JP2005091443A - Driving circuit for display apparatus and driving method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a driving circuit for a display apparatus capable of preventing a driving voltage in a light emitting device from increasing. <P>SOLUTION: The driving circuit used for driving the light emitting device in each of pixels is equipped with a light emitting device driving unit connected to the light emitting device and supplying a driving current selectively to the light emitting device, and a discharge unit connected to the light emitting device driving unit, and discharging the light emitting device according to a voltage level of a control signal, immediately after the light emitting device driving unit supplies the driving current to the light emitting device. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a driving circuit for a display device. More particularly, the present invention relates to a display device driving circuit and a driving method capable of preventing an increase in driving voltage of a light emitting device.

  In the early 20th century, dynamic images were joined together to form a movie. Soon after the invention of the cathode ray tube (CRT), commercial broadcasting led to the popularization of television. In recent decades, cathode ray tubes have also been adopted by the computer industry as an interface between humans and personal computers. Most desktop monitors have the same operating principles as CRTs. However, the danger and bulk of radiation due to the electron gun design in the CRT has led to the invention of several other novel display devices such as flat panel displays.

  There are many types of flat panel displays. The most common types of flat panel displays include liquid crystal displays (LCDs), field emission displays (FEDs), organic light emitting diodes (OLEDs) and plasma display panels (PDPs). Organic light emitting diodes (OLEDs), also known as organic electroluminescent displays (OELD), are self-illuminated display devices. The OLED is driven by a DC low voltage and has high visibility, high operating efficiency, high contrast, and small weight. Furthermore, many color tones can be generated from the three primary colors, red (R), green (G) and blue (B) with respect to white. OLEDs are therefore often considered flat panel displays with the most promising developmental potential. With the exception of light weight and high resolution like LCD, self-illumination like LED, quick response, and the production of low temperature light that saves energy, the advantages of OLED are wide viewing angle and beautiful color contrast And low manufacturing costs. Thus, OLEDs have many uses, including LCD or indicator panel backlighting, cell phones, digital cameras, personal digital assistants.

  According to the driving method, the OLED can be divided into a passive matrix driving type and an active matrix driving type. The advantages of passively driven OLEDs include the simplicity of the structure and the absence of thin film transistors (TFTs) as drivers. Therefore, the manufacturing cost of passively driven OLED is relatively low. However, passive OLEDs have a lower image resolution and consume a large amount of energy. Therefore, when a large display panel is manufactured using passive OLED, too much energy is consumed, the usable time is shortened, and the display clarity is lowered. On the other hand, active matrix driven OLEDs are slightly more expensive to manufacture, but with active OLED designs, with a larger viewing angle, higher visibility, and faster response to signals, more Large displays can be manufactured.

  Flat panel displays can be further classified into voltage-driven or current-driven types according to the drive type. In general, the voltage drive type is usually used for a TFT-LCD. Different voltages are used on the various data lines to produce different levels of gray scale, resulting in full colorization. Since voltage-driven TFT-LCDs have been developed for quite some time, TFT-LCDs operate fairly reliably and have low manufacturing costs. The current driven type is often used for OLED displays. Different currents are supplied to the data lines, producing different gray scales, resulting in full colorization. However, when current is used as a medium to drive a pixel, new circuit and IC designs are required. Since such design and development costs are high, if the voltage driving circuit for driving the TFT-LCD is used to drive the OLED, the cost is considerably reduced.

FIG. 1 is a diagram illustrating a pixel driving circuit in a conventional display device. As shown in FIG. 1, the pixel 100 includes a drive circuit 110 and a light emitting device (OLED) 120. The drive circuit includes a first thin film transistor (TFT1), a capacitor (C), and a second thin film transistor (TFT2). The transistor TFT2 is a driving thin film transistor that generates a current for driving the OLED 120 to emit light. The drain terminal of the transistor TFT1 is coupled to the data voltage ( _Vdata ) terminal, the gate terminal of the transistor TFT1 is coupled to the scan voltage ( _Vscan ) terminal, and the source terminal of the transistor TFT1 is connected to the first terminal of the capacitor (C) and the transistor. Coupled to the gate terminal of TFT2. The drain terminal of transistor TFT 2 is coupled to the positive voltage (V _dd ) terminal, and the source terminal of transistor TFT 2 is coupled to the positive terminal of OLED 120. The second terminal of the capacitor (C) is coupled to the voltage (V _ss1 ) terminal. A negative voltage or a ground potential is applied to the voltage terminal (V _ss1 ). The negative terminal of OLED 120 is coupled to the voltage (V _ss ) terminal. A negative voltage or a ground potential is also applied to the voltage (V _ss ) terminal.

