Patent Publication Number: US-2006007070-A1

Title: Driving circuit and driving method for electroluminescent display

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention generally relates to the field of electroluminescent displays, and more particularly to a driving circuit implemented to drive the operation of a light-emitting device of the electroluminescent display.  
      2. Description of the Related Art  
      Electroluminescent displays are subjected to intense researches and developments in the field of emissive displays. Compared to other types of emissive displays such as the plasma displays, the electroluminescent display provides many advantages such as a lower power consumption, a reduced size, and a high image brightness. An electroluminescent display system conventionally includes a mesh of scan and data lines that define an array of pixels in each of which is coupled one light-emitting device. The light-emitting device particularly can be an organic light-emitting device (OLED), and is usually driven by a driving circuit associated to each pixel.  
       FIG. 1A  is a schematic view of a coneventional driving circuit implemented in one pixel. The pixel driving circuit includes two transistors  102 ,  104 , a storage capacitor  108 , and an organic light-emitting device  106 . The transistors can be any type of transistor, such as thin film transistor or the like. The transistors  102 ,  104  can be NMOS transistors in the following description. The transistor  102  has a gate connected to a scan line SCAN, and a source connected to a data line DATA. The transistor  104  has a gate connected to the drain of the transistor  102 , a source connected to a power voltage V dd , and a drain connected to an electrode terminal of the OLED  106 . The other electrode terminal of the OLED  106  is connected to a common voltage V ss . The storage capacitor  108  is coupled between the drain and the gate of the transistor  104 .  
      In operation, a high voltage level of the scan line SCAN turns on the transistor  102 , and charge the storage capacitor  108  with the data line voltage DATA. As a result, the charged storage capacitor  108  turns on the transistor  104  that accordingly experiences the flow of an electric current towards the OLED  106 . The transistor  104  conventionally works in a saturation range, and the electric current I delivered to the OLED  106  can be expressed as follows: 
 
 I=k ( V   A   −V   B   −V   th ) 2   (1) 
          wherein k is a constant coefficient, V A  is the gate voltage of the transistor  104  at node A, V B  is the source voltage of the transistor  104  at node B, and V th  is the threshold voltage of the transistor  104 .        

