JP4533423B2 - Drive circuit and image forming apparatus using the same - Google Patents

Description

  The present invention relates to a load driving circuit that can be used in an image forming apparatus represented by a television receiver, a digital camera, a digital video camera recorder, a computer monitor, an electrophotographic printer, and the like, and more specifically. The present invention relates to a driving circuit for a light emitting element that can be used in a display or an exposure device that uses the light emitting element as a load.

  As an example of the load, a light-emitting element, in particular, an organic EL (electroluminescence) element, is a planar self-luminous element having a thin film stack capable of emitting light with high luminance. This EL element enables high-efficiency light emission at a low voltage by increasing the number of functional layers of organic layers (see Non-Patent Documents 1 and 2). Since the organic EL element can obtain a substantially linear emission intensity with respect to the current, a constant current driving method has been proposed.

  FIG. 8 shows a circuit configuration example of one pixel of a display element using a conventional EL element. In the figure, 1, 3 and 4 are thin film transistors (TFTs), 2 is a capacitor, 5 is an EL element, 6 is an ammeter, and 7 is a power source. The operation of the circuit will be described with reference to the timing chart of FIG.

  During a predetermined writing period, the source potential Vsig of the n-type TFT 1 is set to a display signal corresponding to the luminance displayed by the pixel in the next frame, and as shown in FIG. The gate potential Vg <b> 1 becomes H (high level), the TFT 1 is turned on, and charges corresponding to the display signal are accumulated in the capacitor 2. Next, at t2, Vg1 becomes L (low level), the TFT1 is turned off again, and at the same time, the gate potential Vg2 of the n-type TFT4 becomes H, and the TFT4 is turned on. A corresponding current (display current) flows, is supplied to the EL element 5, and emits light at a luminance corresponding to the display signal until the next writing is performed. Reference numeral 6 denotes an ammeter, which is not necessary for an actual driving circuit, but is illustrated here for explaining the operation.

  However, even when the organic EL element emits light at a constant current, it is known that the impedance changes due to deterioration of the stacked organic layers, and the luminance decreases with time as shown in FIG. . FIG. 10 shows a general tendency, and the actual change over time of the characteristics of the organic EL element is not limited to this figure.

  Therefore, a method for measuring the driving time and changing the luminance, a method for adjusting the driving voltage by detecting the luminance with a sensor, and the like have been proposed.

JP 59-55487 A "Applied Physics Letters" Volume 51, 1987, 913 Journal of Applied Physics, Volume 65, 1989, 3610

  The proposal for the luminance reduction due to the deterioration of the organic EL element described above requires means and a sensor for storing the driving time, and it is difficult to compensate for the luminance change in units of frames for each pixel.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a drive circuit that can be stably driven for a long time even with a load whose impedance and resistance change due to deterioration over time, and an image forming apparatus using the drive circuit. There is to do.

  Another object of the present invention is to detect and correct (compensate for) a decrease in luminance due to deterioration over time of a light emitting element for each pixel even in the case of a light emitting element whose load deteriorates over time. ) To realize stable image formation for a long time.

The driving circuit of the present invention includes a first driving transistor for supplying a driving current to the light emitting element according to a gate potential,
A capacitor connected to the gate of the first drive transistor and holding the gate potential in response to an input signal;
A correction transistor having a gate connected to an input terminal of the light emitting element via a first switching transistor;
Which is connected to the source of the correcting transistor, and a nonlinear element having a diode characteristic,
A second drive transistor having a gate connected to the gate of the first drive transistor and supplying a current to the nonlinear element according to a gate potential;
A second switching transistor disposed between the drain of the correction transistor and the gates of the first drive transistor and the second drive transistor;
A drive circuit comprising:
The first switching transistor is turned on , the second switching transistor is turned off , the same current flows through the nonlinear element and the light emitting element, and the source potential of the correction transistor is set as the input terminal of the light emitting element. set the threshold voltage by a low value of the correcting transistor than said first off the switching transistor, and turning on the second switching transistor, a gate of the correcting transistor - depending on the source voltage The current is supplied to the capacitor.