FIG. 2 is a timing diagram illustrating the voltage / time relationship for voltages V _dd , V _scan and V _data in the conventional drive circuit shown in FIG. The drain current equation for operating the second thin film transistor (TFT2) in the saturation region is given by:

I _ds = (1/2) × k2 × (V _GS −V _th2 ) ²
= (1/2) × k2 × (V _g2 −V _d2 −V _th2 ) ²

Here, k2 = μ _n × C _ox × (W / L) ² , μ _n is the electron mobility, C _ox is the gate capacitance per unit area (μ _n and C _ox have constant values) , (W / L) ² is the channel width / length of the second thin film transistor (TFT2), V _g2 is the gate voltage of the second thin film transistor (TFT2), and V _d2 is the drain voltage of the second transistor (TFT2). V _th2 is the threshold voltage of the second thin film transistor (TFT2).

According to the above formula, V _d2 = V _ss + VOLED, where VOLED is a voltage applied to the light emitting device 120. When the value of the voltage VOLED is unstable, the current for driving the light emitting device 120 changes. Ultimately, this may lead to a reduction in the usable time of the light emitting device 120.

  Accordingly, an object of the present invention is to provide a driving circuit and a driving method for a display device.

  The drive circuit is incorporated into the original drive circuit of the display device so that each light emitting device drive unit has an additional thin film transistor. The gate of each thin film transistor is connected to the next scan line. When the driving circuit is switched one by one on the scan line, the additional thin film transistor is discharged from the light emitting device immediately after switching in the next scan line. Accordingly, an increase in the driving voltage of the light emitting device is prevented, and the operable time of the device increases. In addition, the drain terminal of the additional thin film transistor can be coupled to a ground potential or a negative voltage. When the drain terminal is coupled to a negative voltage, the discharge efficiency from the light emitting device is improved and the operable time of the device is further increased.

  To achieve these and other advantages, according to the objects of the present invention, as embodied and broadly described herein, the present invention provides a drive circuit and a display device using the drive circuit. The drive circuit drives the light emitting device. The display device drive circuit includes a light emitting device drive unit and a discharge unit. The light emitting device driving unit for selectively supplying a driving current to the light emitting device is coupled to the light emitting device. As soon as the light emitting device driving unit supplies current and drives the light emitting device, the discharge unit for discharging the light emitting device according to the voltage level of a control signal is coupled to the light emitting device driving unit.

  The driving circuit further includes a light emitting device selection unit coupled to the light emitting device driving unit to receive a scan signal and a data signal. When the scan signal and the data signal are at the logic level ‘1’, the light emitting device selection unit activates the light emitting device driving unit so that the light emitting device driving unit supplies a driving current to the light emitting device.

  According to the present invention, the drive circuit control device is triggered by the scan signal of the next pixel. When the scan signal of the next pixel is at a logic level '1', the discharge unit discharges the light emitting device. In addition, the discharge unit is coupled to a ground potential or a negative voltage so that the light emitting device can discharge to the ground or a negative voltage line.

  The present invention also provides a method of driving a display device. The display device has a plurality of pixels. The light emitting device in each pixel is driven using the driving device. A drive current is selectively supplied to drive one of the light emitting devices described above. According to the voltage level of the control signal, the light emitting device discharges when being driven by a driving current.

  In the above-described method, a driving current is received by a specific light emitting device according to a scan signal and a data signal supplied to the display device. When the scan signal and the data signal are at the logic level “1”, the driving current is received by the light emitting device. On the other hand, a control signal for discharging the light emitting device is obtained from a scan signal of the next pixel.

  Both the above general description and the following detailed description are exemplary, and are intended to provide a further description of the claimed invention.

  Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used in the drawings and the description to refer to the same or like parts.

