JPH05198844A - Light emitting device - Google Patents

Abstract

PURPOSE:To provide a light emitting device in which emission of an LED by a weak current flowing even in a state that an LED driving current is interrupted is prevented and a high gradation electrophotograph of high quality is performed. CONSTITUTION:When an output signal from a strobe circuit 30 is high, a transfer gate 1 is turned ON, a gate of an output transistor 2 is biased by a current control voltage 32, and a driving current is supplied to an LED 3. When the output signal from the circuit 30 is low, a transfer gate 31 is turned ON, and the transistor 2 is turned OFF. Further, a transfer gate 22 is turned ON, a reverse bias voltage 31 is applied to the LED 3 to become a reversely biased state (VBIAS>VLED), thereby becoming a completely non-light emitting state.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light emitting device, and more particularly to a light emitting device using an LED array applied to an electrophotographic optical printer.

[0002]

2. Description of the Related Art Conventionally, as an electrophotographic optical printer, a printer using a laser beam as a light source has been known.
An optical printer using an ED array as a light source has been proposed. In such an optical printer, it is necessary to use a light emitting device for appropriately modulating the light emission of the LED array to form an image on the photoconductor.

FIG. 5 is a schematic configuration diagram of such a conventional light emitting device. In the figure, 52 is a mother board, 54 is an input board, 55 is an input connector, 53 is a spreader board assembled on the mother board 52, 100 is an LED array drive integrated circuit assembled on the mother board 52, and 101 is the mother board 52. It is the LED array assembled above.

Here, the LED array 101 has hundreds and ten LEDs in a chip, and they are arranged in one line according to the width of the paper to be printed. And input board 54
The spreader board 53, the spreader board 53 and the LED array driving integrated circuit 100, and the LED array driving integrated circuit 100 and the LED array 101 are electrically connected by wire bonding 51. Although not shown, a converging rod lens array is arranged above the LED array 101 so that the light from the LED array 101 is focused on a photoconductor in the printer.

FIG. 6 shows an internal configuration diagram of an LED array driving integrated circuit 100 used in a conventional light emitting device.
Reference numeral 13 is a shift register circuit, 14 is a latch circuit, 15 is a strobe circuit, and 12 is a constant current drive circuit. Also,
Reference numeral 203 is a clock pad, 204 is a print data input pad, 205 is a print data output pad, 207 is a latch pad, 208 is a strobe input pad, 202
Is a current control pad, 200 is an output pad, and 209 is a power supply pad.

Here, the print data input pad 20
4, print data output pad 205, latch pad 2
07, strobe input pad 208, clock pad 2
03, current control pad 202, and power supply pad 209 are spreader board 5 by wire bonding 51.
3, the output pad 200 is connected to the LED pad 30 of the LED array 101 by the wire bonding 51.
0, and the back surface electrode 301 of the LED array 101 is connected to the LED power supply conductor surface on the mother board 52.

The operation of the above arrangement will be described below.

A power supply and a driving serial data signal, a clock signal, and a control signal which are externally given to this device are transmitted to the spreader board 53 through the input connector 55 and the input board 54, and the spreader board 53 drives each LED array. It is transmitted to the integrated circuit 100. LE
The D driving power is supplied from the back surface electrode of the LED array 101 via the spreader board 53. The LED array driving integrated circuit 100 parallelizes the driving data, that is, the light emission data, which are input in series, and the LED array 1 is driven by the control signal.
Each LED 3 of 01 is driven to emit light.

Next, the operation of the LED array driving integrated circuit 100 will be described in more detail with reference to FIG. The driving serial data signal is input from the spreader board 53 to the print data input pad 204 of the shift register circuit 13, and the spreader board 53 outputs the clock pad 203.
The shift register circuit 13 is transferred from the left to the right by the clock signal input to. The print data signal transferred to the right of the shift register circuit 13 is transferred from the print data output pad 205 to the spreader board 5
The data is input to the print data input pad 204 of the LED array driving integrated circuit 100 on the right next to the print data input pad 204. The print data signal is latched by the control signal input from the latch pad 207 in the latch circuit 14 when one line of the printer is completed. Each LED when latched
The ON / OFF data of is completed.

Then, the energized time, that is, the exposure time, of the latched signal is determined by the control signal input from the strobe pad 208 in the strobe circuit 15.
The constant current drive circuit 12 outputs a drive current controlled by the voltage supplied from the current control pad 202 in response to the exposure signal of each channel output from the strobe circuit 15, and drives and emits each LED3 of the LED array 101. ..

