1. Field of the Invention
The present invention generally relates to a pixel circuit of a light emitting diode display, and more particularly to a pixel circuit of a light emitting diode (LED) display and a driving method thereof for totally solving issues of LED current drop, luminous efficiency drop and IR drop due to the larger size of the display.
2. Description of Prior Art
The LCD is the mainstream of display technology. However, the OLED display is considered to replace the LCD by related industry and is going to be the next generation display. Comparing with the LCD, the OLED display possesses tons of advantages. For example, self lighting, wide view angle, rapid response time, high brightness, high luminous efficiency, low operating voltage, thin panel, flexibility, few processes, low cost and et cetera.
However, the biggest difference between the OLED and the LCD is that the brightness of the OLED is determined by the current. Therefore, for precisely determining the brightness of the pixel requires to precisely controlling the current IOLED. Comparing with the LCD which merely requires the exact control to the voltage level of the pixel for determining the brightness, the precise control to the current IOLED is much difficult.
Please refer to
I
OLED=½*W/L*μN*COX(VGS−VTH)2 (equation 1)
COX represents the unit-area capacitance. W and L represent the width and the length of the thin film transistor T2. IOLED represents the current as a voltage Vdata is supplied to the thin film transistor T2. When the usage time of the OLED extends, some reasons why the IOLED in the foregoing formula changes are: the threshold voltage VTH of the thin film transistor T2 increases or the mobility μN decreases. Then, the IOLED drops and leads to the brightness decay of the OLED.
Furthermore, the material of the OLED certainly ages after the long usage time. The voltage of the OLED increases and the luminous efficiency of the OLED drops. The increase of the OLED voltage also can affect the operation of the thin film transistor. Such as the N type thin film transistor T2 shown in
Furthermore, the luminous efficiency of the OLED drops because the material of the OLED certainly ages after the long usage time. Due to the luminous efficiency drop, even the same current flowing through the OLED, the expected brightness cannot be realized. Besides, the luminous efficiency drops of the tricolor are different and more serious issue of the color cast occurs.
Moreover, with the size of the display becomes larger, an IR drop issue occurs. Please refer to
Consequently, there is a need to develop a pixel circuit of a light emitting diode (LED) display and a driving method thereof for totally solving issues of the LED current drop, the luminous efficiency drop and the IR drop due to the larger size of the display.
An objective of the present invention is to provide a pixel circuit of a light emitting diode (LED) display and a driving method thereof to totally solve the issues the LED current drop, the luminous efficiency drop and the IR drop due to the larger size of the display.
For realizing the aforesaid objective, the present invention provides a pixel circuit of a light emitting diode display. The light emitting diode display has a data line, an emit line and a scan line, respectively coupled to the pixel circuit and has an operating voltage and a grounding voltage provided to the pixel circuit. The pixel circuit of the light emitting diode display comprises: a first thin film transistor, employed as being a driving thin film transistor and having a first end and a second end, and the first end of the first thin film transistor is source; a light emitting diode, having a first end and a second end, and the first end of the light emitting diode is anode to be coupled to the first end of the first thin film transistor to be driven by the first thin film transistor; a second thin film transistor, having a first end and a second end, and a gate of the second thin film transistor is coupled to the emit line and the first end of the second thin film transistor is supplied with the operating voltage, and the second end of the second thin film transistor is coupled to the second end of the first thin film transistor where a first node is formed therebetween; a third thin film transistor, having a first end and a second end, and a gate of the third thin film transistor is coupled to the scan line, and the first end of the third thin film transistor is coupled to the first node, and the second end of the third thin film transistor is coupled to a gate of the first thin film transistor where a second node is formed therebetween; a fourth thin film transistor, having a first end and a second end, and a gate of the fourth thin film transistor is coupled to the scan line, and the first end of the fourth thin film transistor is coupled to the data line to control an input interval of the data line; a fifth thin film transistor, having a first end and a second end, and a gate of the fifth thin film transistor is coupled to the emit line, and the first end of the fifth thin film transistor is coupled to the second end of the fourth thin film transistor where a third node is formed therebetween, and the second end of the fifth thin film transistor is coupled to the second end of the light emitting diode; and a compensation capacitance, having a first end and a second end, and the first end of the compensation capacitance is coupled to the third node, and the second end of the compensation capacitance is coupled to the second node; wherein the second thin film transistor initializes voltage levels of the first node and the second node to be maintained at the operating voltage, and the third thin film transistor saves a compensation voltage of the second node in the compensation capacitance, and the fifth thin film transistor constantly discharges to the first end of the compensation capacitance to maintain a voltage level of the third node.
