Active matrix organic electroluminescence display driving circuit

A driving circuit of active matrix organic electroluminescence display is disclosed. Each pixel includes four TFTS and two capacitors. A gate of scan TFT is controlled by the scan line of the row where the pixel is located and a drain of scan TFT is connected to the data line of the column where the pixel is situated. Reset TFT and detect TFT are controlled by one threshold-lock line. One capacitor Cd is used to store data voltage (Vdata) of image signals and the other capacitor Ct is used to store threshold voltage (Vth) of drive TFT. Therefore, the sum of capacitors Cd and Ct will drive TFT to output a corresponding current to the organic electro luminescence element.

FIELD OF THE INVENTION

This invention relates to a driving circuit of active matrix organic electroluminescence display. More particularly, the invention is directed to a driving device and method that improve the non-uniform phenomena on an active matrix organic light-emitting diode display panel.

BACKGROUND OF THE INVENTION

OLED Display can be classified according to its driving method, passive-matrix (PMOLED) and active-matrix (AMOLED). AMOLED uses TFT (Thin Film Transistor) with a capacitor for storing data signals that can control OLED levels of brightness.

Manufacturing procedure of PMOLED is simpler in comparison and is less costly of the two; however, it is limited in its size (<5 inch) because of its driving mode and has a lower-resolution display application. In order to produce an OLED display with higher resolution and larger size, utilizing active-matrix driving is necessary. The so-called AMOLED uses TFT (Thin Film Transistor) with a capacitor for storing data signals, so that pixels can maintain its brightness after line scanning; on the other hand, pixels of passive matrix driving only light up when scan line selects them. Therefore, with active matrix driving, the brightness of OLED is not necessarily ultra-bright, resulting in longer lifetime, higher efficiency and higher resolution. Naturally, TFT-OLED with active matrix driving is suitable for display application of higher resolution and excellent picture due to the unique qualities of OLED.

LTPS (Low Temperature Poly-Silicon) and a-Si (amorphous Silicon) are both technologies of TFT integrating on glass substrate. The obvious differences are electric characteristics and complexity of process. Although LTPS-TFT possesses higher carrier mobility and higher mobility means more current can be supplied, the process is much more complex. However, the process of a-Si TFT is simpler and maturer, except for low carrier mobility. Therefore, a-Si process has better competitive advantages in cost.

Due to limitations of LTPS process capability, threshold voltage and mobility of TFT elements produced vary leading to different properties of each TFT element. When the driving system achieves gray scale by analog voltage modulation, OLED produces different output current despite of the same data voltage signal input due to different TFT characteristics of various pixels. Therefore, luminance of OLED varies. Images of erroneous gray scale will show up on OLED panel and damage image uniformity seriously.

The most urgent problem of AMOLED to be solved currently is how to reduce bad impact of uneven LTPS-TFT characteristics. Such issue requires immediate solution for follow-up development and application since images on the display tell the difference.

U.S. Pat. No. 6,229,506 discloses an Active Matrix Light Emitting Diode Pixel Structure And Concomitant Method. A 4T2C (4 TFTS and 2 capacitors) pixel circuit is proposed as shown inFIG. 4. An Auto-Zero mechanism is applied to compensate for the threshold voltage differences of TFT elements to improve uniformity of images. Driving sequences of control signals include Auto-Zero Phase510, Load Data Phase520and Illuminate Phase530. Refer toFIG. 5for the sequences of control signals inFIG. 4.

Transistors T3and T4are off and Transistor T2is on prior to Auto-Zero Phase510. The current passing through OLED460at this moment is current of the previous frame and controlled by Vsg of Transistor T1(voltage difference between source and gate; i.e., voltage difference of both ends of Cs).

After entering Auto-Zero Phase510, Transistor T4is on and then Transistor T3is on, too so that Drain and Gate of Transistor T1can be connected as a diode. As Transistor T2is off, gate voltage of Transistor T1will increase, which equals to Vdd minus threshold voltage (Vth) of Transistor T1. That is to say, the voltage difference stored at both ends of capacitor Cs is the threshold voltage of Transistor T1. After placing Transistor T3off, threshold voltage (Vth) of Transistor T1can be stored into capacitor Cs and Auto-Zero Phase510is completed.