According to another aspect of the present invention, there is provided an image forming apparatus having a pixel circuit group in which a plurality of pixel circuits each including the driving circuit and a light emitting element as a load of the driving circuit are arranged.
The pixel circuit group is arranged in a two-dimensional matrix, and a display unit that forms an image by light emission of the light emitting element in the pixel circuit group;
A column driving circuit for supplying an image signal to the pixel circuit group;
An image data supply circuit for supplying image data to the column drive circuit;
A decoder for decoding the compressed image data stored in the storage medium and supplying the decoded image data to the image data supply circuit;
It is characterized by comprising.

Furthermore, the present invention provides an image forming apparatus having a pixel circuit group in which a plurality of pixel circuits each having the above driving circuit and a light emitting element as a load of the driving circuit are arranged.
A photoreceptor,
A charger for charging the photoreceptor;
An exposure device for forming a latent image on a photosensitive member by light emission of the light emitting element in the pixel circuit group, the pixel circuit group being arranged in at least a one-dimensional matrix;
A developer,
Comprising
A column driving circuit for supplying an image signal to the pixel circuit group;
An image data supply circuit for supplying image data to the column drive circuit;
It is characterized by comprising.

  According to the present invention, even a load that deteriorates with time and changes its impedance and resistance can be stably driven in the long term by applying feedback.

  For example, when a light-emitting element having a characteristic that the luminance deteriorates with time and decreases in luminance as a load, the luminance can be corrected for each pixel, for example, in units of frames. Therefore, deterioration of the light emitting element over time does not affect the image, and a stable image can be displayed over a long period of time. For this reason, the present invention is preferably used in an image forming apparatus such as a display or an electrophotographic image forming apparatus.

  First, in order to facilitate understanding of the operation of the drive circuit of the present invention, the basic operation will be described with reference to the drawings.

  In the circuit shown in FIG. 8, the luminance of the light emitting element 5 as a load supplied with a predetermined current decreases with time as shown in FIG. 10, and the voltage between both terminals of the light emitting element 5 increases. This is because the impedance of the light emitting element 5 increases due to deterioration of the organic layer. In the embodiment of the present invention, the voltage increase at this time is detected as the amount of impedance change of the light emitting element 5, fed back to the TFT 3 as the driving transistor, and the amount of current supplied to the light emitting element 5 by the TFT 3 is adjusted. The current flowing through the light emitting element 5 is corrected to correct the luminance of the light emitting element.

As shown in FIG. 10, the voltage change is a curve. On the other hand, the luminance change draws a curve that is substantially reverse to the voltage rise. Therefore, a circuit shown in FIG. 11 was prototyped. In the figure, 61 is a capacitor, 62 is a TFT, 63 is a variable bias voltage, and 64 is a voltmeter. TFT62 is an n-channel type, the potential of the control terminal of the TFT62 provided on the common side of the capacitor 61, the variable bias voltage 63, a variable bias voltage Vgbias in the range up to use the current value of TFT 6 2 Threshold Changed. As a result, the source-drain voltage of the TFT 62 exhibited the characteristics shown in FIG. This characteristic is opposite to the source / drain current characteristic of the p-type TFT 3 of FIG. The voltage of the capacitor 61 is proportional to the amount of charge. That is, it is proportional to the energization time of the source / drain current of the TFT 62. Therefore, if the circuit of FIG. 11 is incorporated in the circuit of FIG. 8 and the source-drain voltage of the TFT 62 is used as the gate voltage of the TFT 3 of the circuit of FIG. 8, a current that changes substantially linearly is applied to the control terminal of the TFT 62. Thus, it should be possible to linearly compensate for the luminance reduction of the light emitting element 5.

  Here, as a load that can be used in the present invention, an LED made of an inorganic material, an LED made of an organic material (this is often called an organic EL), an electron-emitting device, an electron-emitting device, and a light emitter A light emitting device composed of In particular, a light-emitting element whose luminance can be adjusted by a current value is preferable.