The present invention provides another pixel circuit of a light emitting diode display. The pixel circuit of the light emitting diode display comprises a first thin film transistor, employed as being a driving thin film transistor and having a first end and a second end, and the first end of the first thin film transistor is source; a light emitting diode, having a first end and a second end, and the first end of the light emitting diode is supplied with the operating voltage, and the second end of the light emitting diode is cathode to be coupled to the first end of the first thin film transistor to be driven by the first thin film transistor; a second thin film transistor, having a first end and a second end, and a gate of the second thin film transistor is coupled to the emit line and the first end of the second thin film transistor is supplied with the grounding voltage, and the second end of the second thin film transistor is coupled to the second end of the first thin film transistor where a first node is formed therebetween; a third thin film transistor, having a first end and a second end, and a gate of the third thin film transistor is coupled to the scan line, and the first end of the third thin film transistor is coupled to the first node, and the second end of the third thin film transistor is coupled to a gate of the first thin film transistor where a second node is formed therebetween; a fourth thin film transistor, having a first end and a second end, and a gate of the fourth thin film transistor is coupled to the scan line, and the first end of the fourth thin film transistor is coupled to the data line to control an input interval of the data line; a fifth thin film transistor, having a first end and a second end, and a gate of the fifth thin film transistor is coupled to the emit line, and the first end of the fifth thin film transistor is coupled to the second end of the fourth thin film transistor where a third node is formed therebetween, and the second end of the fifth thin film transistor is coupled to the first end of the light emitting diode; and a compensation capacitance, having a first end and a second end, and the first end of the compensation capacitance is coupled to the third node, and the second end of the compensation capacitance is coupled to the second node; wherein the second thin film transistor initializes voltage levels of the first node and the second node to be maintained at the grounding voltage, and the third thin film transistor saves a compensation voltage of the second node in the compensation capacitance, and the fifth thin film transistor constantly charges to the first end of the compensation capacitance to maintain a voltage level of the third node.
Furthermore, the present invention also provides a driving method of a pixel, employed for a pixel circuit having a data line, an emit line and a scan line, respectively coupled to the pixel circuit and having an operating voltage and a grounding voltage provided thereto, and the pixel circuit comprising a first thin film transistor, a light emitting diode, a second thin film transistor, a third thin film transistor, a fourth thin film transistor, a fifth thin film transistor and a compensation capacitance, and a first end of the first thin film transistor is coupled to the first end of the light emitting diode to drive the light emitting diode, and a second end of the second thin film transistor is coupled to the second end of the first thin film transistor wherein a first node is formed therebetween, and a second end of the third thin film transistor is coupled to a gate of the first thin film transistor where a second node is formed therebetween, and a first end of the fifth thin film transistor is coupled to a second end of the fourth thin film transistor where a third node is formed therebetween, and a first end of the compensation capacitance is coupled to the third node, and a second end of the compensation capacitance is coupled to the second node, the driving method comprising steps of: providing the grounding voltage to the emit line and the scan line and conducting the first thin film transistor, the second thin film transistor, the third thin film transistor, the fourth thin film transistor and the fifth thin film transistor to initialize voltage levels of the first node and the second node to be maintained at the operating voltage; providing the operating voltage to the emit line and cutting off the second thin film transistor and the fifth thin film transistor to provide a pixel data voltage to the data line to make the first node and the second node are discharged through the first thin film transistor and the light emitting diode; and providing the operating voltage to the scan line and the grounding voltage to the emit line, and cutting off the third thin film transistor and the fourth thin film transistor, and conducting the second thin film transistor and the fifth thin film transistor to utilize the compensation capacitance for coupling the voltage level of the third node with the voltage level of the second node to be provided to the first thin film transistor for driving the light emitting diode.