On Load Data Phase520, when voltage difference of Date Line410is ÄV, it can couple to the gate of Transistor T1through Transistor T4and capacitor Cc. Thus, voltage difference stored at both ends of capacitor Cs will be ÄV×[Cc/(Cc+Cs)] adding Vth that is stored in capacitor Cs previously. That is, Vsg of Transistor T1includes Vth of Transistor T1, which makes output current of Transistor T1relate to voltage change (ÄV) of Data Line410only, instead of being affected by Vth of transistor in every pixel.

Last when Illuminate Phase530begins, Transistor T4is off and Transistor T2is on. Output current of Transistor T1at the present frame will flow through OLED460to illuminate.

Though this 4T2C pixel circuit may compensate for the threshold voltage (Vth) differences of transistor elements in each pixel and improve integral uniformity of images; however, other control lines like Auto-Zero Line430and Illuminate Line440are required in addition to Data Line410, Scan Line420and Supply Line (Vdd)450. Capacitor Cs has to record all threshold voltages and part of data voltages loaded. Besides, capacitance coupling approach is used to load data, which not only makes driving method more complicated, but also increases manufacturing cost when non-standard data driving IC is required.

To solve the same problem, Philips also published a thesis with the subject of A Comparison of Pixel Circuits for Active Matrix Polymer/Organic LED Displays. One 4T2C pixel circuit is presented in the thesis asFIG. 6shows. It skillfully changes the location of connecting two capacitors in the pixel circuit of the U.S. Pat. No. 6,229,506 (FIG. 4) to solve the defects causing by complexity and impracticability. However, control lines like Auto-Zero Line630and Illuminate Line640are also required in addition to Data Line610, Scan Line620and Supply Line (Vdd)650.

The sequences of driving control signals are the same as the U.S. Pat. No. 6,229,506 since they consist of Auto-Zero Phase510, Load Data Phase520and Illuminate Phase530. Please refer toFIG. 5and the sequences of control signals inFIG. 6.

On Auto-Zero Phase510, Transistor T64is off and then Transistor T63is on so that Drain and Gate of Transistor T61can be connected as a diode. As Transistor T62is off, gate voltage of Transistor T61will increase, which equals to Vdd minus threshold voltage (Vth) of Transistor T61. That is to say, the sum of voltage stored at capacitors C1and C2is the threshold voltage (Vth) of Transistor T61. After placing Transistor T63off, Auto-Zero Phase510is completed.

Data voltage is conducted through connection of Transistor T64. Data voltage is stored in Capacitor C1and a certain proportion of Vth previously stored at both ends of Capacitor C2is still maintained, which equals to [C1/(C1+C2)]×Vth. Thus, the sum of capacitors C1and C2is (Vdd−Vdata+[C1/(C1+C2)]×Vth); i.e., Vsg of Transistor T61contains part of Vth of Transistor T61, which may not only reduce the correlation between the output current and threshold voltage of Transistor T61, but also compensate for part of the threshold voltage (Vth) difference resulted from process factors. The threshold voltage of Transistor T61in the thesis is memorized by two capacitors (C1& C2). Part of threshold voltage data stored in one of the capacitors will get lost while loading data voltage. Therefore, this approach can only make up for part of threshold voltage difference resulted from process.

SUMMARY OF THE INVENTION

The main purpose of this invention is to solve the aforementioned problems existed for a long time. As the critical component parts of AMOLED like TFT-OLED Data IC are not well developed, the well developed technology of TFT-LCD Source IC can be applied to support TFT-OLED application. However, TFT-LCD Source IC adopts voltage modulation; thus, design of a voltage driving circuit is required.

Hence, a voltage type of AMOLED driving circuit that can compensate for TFT threshold voltage variations is presented in this invention so as to improve image defects resulted from uneven characteristics of TFT.