  The transistor used in the present invention may be an insulated gate transistor, specifically, a MOS transistor using bulk silicon, but a thin film transistor (TFT) having a semiconductor layer on an insulating surface of a substrate is preferably used. The TFT may be a TFT using a so-called amorphous semiconductor, a TFT using a polycrystalline semiconductor, or a TFT using a single crystal semiconductor, but a TFT using a polycrystalline semiconductor, particularly a low-temperature polysilicon TFT is preferably used. .

  A specific circuit configuration example is shown below.

[Embodiment 1]
FIG. 1 shows a pixel circuit of one embodiment of the display element of the present invention. As the load, the light emitting element 5 is used. In the figure, 1, 3, 4, 8, 9, and 12 are TFTs, only TFT 3 is p-type, and the other TFTs are n-type. 2 and 11 are capacitors, 5 is a light emitting element, 6 is an ammeter, 7 is a power source, and 10 is a variable or fixed bias voltage source. In the figure, TFT3 is a drive transistor, and TFT9 is a second transistor. The ammeter 6 is not necessary in an actual drive circuit.

  The load driving circuit of this embodiment is a voltage programming type, and an input signal composed of a voltage corresponding to display luminance is applied to each pixel circuit as a display signal Vsig. The operation of the pixel will be described with reference to the timing chart of FIG.

  A display signal Vsig corresponding to the luminance to be displayed in the next frame is input to the input terminal of the n-channel TFT 1 as the address transistor, and the gate voltage Vg1 of the TFT 1 as the address transistor becomes H at the determined time t1. Then, the TFT 1 is turned on, and the electric charge according to the voltage value of the display signal is accumulated in the storage capacitor 2, and the gate of the p-channel TFT 3 as the driving transistor becomes the electric potential according to the display signal.

  At time t2, Vg1 becomes L and TFT1 is turned off. At the same time, Vg2 becomes H and TFT4 as a switching transistor is turned on. Accordingly, the TFT 3 supplies a current (display current Iout) having a value corresponding to the gate potential to the light emitting element 5 through the TFT 4. At the time t2, Vg4 also becomes H, and the TFT 8 as the second switching transistor is turned on, and the gate potential of the TFT 9 as the correction transistor is equal to the input terminal (anode) potential of the light emitting element 5. Become. Here, if the source of the TFT 9 is set to (the anode potential of the light emitting element 5 with respect to the display current−the threshold value of the TFT 9), that is, the predetermined potential is Ps, and the anode terminal potential of the light emitting element with respect to the driving current is Pi. When the threshold voltage of the correction transistor 9 is Vth, Ps = Pi−Vth is set, and the source potential of the correction transistor is set to a predetermined potential Ps, so that the voltage increased due to the degradation is reduced to the TFT 9. Source / drain current (correction signal).

  Thus, after the source / drain current of the TFT 9 is determined, at time t3, Vg4 is set to L and the TFT 8 is turned off. At the same time, Vg3 is set to H and the TFT 12 as the switching transistor is turned on. Shed from. As a result, the gate potential of the TFT 3 decreases, the amount of current that the TFT 3 supplies to the light emitting element 5 increases (Δi), and light is emitted with the same luminance as before deterioration. Since the relationship between the current and the luminance is linear, the luminance is corrected by the relationship shown in FIG.

  Specifically, the power supply voltage of the power supply 7 is 10 V, the voltage input to and held in the storage capacitor 2 is about 7.3 V, the set output voltage of the variable bias voltage source 10 is about 2.5 V, and the capacity 11 is about 5 V. In the case of the pixel circuit in which the light emission has been detected, the luminance of the organic EL element as the light emitting element decreases and the resistance increases as the light source is used for a long period of time, and the anode voltage of the light emitting element increases accordingly. When the TFT 8 is turned on and detected, about 6 V is detected in the capacitor 11, so that the gate voltage of the TFT 9 as a correction transistor rises, so that more current is to flow. Therefore, when the TFT 12 is turned on, the voltage held in the holding capacitor 2 falls to a value lower than 7.3 V, and the gate voltage of the TFT 3 as the driving transistor is lowered, so that the TFT 3 tries to pass a larger current. In this way, a larger driving current flows through the organic EL element than before long-term use, and light can be emitted with the same luminance as before use even after long-term use.