  The present invention relates to a driving circuit for a display device. The drive circuit included in the display device drives the light emitting device to generate light. The light emitting device can be, for example, an organic light emitting diode (OLED). The light emitting device emits light energy when electrons and holes in the device recombine in pairs. Therefore, after a voltage is applied to the light emitting device for some time, the material comprising the light emitting device may degrade and may have a greater electrical resistance due to extended charge accumulation. This often results in an increase in drive voltage. Due to the increase in driving voltage for the light emitting device, the current sent to drive the light emitting device is reduced and the performance of the device is degraded. Eventually, the useful life of the light emitting device decreases.

  The drive circuit of the present invention is mounted in the original drive circuit of the display device. A thin film transistor is incorporated in the driving circuit of each pixel. The gate of each thin film transistor is connected to the next scan line. When the driving circuit is switched one by one on the scan line, the additional thin film transistor is discharged from the light emitting device immediately after switching in the next scan line. Accordingly, an increase in the driving voltage of the light emitting device is prevented, and the operable time of the device increases. In addition, the drain terminal of the additional thin film transistor can be coupled to a ground potential or a negative voltage. When the drain terminal is coupled to a negative voltage, the discharge efficiency from the light emitting device is improved and the operable time of the device is further increased.

  FIG. 3 is a diagram showing a driving circuit relating to a certain pixel in a display device according to a preferred embodiment of the present invention. As shown in FIG. 3, the pixel 300 includes a drive circuit 310 and a light emitting device 320. The light emitting device 320 can be an organic light emitting diode (OLED), also known as an organic electroluminescent display (OELD), or a molecular light emitting diode. The driving circuit 310 further includes a light emitting device selection unit 311 and a light emitting device driving unit 313. The light emitting device selection unit 311 includes, for example, a first thin film transistor (TFT1) and a capacitor (C). The light emitting device driving unit 313 includes, for example, a second thin film transistor (TFT2). A second thin film transistor (TFT 2) known as a driving thin film transistor generates a driving current and drives the light emitting diode 320.

The drive circuit 310 of the present invention further includes a discharge unit 315. The discharge unit 315 is connected to the drain terminal of the second thin film transistor (TFT2). Each pixel in the display device including the pixel 300 has a data line and a scan line. The timing relationship between various voltage terminals V _dd , V _scan and V _{data in} the drive circuit 310 is the same as that shown in FIG. The high voltage level and the low voltage level appear once in each scan line of the corresponding pixel. A complete cycle that is the sum of the times at the high and low voltage levels is called a time frame (indicated by the letter T in FIG. 2). For example, the time frame is the well-known 1/60 second. That is, the light emitting device operates at a frequency of 60 Hz, and each time frame constitutes one pixel image.

  The discharge unit 315 is connected to the control signal. The discharge unit 315 is activated by the control signal. For example, when the control signal is at a high voltage level, that is, a logic level ‘1’, the light emitting device 320 is discharged. The discharge period is largely determined by design requirements. In the preferred embodiment of the present invention, the control signal for the discharge unit 315 is provided by the next scan line corresponding to the current driver circuit 130. The light emitting device 320 discharges when the next scan line voltage is at a high voltage level. Therefore, the influence of charge accumulation in the light emitting device 320 is minimized, and the operable time of the device is extended.

  The discharge unit 315 according to the present invention may be realized by using, for example, a third thin film transistor (TFT3). However, this is not the only means of realization. The discharge unit 315 may comprise a collection of various devices that perform similar functions. The main criterion for any circuit setup is the ability for high voltage levels to trigger the circuit to discharge excess charge from the light emitting device 320.

Assume that n scan lines correspond to the pixels 300. Connecting the first drain terminal of the thin film transistor (TFT 1) to the data voltage _{V data.} The gate terminal of the first thin film transistor (TFT1) is connected to the nth scan line having the scan voltage V _sn . The source terminal of the first thin film transistor (TFT1) is connected to the first terminal of the capacitor (C) and the gate terminal of the second thin film transistor (TFT2). The second terminal of the capacitor (C) is connected to the voltage source V _ss1 . The voltage source V _ss1 is _set to either a negative voltage or a ground potential. The source terminal of the second thin film transistor (TFT2) is connected to the positive voltage source _Vdd . The drain terminal of the second thin film transistor (TFT2) is connected to the positive terminal of the light emitting device 320 and the source terminal of the third thin film transistor (TFT3). The drain terminal of the third transistor (TFT3) is connected to the voltage source _Vdrv . The gate terminal of the third thin film transistor (TFT3) is connected to the scan voltage source V _{sn + 1} of the next scan line (that is, the (n + 1) th scan line). The negative terminal of the light emitting device 320 is connected to the voltage source V _ss . The voltage source V _ss applies either a negative voltage or a ground potential.