The operation of the constant current drive circuit 12 in the LED array drive integrated circuit 100 will be described with reference to FIG.
The output signal 30 of the strobe circuit 15 turns the transfer gate (1) 1 to O when the signal is in the high (ON) state.
The transfer gate (2) 21 is turned off (interrupted) by the inverter 4 by setting it to N (conductive). At this time, the gate of the output transistor 2 is biased by the current control voltage 32 supplied through the current control pad 202,
A current controlled by this voltage is passed through the LED 3, which is a load. At this time, the output current exhibits a constant current characteristic due to the electrical characteristics of the output transistor 2. On the other hand, when the output signal 30 of the strobe circuit is low (OFF), the transfer gate (1) 1 is turned off (interrupted), and the transfer gate (2) 21 is turned on (conducted) via the inverter 4.
At this time, the gate of the output transistor 2 becomes the ground potential because the transfer gate (1) 1 is opened and the transfer gate (2) 21 is turned on. Since the threshold voltage of the output transistor 2 is designed to be higher than zero volt, no current flows through the load of the output transistor 2, that is, the LED 3 in this state. FIG. 8 schematically shows the operation of the transfer gate. It is explained that the transfer gate (1) 1 and the transfer gate (2) 21 attached to each output transistor 2 are interlocked switches controlled by the output signal 30 of the strobe circuit.

[0012]

In the constant current drive circuit 12 in the LED array drive integrated circuit 100 in the conventional light emitting device described above, the output transistor 2 does not pass a current, that is, the gate and source of the output transistor 2 Consider the time when the inter-potential is zero. At this time, since the gate-source potential of the output transistor 2 is below the threshold value, no current should flow between the drain and source. However, for example, when a MOS type is used for the output transistor, a weak current flows between the drain and the source even if the output transistor is below the threshold by operating in the weak inversion region. That is, the LED emits light even though the current is turned off. The weak light emission does not cause much problem when the electrophotographic print output is binary, that is, black and white. However, in order to obtain a high-gradation black-and-white or high-gradation color electrophotographic print output, it is necessary to change the exposure energy, that is, the light emission energy of the LED, on the photoconductor. An example of a method of changing the emission energy at this time is shown in FIG. In the example of FIG. 9, the emission time is changed to create a state where the emission energy is large and a state where the emission energy is small. In the case where the light emission energy is small, if there is a weak light emission when the drive current is cut off, the energy for this light emission is additionally added, and the desired light emission energy cannot be obtained. As a result, high quality black and white gradation or high gradation color electrophotographic print output cannot be obtained.

In the conventional light emitting device, the above-mentioned problems have not been sufficiently taken into consideration, and it has been difficult to obtain a high quality halftone or color output electrophotographic printout.

The present invention has been made in view of the above conventional problems, and an object thereof is to prevent the LED from emitting light due to a weak current flowing in a state where the driving current is off, and to achieve high quality and high gradation. It is an object of the present invention to provide a light emitting device capable of printing out a color electrophotographic image of black and white or high gradation.

[0015]

In order to achieve the above object, a light emitting device according to a first aspect of the invention is an LED array in which a plurality of LED elements are arranged, and each of the LEDs arranged near the LED array. A light emitting device including an LED array driving integrated circuit chip for driving an element, comprising:
The D array driving integrated circuit integrally includes a switch circuit that applies a reverse bias voltage to each LED element when the LED elements are not driven, and a bonding pad that supplies the reverse bias voltage to the switch circuit. It is characterized by being incorporated.

Further, in order to achieve the above object, a light emitting device according to a second aspect of the present invention is an LED array in which a plurality of LED elements are arranged, and each LED element arranged near the LED array is driven. In a light emitting device including an LED array driving integrated circuit chip, the LED array driving integrated circuit includes a switch circuit that applies a reverse bias voltage to each LED element when the LED elements are not driven, and the switch circuit includes the switch circuit. A voltage generation circuit that supplies a reverse bias voltage is integrally incorporated.

[0017]

According to the above construction, the LED emits light due to the weak current flowing in the disconnection state of the drive current as in the prior art, and the LED emits light by applying the reverse bias to the LED when the drive current is disconnected. Is to prevent.

[0018]

Embodiments of the present invention will be described below with reference to the drawings.