  In the present embodiment, a coefficient is applied to the correction signal by adjusting the size of the TFT 9 and changing the gate voltage-drain current characteristics of the TFT, and the Vgbias and Vout of the TFT 62 (corresponding to the TFT 9) shown in FIGS. By changing the relationship, it is possible to keep the luminance composed of the voltage-luminance characteristics shown in FIG. 10 constant.

[Embodiment 2]
FIG. 3 shows a pixel circuit according to a second embodiment of the display element of the present invention. In the figure, 13 is a non-linear element having diode characteristics, and 14 is a p-type TFT. As the load, the light emitting element 5 is used.

  In the pixel circuit of this embodiment, the variable bias voltage 10 of the pixel circuit of the first embodiment is used as a non-linear element 13, and a current mirror circuit is configured by TFT3 and TFT14 which are driving transistors. The operation of this pixel circuit will be described with reference to the timing chart of FIG.

  When the display signal is determined at time t1, Vg1 becomes H and the TFT1 is turned on, charges corresponding to the display signal are accumulated in the capacitor 2, and the gate potentials of the TFTs 3 and 14 are set. Next, at time t2, Vg1 becomes L, and at the same time, Vg2 and Vg4 become H, TFT1 is turned off, and TFT4 and 8 are turned on at the same time. As a result, a current corresponding to the display signal is supplied from the TFT 3 to the light emitting element 5 and the gate of the TFT 9 via the TFT 4. Here, the current mirror circuit composed of the p-type TFTs 3 and 14 causes a current having the same value as the display current supplied to the light-emitting element 5 to flow to the non-linear element 13. The voltage is set to the forward potential of the non-linear element 13 (designed in advance to the anode potential of the light emitting element 5 with respect to the display current−the threshold value of the TFT 9). That is, when the predetermined potential is Ps, the anode terminal potential of the light emitting element with respect to the driving current is Pi, and the threshold voltage of the correction transistor 9 is Vth, Ps = Pi−Vth, and the source potential of the correction transistor is The diode 13 as a nonlinear element is designed so as to have a predetermined potential Ps. As a result, the voltage that has risen due to deterioration can be extracted as the source / drain current (correction signal) of the TFT 9.

  After the source / drain current of the TFT 9 is determined, Vg4 is set to L at time t3 and the TFT8 is turned off. At the same time, Vg3 is set to H and the TFT12 is turned on to supply the source / drain current of the TFT9 to the capacitor 2. As a result, the gate potential of the TFT 3 decreases, the amount of current that the TFT 3 supplies to the light emitting element 5 increases, and the light emitting element 5 emits light with the same luminance as before deterioration. Since the relationship between the current and the luminance is linear, the luminance is corrected by the relationship shown in FIG.

  Also in this embodiment, it is possible to apply a coefficient to the correction signal by adjusting the size of the TFT 9.

[Embodiment 3]
FIG. 4 shows a pixel circuit of a third embodiment of the display element of the present invention. In the figure, 3, 14, 15, and 16 are p-type TFTs, and 1, 8, 9, 12, and 17 are n-type TFTs. As the load, the light emitting element 5 is used.

  The display element is a current programming type, and a display signal Idata consisting of a current corresponding to display luminance is applied to each pixel circuit as an input signal. The operation of the pixel will be described with reference to the timing chart of FIG.