To achieve the objective above, a driving device of each pixel presented in this invention includes 4 TUFTS and 2 capacitors, which are 1 scan TFT, 1 reset TFT, 1 detect TFT, 1 drive TFT, 2 capacitors (Cd & Ct) and 1 organic electro-luminescence element. The gate of scan TFT is controlled by the scan line of the row where the pixel is located and the drain of scan TFT is connected to the data line of the column where the pixel is situated. Reset TFT and detect TFT are controlled by one threshold-lock line. Capacitor Cd is used to store data voltage (Vdata) of image signals and capacitor Ct is used to store threshold voltage (Vth) of Drive TFT. Therefore, the sum of voltage stored in capacitors Cd and Ct will force Drive TFT to output an corresponding current to the organic electro luminescence element.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Refer toFIG. 1for the circuit of each pixel in this invention. As the Figure shows: the driving circuit of pixel200includes 4 TFTS and 2 capacitors connected as follows:

Gate of a Scan TFT210connected to one Scan Line120and drain connected to a Data Line110.

Gate of a Reset TFT220connected to a Threshold-Lock130, source connected to a Supply Line150and drain connected to source of Scan TFT210.

Two ends of Capacitor Cd installed between source of Scan TFT210and source of Reset TFT220.

Source of Drive TFT240connected to Supply Line150.

Gate of Detect TFT230connected to Threshold-Lock130, drain connected to the gate of Drive TFT240and source connected to the drain of Drive TFT240.

Two ends of Capacitor Ct installed between drain of Reset TFT220and gate of Drive TFT240.

Anode of an organic electro luminescence element250connected to the drain of Drive TFT240and cathode connected to a Common Line140.

Refer toFIG. 2for connection and control of a pixel circuit in this invention. As the Figure shows: a joint where a scan line120(S1, S2, S3. . . Sn) and a data line110(D1, D2, D3. . . Dm) meet is a pixel200. Refer toFIG. 1,FIG. 2. The gate of Scan TFT210is controlled by Scan Line120of the row where Pixel200is located, and the drain of Scan TFT210is connected to Data Line110of the column where Pixel200is situated. Reset TFT220and Detect TFT230are controlled by Threshold-Lock130. Capacitor Cd is used to store data voltage (Vdata) of image signals and Capacitor Ct is used to store threshold voltage (Vth) of Drive TFT240. Therefore, the sum of voltage stored in capacitors Cd and Ct will force Drive TFT240for an output of corresponding current to the organic electro luminescence element250.

Reset TFT220and Detect TFT230of each Pixel200circuit on Display Substrate100are controlled by the same Threshold-Lock130and cathode of organic electro luminescence element250in every Pixel200is jointly connected to a Common Line140, which is connected to the grounding end of the system via an external switch170controlled by a display signal line160. Source of Drive TFT240in each Pixel200circuit is jointly connected to a supply line (Vdd)150.

Actuation procedures of this invention are described as follows:

Refer toFIG. 3for the sequences of control signals in this invention. A cycle of driving signals can be divided into three phases. First, Threshold-Lock Phase310:

Signals of Threshold-Lock130will trigger Reset TFT220and Detect TFT230in every pixel circuit on. When Reset TFT220is on, Capacitor Cd storing voltage of image data will discharge. Display Signal Line160controls Switch170outside of Substrate100and makes it off. Thus, an open circuit exists between Common Line140and the grounding end of the system. Current of Drive TFT240stops flowing through organic electro luminescence element250, but through Detect TFT230that is on at this moment, which forces Drive TFT240to detect threshold voltage. As current of Drive TFT240passes by Detect TFT230, Capacitor Ct and Reset TFT220, voltage stored in Capacitor Ct becomes smaller and smaller, which also makes current of Drive TFT240become smaller until no current is left.