First Embodiment FIG. 1 shows the LE in the light emitting device according to the first embodiment of the present invention.
A circuit diagram of the constant current drive circuit 12 of the D array drive integrated circuit 100 is shown, and FIG. 2 schematically illustrates the operation of the constant current drive circuit 12.

In FIG. 1, 100 is an LED array driving integrated circuit, 12 is a constant current driving circuit, 1 is a transfer gate (1), 21 is a transfer gate (2), 2
2 is a transfer gate (3), 2 is an output transistor, 4 is an inverter, 200 is an output pad, 201 is a reverse bias pad, 202 is a current control pad, 101
Is an LED array, 3 is an LED in the LED array, 300
Is an LED pad, 301 is a back surface electrode, the output pad 200 and the LED pad 300 are connected by wire bonding, the back surface electrode 301 is adhered to the conductor surface of the mother board 52, and the LED drive power 33 is supplied from the outside by the input connector 55. , Via the input board 54. The reverse bias pad 201 is wire-bonded to the spreader board 53, and receives the supply of the reverse bias voltage 31 from the outside via the input connector 55 and the input board 54. The current control pad 202 is wire-bonded to the spreader board 53, and receives the supply of the current control voltage 32 from the outside from the spreader board 53.

Next, the operation of this circuit will be described. The output signal 30 of the strobe circuit turns the transfer gate (1) 1 ON (conducts) when the signal is high (ON),
The transfer gate (2) 21 is turned off (cut off) by the inverter 4, and the transfer gate (3) 22 is turned off (cut off). At this time, the gate of the output transistor 2 is biased by the current control voltage 32 supplied via the current control pad 202, and a current controlled by this voltage is passed through the LED 3 which is a load. At this time, the output current exhibits a constant current characteristic due to the electrical characteristics of the output transistor 2. On the other hand, the output signal 3 of the strobe circuit
Transfer gate (1) when 0 is low (OFF)
Turn off (shut off) 1 and transfer gate (2)
21 is turned on (conductive) through the inverter 4, and the transfer gate (3) 22 is turned on (conductive).
At this time, the gate of the output transistor 2 becomes the ground potential when the transfer gate (1) 1 is opened and the transfer gate (2) 21 becomes conductive, and further the transfer gate (3) 22 becomes conductive. As a result, the cathode of LED3 has a reverse bias voltage of 31
It becomes the electric potential of. Now, the reverse bias voltage 31 is set higher than the voltage of the LED drive power source 33. Then, when the output signal 30 of the strobe circuit is low (OFF), the output transistor 2 does not operate, the cathode of the LED 3 becomes the reverse bias potential 31, and the LED 3 becomes the reverse bias state. The LED 3 in the reverse bias state does not emit light at all. Therefore, the LED 3 is completely turned off.

As described above, in the circuit of the present invention, when the LED 3 is in the OFF state, a reverse bias is applied to the LED 3 so that the LED does not emit light completely.

FIG. 2 schematically shows the operation of the transfer gate. It is explained that the transfer gate (1) 1, the transfer gate (2) 21, and the transfer gate (3) 22 are interlocking switches controlled by the output signal 30 of the strobe circuit.

Here, in this embodiment, the LED array 101 is used.
As the polarity of the LED, the direction in which the current flows in the output pad 200 of the LED driving integrated circuit 100 is used, but the polarity of the LED is opposite, that is, the direction in which the current flows out from the output pad. The circuit can be realized by the same idea as this embodiment. Second Embodiment FIG. 3 shows a circuit in the LED array driving integrated circuit 100 of the second embodiment of the present invention.

In the present embodiment, the reverse bias voltage 31 supplied from the outside in the first embodiment is obtained by the voltage generating circuit 16 including the ring oscillator 17 and the detector 18 in the LED array driving integrated circuit 100. This eliminates the need for external voltage supply, reduces the number of pads, and eliminates the need for one wire bond, and the external reverse bias voltage 31
There is an advantage that there is no need to prepare.

Third Embodiment FIG. 4 shows a block diagram of an LED array driving integrated circuit 100 according to a third embodiment of the present invention.

