This application claims the benefit of priority to Taiwan Patent Application No. 100129312, filed on Aug. 16, 2011, in the Taiwan Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a compensation circuit of an organic light emitting diode, in particular to the compensation circuit capable of maintaining a stable brightness of the organic light emitting diode.
2. Description of the Related Art
Active-matrix organic light-emitting diode (AMOLED) display device featuring thin thickness, light weight, self-luminescence, low driving voltage, high efficiency, high contrast, high color saturation, quick response speed, and high flexibility is considered as the most promising rising display technology after the development of thin film transistor liquid crystal display device (TFT-LCD).
Since the brightness of the organic light emitting diode (OLED) components depends on the magnitude of current, it is necessary to control the current accurately in order to control pixel brightness accurately, thus incurring a much higher level of difficulty than the conventional TFT-LCD that controls the pixel brightness by controlling the voltage level of writing in pixels only.
In fact, the AMOLED also encounters many problems. With reference to
Due to the ageing and deterioration of the OLED, the cross voltage of the OLED will rise and the light emitting efficiency of the OLED will drop gradually after a long time of operation. The rising cross voltage of the OLED may affect the operation of the thin film transistor. The N-type thin film transistor is used for example. If the OLED is coupled to a source of the thin film transistor, and the cross voltage of the OLED rises, then the voltage source between the gate and source of the thin film transistor will be affected directly, and the current passing through will be affected directly, too. As to the light emitting efficiency, a long-time operation will cause the ageing and deterioration of the light emitting efficiency. An expected brightness cannot be achieved, even if the same current is passed through. The drop of the light emitting efficiency for three colors such as red (R), green (G) and blue (B) is different from one another, so that a color shift issue arises, and this problem is not a problem that cannot be solved easily, since the material cannot be improved easily.
As the size of the panel increases, the signal line becomes longer, and the internal resistance becomes increasingly more significant, the uniform brightness of the panel will be affected. This phenomenon is called an I-R Drop. With reference to
In view of the problems of the prior art, it is a primary objective of the present invention to provide a compensation circuit of an organic light emitting diode to overcome the problems including the brightness attenuation, the light emitting efficiency drop and the I-R drop of the conventional OLED.
To achieve the aforementioned objective, the present invention provides a compensation circuit of an organic light emitting diode, comprising a first capacitor, a second capacitor, a stabilizer unit, a third transistor, an organic light emitting diode and a driver unit. The first capacitor has an end which is a first node, and another end which is a second node. The second capacitor is coupled to a first power supply and a first node. The stabilizer unit is coupled to a first power supply, a second power supply, a first control signal and a second control signal, and the stabilizer unit includes a first transistor, a second transistor and a photodiode. The first transistor is coupled to the second transistor with a joint which is the first node. The second transistor is coupled to the photodiode. The third transistor is coupled to the first node, a data voltage and a third control signal. The organic light emitting diode is coupled to the first power supply or the second power supply. The driver unit is coupled to the first power supply or the second power supply, the second node, the organic light emitting diode, the second control signal and a fourth control signal. The driver unit includes a fourth transistor, a fifth transistor and a sixth transistor, and an end of the fourth transistor is coupled to an end of the fifth transistor with a joint which is the second node. Another end of the fourth transistor is coupled to another end of the fifth transistor, and the sixth transistor.
Wherein, the stabilizer unit is coupled to the first power supply and the first control signal through the first transistor, and coupled to the second control signal through the second transistor, and coupled to the second power supply through the input terminal of the photodiode; the driver unit is coupled to the second control signal through the fourth transistor, and coupled to the organic light emitting diode through the fifth transistor, and the organic light emitting diode is coupled to the first power supply, and the driver unit is coupled to the fourth control signal and the second power supply through the sixth transistor.
Wherein, the first transistor, the second transistor, the third transistor, the fourth transistor, the fifth transistor and the sixth transistor are a first P-type thin film transistor, a second P-type thin film transistor, a third P-type thin film transistor, a fourth P-type thin film transistor, a fifth P-type thin film transistor and a sixth P-type thin film transistor respectively.
Wherein, the first P-type thin film transistor is used for charging the first power supply to the first node; the second P-type thin film transistor is used for controlling time for the photodiode to discharge the first node; the third P-type thin film transistor is used for controlling the time of inputting the data voltage; the fourth P-type thin film transistor stores a voltage in the first capacitor at a compensation stage; the fifth P-type thin film transistor is used for driving the organic light emitting diode; the sixth P-type thin film transistor is used for charging a voltage of the second power supply plus a voltage difference of the sixth P-type thin film transistor to the second node at an initial reset stage.
Wherein, the stabilizer unit is coupled to the second power supply and the first control signal through the first transistor, and coupled to the second control signal through the second transistor, and coupled to the first power supply through an output terminal of the photodiode; the driver unit is coupled to the second control signal through the fourth transistor, and coupled to the organic light emitting diode through the fifth transistor, and the organic light emitting diode is coupled to the second power supply, and the driver unit is coupled to the fourth control signal and the first power supply through the sixth transistor.
Wherein, the first transistor, the second transistor, the third transistor, the fourth transistor, the fifth transistor and the sixth transistor are a first N-type thin film transistor, a second N-type thin film transistor, a third N-type thin film transistor, a fourth N-type thin film transistor, a fifth N-type thin film transistor and a sixth N-type thin film transistor respectively.