In the present invention, the gate of the first thin film transistor (the second node) is discharged through the first thin film transistor and the light emitting diode in the compensation and data writing stage operation. The voltage VB drops from VDD to (VDD−VDischarge). As mentioned in formula 1, the threshold voltage VTH of the first thin film transistor increases and the mobility μN decreases as the usage time of the display pixel extends, the size of the display becomes larger to induce IR drop and voltage VSS becomes larger to induce the discharge current drop, the present invention is capable of decreasing the voltage VDischarge and increases the voltage VB and therefore compensating the IOLED drop to prevent the brightness decrease of the OLED.
Moreover, the stress times of the first thin film transistor and the fifth thin film transistor are similar. Therefore, the characteristic of the threshold voltage increase exists for both. Because the threshold voltage increases with usage time, the compensation of the OLED luminous efficiency drop can be realized.
Consequently, the pixel circuit of a light emitting diode display and the driving method thereof according to the present invention is capable of totally solving the issues of the LED current drop, the luminous efficiency drop and the IR drop due to the larger size of the display and more beneficial to the development trend of larger size display in the future.
For a better understanding of the aforementioned content of the present invention, preferable embodiments are illustrated in accordance with the attached figures for further explanation:
Please refer to
The first thin film transistor T1 is employed as being a driving thin film transistor of the organic light emitting diode and has a first end and a second end. The first end of the first thin film transistor T1 is source.
The organic light emitting diode has a first end and a second end. The first end of the organic light emitting diode is anode to be coupled to the first end of the first thin film transistor T1 to be driven by the first thin film transistor T1.
The second thin film transistor T2 has a first end and a second end. The gate of the second thin film transistor T2 is coupled to the emit line and the first end of the second thin film transistor T2 is supplied with the operating voltage VDD. The second end of the second thin film transistor T2 is coupled to the second end of the first thin film transistor T1. A first node A is formed between the first thin film transistor T1 and the second thin film transistor T2.
The third thin film transistor T3 has a first end and a second end. The gate of the third thin film transistor T3 is coupled to the scan line. The first end of the third thin film transistor T3 is coupled to the first node A and the second end of the third thin film transistor T3 is coupled to the gate of the first thin film transistor T1. A second node B is formed between the first thin film transistor T1 and the third thin film transistor T3.
The fourth thin film transistor T4 has a first end and a second end. The gate of the fourth thin film transistor T4 is coupled to the scan line (Scan[n]). The first end of the fourth thin film transistor T4 is coupled to the data line (Data) to control an input interval of the data line (Data).
The fifth thin film transistor T5 has a first end and a second end. The gate of the fifth thin film transistor T5 is coupled to the emit line (Emit[n]). The first end of the fifth thin film transistor T5 is coupled to the second end of the fourth thin film transistor T4. A third node C is formed between the fourth thin film transistor T4 and the fifth thin film transistor T5. The second end of the fifth thin film transistor T5 is coupled to the second end of the organic light emitting diode.
The compensation capacitance Cc has a first end and a second end. The first end of the compensation capacitance Cc is coupled to the third node C and the second end of the compensation capacitance Cc is coupled to the second node B.