  Due to the charge accumulation through the continuous operation, the drive voltage for driving the light emitting device 320 increases. To reduce the drive voltage rise, the discharge unit 315 is incorporated into a conventional drive circuit as shown in FIG. 1 by connecting to the next scan line. By using the sequential scan line switching characteristics of the driving circuit, the discharge unit 315 immediately receives an activation signal (scan voltage change from a low voltage level to a high voltage level) from the next scan line. 320 is discharged. By discharging the light emitting device 320, the effect of charge accumulation on the driving voltage of the light emitting device 320 is minimized.

  For example, the third thin film transistor (TFT3) in FIG. 3 functions as a discharge unit. The gate terminal of the transistor (TFT3) is connected to the next scan line. In general, the high and low voltage levels appear once in each scan line of the corresponding pixel, forming a complete cycle or time frame (indicated by the letter T in FIG. 2). The time frame is, for example, 1/60 seconds. That is, the light emitting device operates at a frequency of 60 Hz, and each time frame constitutes one pixel image. When the (n + 1) th scan line is ready to be switched on, the newly added thin film transistor (TFT3) discharges the light emitting device 320 in the pixel 300 corresponding to the nth scan line. Finally, an increase in the driving voltage of the light emitting device 320 is prevented.

The discharge unit 31 described above discharges the light emitting device 30 to the ground. In another embodiment, the discharge unit 31 may be connected to a negative voltage terminal to increase discharge efficiency. For example, the drain terminal of the third thin film transistor (TFT3) may be connected to a voltage source _Vdrv at the ground potential or negative voltage. When the drain terminal is connected to a negative voltage, the discharge rate from the light emitting device increases, and the operable time of the display increases.

When the scan voltage V _sn of the nth scan line is at a high voltage level, the first thin film transistor (TFT1) becomes conductive. The source terminal of the second TFT (TFT 2) picks up the voltage _{V data.} The drain current equation for operating the second thin film transistor (TFT2) in the saturation region is given by:

I _ds = (1/2) × k2 × (V _GS −V _th2 ) ²
= (1/2) × k2 × (V _g2 −V _d2 −V _th2 ) ²

Here, k2 = μ _n × C _ox × (W / L) ² , μ _n is the electron mobility, C _ox is the gate capacitance per unit area (μ _n and C _ox have constant values) , (W / L) ² is the channel width / length of the second thin film transistor (TFT2), V _g2 is the gate voltage of the second thin film transistor (TFT2), and V _d2 is the drain voltage of the second transistor (TFT2). V _th2 is the threshold voltage of the second thin film transistor (TFT2).

Here, V _d2 = V ₃₂₀ + V _ss , and V ₃₂₀ is a voltage at the positive terminal of the light emitting device 320. According to the above equation, the voltage V ₃₂₀ increases with the operation period, resulting in a decrease in the drain current Ids. However, the light emitting device 320 is connected to the voltage _Vdrv by switching on the third thin film transistor (TFT3). Since V _drv is either a ground potential or a negative voltage, the charge accumulated in the light emitting device 320 is discharged, and thus the driving voltage of the light emitting device 320 does not increase with time.

  In summary, the present invention incorporates a discharge unit in the original drive circuit of the display device so that the light emitting device can discharge when activated by the scan voltage from the next scan line. In this way, an increase in driving voltage of the light emitting device due to charge accumulation is prevented, and the light emitting device can maintain a constant brightness throughout the operation.

  It will be apparent to those skilled in the art that various modifications and variations to the structure of the present invention can be made without departing from the scope and spirit of the invention. In view of the above, the present invention is intended to cover modifications and variations of this application provided they come within the scope of the claims and their equivalents.

It is a figure which shows the drive circuit of the pixel in a conventional display apparatus. FIG. 3 is a timing diagram illustrating a voltage / time relationship regarding voltages V _dd , V _scan , V _data, and V _g2 in the conventional drive circuit illustrated in FIG. 1. FIG. 3 is a diagram illustrating a pixel driving circuit in a display device according to a preferred embodiment of the present invention.