Wherein, the first N-type thin film transistor is used for discharging the first node to the second power supply; the second N-type thin film transistor is used for controlling time for the photodiode to charge the first node; the third N-type thin film transistor is used for controlling time of inputting the data voltage; the fourth N-type thin film transistor stores a voltage in the first capacitor at a compensation stage; the fifth N-type thin film transistor is used for driving the organic light emitting diode; the sixth N-type thin film transistor is used for charging a voltage equal to the first power supply minus a voltage difference of the sixth N-type thin film transistor to the second node at an initial reset stage.
In summation, the compensation circuit of an organic light emitting diode of the present invention uses the voltage at the node of the control circuit to drop the light emitting efficiency of the organic light emitting diode, and uses the compensation circuit to increase the IOLED to provide a brighter OLED component and achieve the compensation effect, so as to maintain the brightness stability of the OLED.
With reference to
In all TFT used as switches, T6 is used in an initial reset stage of the second node B for resetting the second node B to VTH
With reference to
1. Initial reset stage of the first node A: Reset[n] signal is Low, T1 is ON, and the first node A is pre-charged to VDD.
2. Initial reset stage of the second node B: Reset[n] signal is pulled High, T1 is OFF, Scan[n-1] and Emit[n] signals are switched to Low, T2, T4, T5 and T6 are ON, and the second node B is initialized to VTH
3. VTH detecting compensation stage: Scan[n-1] signal is maintained at Low, Emit[n] signal is pulled High, T6 is OFF, T2, T4 and T5 are still ON, the voltage of the second node B is charged to VDD−VTH
4. Writing in the pixel data voltage stage:
Scan[n-1] signal is pulled High, Scan[n] is switched to Low, T2 and T4 are OFF, T3 is ON, the pixel data voltage is written in, the second node B is at a floating state, the voltage of the first node A is changed from VA′ to VData, and its variation is VData−VA′ (which is a negative value), the second node B is changed to (VDD-VTH
5. OLED light emission display stage:
Scan[n] is switched to High, Emit[n] signal is pulled Low, T3 is OFF, T6 is ON, the current for conducting the T5 for driving the OLED depends on the light emitting brightness of the OLED, VGate
As to the I-R Drop, observations show that the pixels of the AMOLED at the input terminal and situated away from the VDD, VSS signals have a change of VDD, VSS to VDD−I*R, VSS+I*R. In other words, the first node A will pre-charge to a lower potential of VDD−I*R, and then the photodiode D is provided for discharging to a higher potential of VSS+I*R. In other words, the cross voltage of the photodiode D decreases, and the discharged current of the first node A also decreases, such that the VA ′not decrease too much due to the I-R Drop to achieve the effect of compensating the I-R Drop.
With reference to
In all TFT used as switches, the T60 is used for resetting the second node B to VDD−VTH
With reference to
1. Initial reset stage of the first node A: Reset[n] signal is High, T10 is ON, the first node A is pre-charged to VSS.
2. Initial reset stage of the second node B: Reset[n] signal is pulled Low, T10 is OFF, Scan[n-1] and Emit[n] signals are switched to High, the T20, T40, T50 and T60 are ON, the second node B is initialized to VDD−VTH
3. VTH detecting compensation stage:
Scan[n-1] signal is maintained at High, Emit[n] signal is pulled Low, T60 is OFF, T20, T40 and T50 are ON, the voltage of the second node B is discharged to VTH
4. Writing in pixel data voltage stage:
Scan[n-1] signal is pulled Low, Scan[n] is switched to High, T20 and T40 are OFF, T30 is ON, the pixel data voltage is written in, and now, the second node B is situated at a floating state, the first node A voltage changes from VA′ to VData, and the change is equal to VData−VA′ which is a positive value, and the second node B is changed to (VTH
5. OLED light emitting display stage:
Scan[n] is switched to Low, Emit[n] signal is pulled High, T30 is OFF, T60 is On, the T50 of the OLED is driven and whose conductive current determines the light emitting brightness of the OLED, and VGate
As to the I-R Drop, observations show that the pixels of the AMOLED at the input terminal and situated away from the VDD, VSS signals have a change of VDD, VSS to VDD−I*R, VSS+I*R. In other words, the first node A will pre-charge to a lower potential of VDD−I*R, and then the photodiode D is provided for discharging to a lower potential of VSS+I*R. In other words, the cross voltage of the photodiode D decreases, and the discharged current of the first node A also decreases, such that the VA′will not decrease too much due to the I-R Drop to achieve the effect of compensating the I-R Drop.
With reference to
In summation of the description above, the compensation circuit of an organic light emitting diode of the present invention can overcome the problems of a conventional OLED with the brightness attenuation, the light emitting efficiency drop and the I-R Drop.
Number | Date | Country | Kind |
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100129312 A | Aug 2011 | TW | national |
Number | Name | Date | Kind |
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8207918 | Kohno et al. | Jun 2012 | B2 |
20040252089 | Ono et al. | Dec 2004 | A1 |
20050285826 | Park et al. | Dec 2005 | A1 |
Number | Date | Country | |
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20130043796 A1 | Feb 2013 | US |