In the first embodiment, the gate of the first thin film transistor T1 (the second node B) is discharged through the first thin film transistor T1 and the organic light emitting diode in the compensation and data writing stage operation. Therefore, the voltage VA of the first node A and the voltage VB of the second node B are discharged and changed from VDD to (VDD−VDischarge). As in cases that the usage time of the display pixel extends, the threshold voltage VTH of the first thin film transistor T1 increases and the mobility μN decreases, the voltage of the organic light emitting diode increases, or the size of the display becomes larger to induce IR drop and voltage VSS becomes larger and causes to induce the discharge current IDischarge drop. In the aforementioned three scenarios, the IOLED drop occurs and the brightness of the organic light emitting diode decrease. However, the present invention enables to decrease the voltage VDischarge and increases the voltage VB, and therefore to compensate the IOLED drop. Moreover, stress times of the fifth thin film transistor T5 and the first thin film transistor T1 for driving the organic light emitting diode are similar. Therefore, the characteristic of the threshold voltage increase exists for both. Because the threshold voltage VTH
Please refer to
Please refer to
The initializing stage
providing the grounding voltage VSS to the emit line (Emit[n]) and the scan line (Scan[n]) and conducting the first thin film transistor T1, the second thin film transistor T2, the third thin film transistor T3, the fourth thin film transistor T4 and the fifth thin film transistor T5 to initialize voltage levels of the first node A and the second node B to be maintained at the operating voltage VDD. At this moment, the VData is VSS and the voltage at the third node C is the smaller one of the VSS+VTH
The compensation and data writing stage
providing the operating voltage VDD to the emit line (Emit[n]) and cutting off the second thin film transistor T2 and the fifth thin film transistor T5 to provide a pixel data voltage to the data line (Data). At this moment, the voltage VC of the third node C becomes VData. The first node A and the second node B are discharged to the grounding voltage VSS through the first thin film transistor T1 and the organic light emitting diode (OLED). The voltage VA of the first node A and the voltage VB of the second node B are changed from VDD to VDD−VDischarge. Meanwhile, the discharge is controlled within a predetermined interval to prevent the first node A and the second node B discharged completely. With the incomplete discharge characteristic according to the present invention, the effect of the mobility μN drop can be compensated (complete discharge cannot compensate the mobility μN drop). Moreover, the incomplete discharge characteristic also can shorten the react time of the display in advance;
The OLED lighting stage
providing the operating voltage VDD to the scan line (Scan[n]) and the grounding voltage VSS to the emit line (Emit[n]), and cutting off the third thin film transistor T3 and the fourth thin film transistor T4, and conducting the second thin film transistor T2 and the fifth thin film transistor T5. The second node B becomes a floating status and the voltage VC of the third node C is changed from VData to VSS+VTH
In the foregoing equation 2 of the current IOLED of the organic light emitting diode, VTH
Please refer to
The first thin film transistor T1 is employed as being a driving thin film transistor of the organic light emitting diode OLED and has a first end and a second end. The first end of the first thin film transistor T1 is source.
The organic light emitting diode has a first end and a second end. The first end of the organic light emitting diode (OLED) is supplied with the operating voltage VDD. The second end is cathode to be coupled to the first end of the first thin film transistor T1 to be driven by the first thin film transistor T1.
The second thin film transistor T2 has a first end and a second end. The gate of the second thin film transistor T2 is coupled to the emit line and the first end of the second thin film transistor T2 is supplied with the grounding voltage VSS. The second end of the second thin film transistor T2 is coupled to the second end of the first thin film transistor T1. A first node A is formed between the first thin film transistor T1 and the second thin film transistor T2.
The third thin film transistor T3 has a first end and a second end. The gate of the third thin film transistor T3 is coupled to the scan line. The first end of the third thin film transistor T3 is coupled to the first node A and the second end of the third thin film transistor T3 is coupled to the gate of the first thin film transistor T1. A second node B is formed between the first thin film transistor T1 and the third thin film transistor T3.