  A display signal corresponding to the luminance to be displayed in the next frame is input to the input terminal of the n-channel TFT 1 serving as the addressing transistor, and the gate potential Vg 1, 6 of the TFT 1, 17 becomes H at the determined time t 1 and at the same time the TFT 16 The gate potential Vg5 becomes L, the TFTs 1, 17 and 16 are turned on, charges corresponding to the current value of the display signal are accumulated in the capacitor 2, and the gates of the TFTs 3 and 14 become potentials corresponding to the display signal.

  At time t2, Vg1 and 6 become L, Vg5 becomes H, and the TFTs 1, 17 and 16 are turned off. At the same time, Vg2 becomes L and Vg4 becomes H, the TFTs 8 and 15 are turned on, and a current corresponding to the display signal is supplied from the TFT 3 to the gates of the light emitting element 5 and the TFT 9 via the TFT 15. Here, the current mirror circuit composed of the p-type TFTs 3 and 14 causes a current having the same value as the display current supplied to the light-emitting element 5 to flow to the non-linear element 13. The voltage is set to the forward potential of the non-linear element 13 (designed in advance to the anode potential of the light emitting element 5 with respect to the display current−the threshold value of the TFT 9). That is, when the predetermined potential is Ps, the anode terminal potential of the light emitting element with respect to the driving current is Pi, and the threshold voltage of the correction transistor 9 is Vth, Ps = Pi−Vth, and the source potential of the correction transistor is The diode 13 as a nonlinear element is designed so as to have a predetermined potential Ps. As a result, the voltage that has risen due to deterioration can be extracted as the source / drain current (correction signal) of the TFT 9.

  At time t3, Vg2 is H and Vg4 is L and the TFTs 8 and 15 are turned off. At the same time, Vg3 is H and Vg5 is L and the TFTs 9, 12, and 16 are turned on. As a result, the source / drain current of the TFT 9 flows from the capacitor 2 and the gate potential of the TFT 3 decreases.

  At time t4, Vg3 becomes L, Vg5 becomes H, and the TFTs 12 and 16 are turned off. At the same time, Vg2 becomes L and the TFT 15 is turned on, and a current obtained by adding a correction signal for deterioration to the display current is supplied to the light emitting element 5. The light emitting element 5 emits light with the same luminance as before deterioration. Since the relationship between the current and the luminance is linear, the luminance is corrected by the relationship shown in FIG.

  Also in this embodiment, it is possible to apply a coefficient to the correction signal by adjusting the size of the TFT 9.

  As in each of the embodiments described above, the switching transistors TFTs 8 and 12 are turned on every predetermined period, for example, one frame period or several frame periods, and the impedance of the load (it can also be regarded as resistance or anode voltage). And the drive current is corrected based on the detected current, so that the load can be driven with a current necessary to develop a desired phenomenon. A typical example is a pixel circuit using an organic EL element.

(Embodiment 4)
The image forming apparatus of the present embodiment shown in FIG. 6 uses a large number of the pixel circuits of the first to third embodiments described above, and the pixel circuit groups are arranged in a two-dimensional matrix. The display unit 41 forms an image by light emission of the light emitting element. The column driving circuit 42 supplies an image signal (Vsig or Idata) to the pixel circuit group. The display unit 41 is driven and controlled by a column drive circuit 42 and a row selection circuit 46. The image data supply circuit 43 that supplies analog or digital image data DATA to the column drive circuit 42 is preferably capable of performing image processing such as contrast adjustment, gamma adjustment, sharpness adjustment, and scaling. Furthermore, a decoder 45 that decodes the compressed image data JPG stored in the storage medium 44 and supplies it to the image data supply circuit 43 is provided. This apparatus is preferably used as a monitor for a TV receiver, a digital camera, or a digital video camera recorder.

(Embodiment 5)
The image forming apparatus of the present embodiment shown in FIG. 7 uses a large number of the pixel circuits of the first to third embodiments described above, and the pixel circuit groups are arranged in at least a one-dimensional matrix to form a light emitting element array. is doing. This image forming apparatus is an electrophotographic printer, and includes a photosensitive member 51, a charger 52 for charging the photosensitive member 51, and light emission of the light emitting element to the pixel circuit group. And an exposure unit 53 for forming an image. The exposure unit 53 includes the light emitting element array.