In this embodiment, 10 is a print data fetching / reading circuit, 11 is a pulse width modulation circuit, and 206 is a pulse width modulation circuit.
Is the second clock pad. The print data acquisition / readout circuit 10 has a function of inputting and latching light emission time data for each LED, and the pulse width modulation circuit 11
Determines the light emission time of each LED based on the above data.
A clock signal that produces a light emission time is input from the second clock pad. These features are described in US Patent 4,75
0,010 “CIRCUIT FOR GENERATING CENTER PULSEWIDTH M
ODURATED WAVEFORMS AND NON-IMPACT PRINTER USING SA
ME ”and US Patent 4,746,941“ DOT PRINTER WITH TOKE
N BIT SELECTION OF DATA LATCHING ". The present invention, by the functions of the print data acquisition / readout circuit 10 and the pulse width modulation circuit 11 just described, simultaneously emits different emission time widths for each LED. By combining the function of producing high gradation by producing the voltage generating circuit 16 and the constant current driving circuit 12 shown in the second embodiment, a light emitting device having high speed and high gradation performance is obtained. ..

[0028]

As described above, according to the present invention,
It has become possible to provide a light-emitting device that can perform high-quality black-and-white or high-gradation color electrophotography print output with high quality, which makes it possible to perform a complete non-light-emission operation when the LED is off, which has been difficult to realize. Become.

[Brief description of drawings]

FIG. 1 is an LED in a light emitting device according to an embodiment of the present invention.
It is a circuit diagram of a constant current drive circuit unit in the array drive integrated circuit.

FIG. 2 is a diagram for schematically explaining the operation of the constant current drive circuit unit in the embodiment of the present invention.

FIG. 3 is a circuit diagram of a constant current drive circuit unit in an LED array drive integrated circuit in a light emitting device according to a second embodiment.

FIG. 4 is a block diagram showing a configuration of an LED drive integrated circuit according to a third embodiment.

FIG. 5 is a diagram showing an appearance of a printer head in a conventional LED printer.

FIG. 6 is a block diagram showing a configuration of a conventional LED array driving integrated circuit.

FIG. 7 is a circuit diagram of a constant current drive circuit of a conventional LED array drive integrated circuit.

FIG. 8 is a diagram schematically illustrating an operation of a constant current drive circuit of a conventional LED array drive integrated circuit.

FIG. 9 is a diagram for explaining a case where the exposure amount is large and a case where the exposure amount is small in the operation of the light emitting device.

[Explanation of symbols]

 1 Transfer Gate (1) 2 Output Transistor 3 LED 4 Inverter 5 Capacitor 6 Resistor 7 Diode 10 Print Data Capture / Read Circuit 11 Pulse Width Modulation Circuit 12 Constant Current Drive Circuit 13 Shift Register Circuit 14 Latch Circuit 15 Strobe Circuit 16 Voltage Generation Circuit 17 Ring Oscillator 18 Detector 21 Transfer Gate (2) 22 Transfer Gate (3) 30 Strobe Circuit Output Signal 31 Reverse Bias Voltage 32 Current Control Voltage 33 LED Driving Power Supply 34 Integrated Circuit Power Supply 51 Wire Bonding 52 Motherboard 53 Spreader Board 54 Input Board 55 Input connector 100 LED drive integrated circuit 101 LED array 200 Output pad 201 Reverse bias pad 202 Current control pad 20 Clock input pad 204 print data input pad 205 the print data output pad 206 second clock pad 207 latches pad 208 strobe pad 209 power supply pad 300 LED pad 301 back electrode

Claims (3)

[Claims] 1. An LE having a plurality of LED elements arranged therein.
In a light emitting device including a D array and an LED array driving integrated circuit chip arranged in the vicinity of the LED array and driving each of the LED elements, the LED array driving integrated circuit includes: A light emitting device, wherein a switch circuit for applying a reverse bias voltage to each LED element and a bonding pad for supplying the reverse bias voltage to the switch circuit are integrally incorporated. 2. An LE having a plurality of LED elements arranged therein.
In a light emitting device including a D array and an LED array driving integrated circuit chip arranged in the vicinity of the LED array and driving each of the LED elements, the LED array driving integrated circuit includes: A light emitting device comprising: a switch circuit that applies a reverse bias voltage to each LED element; and a voltage generation circuit that supplies the reverse bias voltage to the switch circuit, which are integrally incorporated. 3. The light emitting device according to claim 1, wherein each of the LED array driving integrated circuit chips further includes a pulse width modulation circuit for controlling a light emitting time of the LED element for each LED element. A light-emitting device characterized by being incorporated in.