The fourth thin film transistor T4 has a first end and a second end. The gate of the fourth thin film transistor T4 is coupled to the scan line (Scan[n]). The first end of the fourth thin film transistor T4 is coupled to the data line (Data) to control an input interval of the data line (Data).
The fifth thin film transistor T5 has a first end and a second end. The gate of the fifth thin film transistor T5 is coupled to the emit line (Emit[n]). The first end of the fifth thin film transistor T5 is coupled to the second end of the fourth thin film transistor T4. A third node C is formed between the fourth thin film transistor T4 and the fifth thin film transistor T5. The second end of the fifth thin film transistor T5 is coupled to the second end of the organic light emitting diode (OLED).
The compensation capacitance Cc has a first end and a second end. The first end of the compensation capacitance Cc is coupled to the third node C and the second end of the compensation capacitance Cc is coupled to the second node B. In the second embodiment, the gate of the first thin film transistor T1 (the second node B) is charged through the first thin film transistor T1 and the organic light emitting diode in the compensation and data writing stage operation. Therefore, the voltage VA of the first node A and the voltage VB of the second node B are charged and changed from VSS to (VSS+VCharge). As in cases that the usage time of the display pixel extends, the threshold voltage VTH of the first thin film transistor T1 increases and the mobility μN decreases, the voltage of the organic light emitting diode increases, or the size of the display becomes larger to induce IR drop and voltage VDD becomes smaller and causes to induce the charge current ICharge drop. In the aforementioned three scenarios, the IOLED drop occurs and the brightness of the organic light emitting diode decrease. However, the present invention enables to decrease the voltage VCharge and increases the voltage VB, and therefore to compensate the IOLED drop. Moreover, stress times of the fifth thin film transistor T5 and the first thin film transistor T1 for driving the organic light emitting diode are similar. Therefore, the characteristic of the threshold voltage increase exists for both. Because the threshold voltage VTH
Please refer to
Please refer to
The initializing stage
providing the operating voltage VDD to the emit line (Emit[n]) and the scan line (Scan[n]) and conducting the first thin film transistor T1, the second thin film transistor T2, the third thin film transistor T3, the fourth thin film transistor T4 and the fifth thin film transistor T5 to initialize voltage levels of the first node A and the second node B to be maintained at the grounding voltage VSS. At this moment, the VData is VSS and the voltage at the third node C is the smaller one of the VSS−VTH
The compensation and data writing stage
providing the grounding voltage VSS to the emit line (Emit[n]) and cutting off the second thin film transistor T2 and the fifth thin film transistor T5 to provide a pixel data voltage to the data line (Data). At this moment, the voltage VC of the third node C becomes VData. The first node A and the second node B are charged to the operating voltage VDD through the first thin film transistor T1 and the organic light emitting diode (OLED). The voltage VA of the first node A and the voltage VB of the second node B are changed from VSS to VSS−VCharge. Meanwhile, the charge is controlled within a predetermined interval to prevent the first node A and the second node B charged completely. With the incomplete charge characteristic according to the present invention, the effect of the mobility μP drop can be compensated (complete charge cannot compensate the mobility μP drop). Moreover, the incomplete charge characteristic also can shorten the react time of the display in advance;
The OLED lighting stage
providing the grounding voltage VSS to the scan line (Scan[n]) and the operating voltage VDD to the emit line (Emit[n]), and cutting off the third thin film transistor T3 and the fourth thin film transistor T4, and conducting the second thin film transistor T2 and the fifth thin film transistor T5. The second node B becomes a floating status and the voltage VC of the third node C is changed from VData to VSS−NTH
In the foregoing equation 2 of the current IOLED of the organic light emitting diode, VTH
As is understood by a person skilled in the art, the foregoing preferred embodiments of the present invention are illustrative rather than limiting of the present invention. It is intended that they cover various modifications and similar arrangements be included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structure.
Number | Date | Country | Kind |
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100110224 | Mar 2011 | TW | national |