  In addition, this apparatus includes a developing device 54. Further, a column drive circuit (not shown) in the exposure unit 53 supplies an image signal to the pixel circuit group, and in synchronization with this, the light emitting element array emits light, and the photoconductor 51 rotates. The image data supply circuit 43 that supplies image data to the column drive circuit can be the same as that in the fourth embodiment. However, in this embodiment, only the still image is handled, so the internal configuration is different.

It is a pixel circuit diagram of one embodiment of the present invention. 2 is a timing chart of the operation of the circuit of FIG. It is a pixel circuit diagram of other embodiments of the present invention. It is a pixel circuit diagram of other embodiments of the present invention. 6 is a timing chart of the operation of the circuit of FIG. 1 is a block diagram of an embodiment of an image forming apparatus of the present invention. It is a schematic block diagram of other embodiment of the image forming apparatus of this invention. It is a pixel circuit diagram of the conventional display element. 9 is a timing chart of the operation of the circuit of FIG. It is a figure which shows the time-dependent change concerning the light emitting element of the circuit of FIG. It is a circuit diagram for demonstrating the basic principle of this invention. FIG. 12 is a voltage characteristic diagram in the circuit of FIG. 11.

Explanation of symbols

1,3,4,8,9,12,14,15 TFT
2,11 capacity 5 light emitting element 6 ammeter 7 power supply 10 variable bias voltage 13 nonlinear element 41 display unit 42 column drive circuit 43 image data supply circuit 44 storage medium 45 decoder 51 photoconductor 52 charger 53 exposure unit 61 capacity 62 TFT
63 Variable bias voltage 64 Voltmeter 65 Power supply

Claims (3)

A first driving transistor for supplying a driving current to the light emitting element according to the gate potential;
A capacitor connected to the gate of the first drive transistor and holding the gate potential in response to an input signal;
A correction transistor having a gate connected to an input terminal of the light emitting element via a first switching transistor;
Which is connected to the source of the correcting transistor, and a nonlinear element having a diode characteristic,
A second drive transistor having a gate connected to the gate of the first drive transistor and supplying a current to the nonlinear element according to a gate potential;
A second switching transistor disposed between the drain of the correction transistor and the gates of the first drive transistor and the second drive transistor;
A drive circuit comprising:
The first switching transistor is turned on , the second switching transistor is turned off , the same current flows through the nonlinear element and the light emitting element, and the source potential of the correction transistor is set as the input terminal of the light emitting element. set the threshold voltage by a low value of the correcting transistor than said first off the switching transistor, and turning on the second switching transistor, a gate of the correcting transistor - depending on the source voltage A driving circuit for supplying a current to the capacitor. An image forming apparatus having a pixel circuit group in which a plurality of pixel circuits each including the drive circuit according to claim 1 and a light emitting element as a load of the drive circuit are arranged.
The pixel circuit group is arranged in a two-dimensional matrix, and a display unit that forms an image by light emission of the light emitting element in the pixel circuit group;
A column driving circuit for supplying an image signal to the pixel circuit group;
An image data supply circuit for supplying image data to the column drive circuit;
A decoder for decoding the compressed image data stored in the storage medium and supplying the decoded image data to the image data supply circuit;
An image forming apparatus comprising: An image forming apparatus having a pixel circuit group in which a plurality of pixel circuits each including the drive circuit according to claim 1 and a light emitting element as a load of the drive circuit are arranged.
A photoreceptor,
A charger for charging the photoreceptor;
An exposure device for forming a latent image on a photosensitive member by light emission of the light emitting element in the pixel circuit group, the pixel circuit group being arranged in at least a one-dimensional matrix;
A developing device,
A column driving circuit for supplying an image signal to the pixel circuit group;
An image data supply circuit for supplying image data to the column drive circuit;
An image forming apparatus comprising:

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