Explanation of symbols

100, 300 Pixel 110, 310 Drive circuit 120, 320 Light emitting device 311 Light emitting device selection unit 313 Light emitting device drive unit 315 Discharge unit

Claims (20)

A drive circuit for a display device having a plurality of pixels, wherein the drive circuit is used to drive a light emitting device in each pixel.
A light emitting device driving unit coupled to the light emitting device and selectively supplying a driving current to the light emitting device;
A discharge unit coupled to the light emitting device driving unit, the discharge unit discharging the light emitting device according to a voltage level of a control signal as soon as the light emitting device driving unit supplies a driving current to the light emitting device; Drive circuit.   2. The driving circuit according to claim 1, wherein the light emitting device driving unit is coupled to the light emitting device driving unit, receives a scan signal and a data signal, and when the scan signal and the data signal are at a logic level '1', A drive circuit, further comprising a light emitting device selection unit that enables a drive current to be supplied to the light emitting device.   3. The drive circuit according to claim 2, wherein the control signal uses a scan signal from the next pixel.   4. The drive circuit according to claim 3, wherein the discharge unit discharges the light emitting device when a scan signal in the next pixel is at a logic level “1”, that is, at a high voltage level. 5.   2. The drive circuit according to claim 1, wherein the discharge unit is coupled to a ground potential so that electric charges are discharged from the light emitting device to the ground.   2. The drive circuit according to claim 1, wherein the discharge unit is coupled to a negative voltage so that electric charges are discharged from the light emitting device to a negative voltage terminal.   2. The drive circuit according to claim 1, wherein the discharge unit is a transistor, the transistor is switched on according to a voltage level of the control signal, and the light emitting device is discharged.   8. The drive circuit according to claim 7, wherein the gate terminal of the transistor is connected to a control signal terminal, the drain terminal of the transistor is connected to a ground potential, and the transistor is turned on by the control signal. A drive circuit characterized in that electric charges are discharged to ground.   8. The driving circuit according to claim 7, wherein when the gate terminal of the transistor is connected to a control signal terminal, the drain terminal of the transistor is connected to a negative voltage, and the transistor is turned on by the control signal, A drive circuit characterized in that electric charges are discharged to a negative voltage terminal.   2. The drive circuit according to claim 1, wherein the light emitting device includes an organic light emitting diode (OLED).   The drive circuit according to claim 1, wherein the light emitting device includes a molecular light emitting diode. In a display device having a plurality of pixels, each pixel having a drive circuit that drives a light emitting device in each pixel, the drive circuit includes:
A light emitting device driving unit coupled to the light emitting device and selectively supplying a driving current to the light emitting device;
A discharge unit coupled to the light emitting device driving unit, the discharge unit discharging the light emitting device according to a voltage level of a control signal as soon as the light emitting device driving unit supplies a driving current to the light emitting device; Display device.   13. The display device according to claim 12, wherein the light emitting device driving unit is coupled to the light emitting device driving unit, receives a scan signal and a data signal, and when the scan signal and the data signal are at a logic level '1', A display device, further comprising a light emitting device selection unit that enables a drive current to be supplied to the light emitting device.   14. The display device according to claim 13, wherein the control signal uses a scan signal from the next pixel.   15. The display device according to claim 14, wherein the discharge unit in the drive circuit discharges the light emitting device when a scan signal in the next pixel is at a logic level '1', that is, a high voltage level. Display device.   13. The display device according to claim 12, wherein a discharge unit in the drive circuit is coupled to a ground potential so that electric charges are discharged from the light emitting device to the ground.   13. The display device according to claim 12, wherein a discharge unit in the drive circuit is coupled to a negative voltage so that electric charges are discharged from the light emitting device to a negative voltage terminal. In a driving method of a display device, wherein the display device has a plurality of pixels and drives a light emitting device in each pixel.
Selectively supplying a drive current to one of the light emitting devices;
A step of discharging the light emitting device in accordance with a voltage level of a control signal while the light emitting device is driven by a driving current.   19. The driving method according to claim 18, wherein the step of selectively supplying a driving current to one of the light emitting devices includes a scan signal and a data signal sent to the display device having a logic level of "1", that is, a high voltage level. If there is, a driving method comprising supplying a driving current to the light emitting device.   The driving method according to claim 19, wherein the control signal is supplied by a scan signal of a next pixel in the display device.

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