Optical writing head, characterized in that it comprises a. The optical writing head according to claim 7 , wherein the constant current circuit includes a current mirror. The optical writing head according to claim 7 , wherein the constant current circuit includes a resistance selection circuit and a constant voltage circuit. An optical printer comprising the optical writing head according to claim 7 , 8 or 9 .

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