At last, Capacitor Cd won't store any electric charge (0 voltage on both ends) and voltage difference on both ends of Capacitor Ct will equal to threshold voltage (Vth) of Drive TFT240; i.e. when Capacitor Cd discharges and resets, Capacitor Ct will memorize threshold voltage (Refer toFIG. 1for Pixel200circuit.). In summary, threshold voltage (Vth) of Drive TFT240in every Pixel200circuit will be stored in its own Capacitor Ct after Threshold-Lock Phase310.

Next, signals of Threshold-Lock130will trigger Reset TFT220and Detect TFT230in every Pixel200circuit off for the following Write Phase320.

In Write Phase320, each Scan Line120(S1, S2. . . Sn) will send out scan signals in order. When scan signals shift to Scan Line120, all Scan TFT210on the same scan line will be on and Reset TFT220and Detect TFT230will be off. Data voltage (Vdata) of Data Line110can be stored into Capacitor Cd as Scan TFT210is on; however, threshold voltage (Vth) previously memorized by Capacitor Ct will still be retained as Reset TFT220and Detect TFT230are off. Thus, voltage difference between two ends of Capacitor Cd will be equivalent to supply voltage (Vdd) minus data voltage (Vdata); i.e. voltage at both ends of Capacitor Cd is (Vdd−Vdata).

Therefore, the sum of voltage stored in capacitors Cd and Ct will equal to (Vdd−Vdata+Vth), which will enable Drive TFT240to output corresponding current to organic electro luminescence element250in the following phase (Display Phase330). Consequently, the current (I) can be expressed with a formula as follows:
I=(½)×â×(Vsg−Vth)2
I=(½)×â×(Vdd−Vdata+Vth−Vth)2
I=(½)×â×(Vdd−Vdata)2

From the above equations (â is the Transconductance Parameter of Drive TFT240), the current (I) generated by Drive TFT240is irrelevant to the threshold voltage (Vth) of its own, but only correlated to write data voltage (Vdata). Thus, threshold voltage differences of TFT resulted from process factors can be compensated.

When the last Scan Line120(Sn) completes writing data voltage (Vdata), Display Signal Line160will control Switch170and make it on and Common Line140will be connected to the grounding end of the system for the third stage of Display Phase330.

In Display Phase330, Drive TFT240in each Pixel200circuit will output Current (I) related to written data voltage (Vdata) to organic electro luminescence element250, which produces proper luminance. Output Current (I) is not related to threshold voltage (Vth) of Drive TFT240.

In comparison with the U.S. Pat. No. 6,229,506, the technology of loading data voltage realized in this invention can be applied to TFT-LCD Source IC (Voltage Mode) that is popular currently and avoids complexity.

To compare with the thesis published by PHILIPS with the subject of A Comparison of Pixel Circuits for Active Matrix Polymer/Organic LED Displays, the technology of this invention is to record all threshold voltage into one capacitor (capacitor Ct) to offset the effect of threshold voltage differences.

Furthermore, as the critical component parts of AMOLED like TFT-OLED Data IC are not well developed at present, the well-developed technology of TFT-LCD Source IC is required to support TFT-OLED application. However, TFT-LCD Source IC adopts voltage modulation; thus, design of a voltage driving circuit is required.

Two capacitors (Cd & Ct) are used in this invention to deal with two different things. One capacitor Ct is responsible to record all threshold voltage values (Vth) and the other capacitor Cd is in charge of recording all data voltage values (Vdata). It is different from U.S. Pat. No. 6,229,506 as the capacitor Cs has to record all threshold voltage (Vth) and part of data voltage (Vdata) loaded. It is also different from the thesis released by PHILIPS as capacitors C1and C2record threshold voltage jointly. Part of threshold voltage stored in Capacitor C1will be lost since Capacitor C2only records part of threshold voltage.

To conclude, the AMOLED driving circuit of this invention has the following advantages:

1. As all threshold voltage values (Vth) can be stored in one capacitor Ct (threshold voltage storage capacitor), the effects of threshold voltage differences can be compensated completely.

2. The technology of loading data voltage (Vdata) realized in this invention can be achieved by TFT-LCD Source IC (Voltage Mode) that is popular currently and avoids complexity.