      As illustrated in  FIG. 1B , it can be observed that the voltage V B  timely increases in operating the OLED  106 . One reason of this deviation includes an alteration of the transistor characteristics. As indicated by the expression (1), the increase of V B  results in a reduction of the electric current flowing across the OLED  106  and consequently affects the brightness of the light emitted from the OLED  106 . As a result, the service life of the OLED is adversely reduced.  
      Therefore, there is a need in the art for a pixel driving circuit that can improve the service life of the OLED.  
     SUMMARY OF THE INVENTION  
      The application describes a pixel driving circuit and a pixel driving method which can be implemented in an electroluminescent display system without the prior art problems. The electroluminescent display system includes a plurality of light-emitting devices respectively coupled with scan and data lines.  
      In one embodiment, the pixel driving circuit comprises a pixel driving circuit coupled with a scan line, a data line and one or more light-emitting device, and a voltage clamp circuit coupled between the pixel driving circuit and the light-emitting device. The pixel driving circuit when turned on in response to a scan signal issued on the scan line is configured to deliver to the light-emitting device an electric current set according to a data signal delivered through the data line.  
      The voltage clamp circuit is operable to apply a voltage potential at a connection node between the pixel driving circuit and the light-emitting device when an electric current is delivered to the light-emitting device. In one embodiment, the voltage potential applied at the connection node between the pixel driving circuit and the light-emitting device is constant.  
      In one embodiment, the pixel driving circuit includes a driving transistor having its source and drain coupled between the light-emitting device and a power voltage potential. The driving transistor is operated in a saturation range to deliver an electric current to the light-emitting device.  
      In another embodiment, a method of driving an electroluminescent display comprises operating the driving transistor in a saturated range to deliver an electric current to the light-emitting device of the selected pixel, wherein the electric current varies according to the level of the data signal, and applying a constant voltage bias between the gate and the drain of the driving transistor while the driving transistor is operated in the saturated range.  
      The foregoing is a summary and shall not be construed to limit the scope of the claims. The operations and structures disclosed herein may be implemented in a number of ways, and such changes and modifications may be made without departing from this invention and its broader aspects. Other aspects, inventive features, and advantages of the invention, as defined solely by the claims, are described in the non-limiting detailed description set forth below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1A  is a schematic diagram of a conventional pixel driving circuit implemented in an electroluminescent display according to the prior art;  
       FIG. 1B  is a graph depicting the characteristic bias of the driving circuit implemented in the prior art;  
       FIG. 2A  is a block diagram of a pixel circuit implemented in an electroluminescent display according to an embodiment of the invention;  
       FIG. 2B  is a circuit diagram illustrating a circuit implementation of the pixel circuit in an electroluminescent display according to an embodiment of the invention;  
       FIG. 3A  is a time chart describing the operation of a pixel driving circuit implemented in an electroluminescent display according to an embodiment of the invention; and  
       FIG. 3B  is a flowchart of a method of driving an electroluminescent display according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT(S)  
      The application describes a pixel driving circuit and a driving method implemented in an electroluminescent display. In the following description, numerous details are set forth in order to provide a thorough understanding of the present invention. It will be appreciated by one skilled in the art that many variations of these specific details are possible to achieve the inventive features as described herein.  
       FIG. 2A  is a block diagram of a pixel circuit implemented in an electroluminescent display according to an embodiment of the invention. The electroluminescent display can be exemplary an active matrix organic light-emitting display system. The pixel  200  includes a current driving circuit  202 , a switch circuit  204 , a voltage clamp circuit  206 , a storage capacitor C s , and one or more light emitting device  212 . The light-emitting device can be an organic light-emitting device. The current driving circuit  202  couples the storage capacitor C s  to one or more light-emitting device  212 . The switch circuit  204  operates to charge the storage capacitor C s  with a data signal DATA upon receiving a scan signal SCAN indicating the selection of the light-emitting device  212 . Both current driving circuit  202  and switch circuit  204  are thereby configured to deliver an electric current to the light-emitting device  212  according to the level of the data signal DATA.  
      The voltage clamp circuit  206  connects to the node Y that links the connecting the current driving circuit  202  to the light-emitting device  212 . The voltage clamp circuit  206  is operable to apply a voltage potential to the node Y when an electric current is delivered, according to the data signal stored in the storage capacitor C s , through the current driving circuit  202  to the light-emitting device  212 . The voltage control applied to the node Y can prevent undesirable variations of the electric current delivered to the light-emitting device  212 .  
       FIG. 2B  is a circuit diagram illustrating a circuit implementation of the pixel circuit according to an embodiment of the invention. The light-emitting device  212  is connected between a voltage potential V ss  and a voltage node Y, output of the current driving circuit  202 . The current driving circuit  202  includes a transistor T 1  having its source connected via a switch SW to a power voltage V dd , its drain connected via the node Y to the light-emitting device  212 , and its gate coupled with the storage capacitor C s . The switch circuit  204  includes a transistor T 2  having its gate connected to the scan line SCAN, and its source and drain coupled between the data signal DATA and the storage capacitor C s . The voltage clamp circuit  206  includes a transistor T 3  having its gate connected to the scan line SCAN, and its source and drain coupled between a reference voltage V ref  and the node Y.  
       FIG. 3A  is a time chart depicting the operation of the pixel driving circuit of  FIG. 2B  according to an embodiment of the invention. A high voltage level of the scan signal SCAN turns both transistors T 2 , T 3  to a conducting state, while the switch SW is open. Accordingly, the storage capacitor C s  is charged with the data signal voltage DATA and no electric current flows through the transistor T 1  to the light-emitting device  212 . The voltage node X is equal to about the data signal voltage DATA and the voltage node Y is equal to about V ref . A low voltage level of the scan signal SCAN turns both transistors T 2 , T 3  to a non-conducting state while the switch SW is turned on, i.e. in a conducting state. Accordingly, an electric current flows through the transistor T 1  to the light-emitting device  212 , and the gate-drain voltage (V XY ) of the transistor T 1  is equal to about (DATA−V ref ). With the transistor T 1  operating in a saturation range, the electric current I delivered to the light-emitting device  212  can be expressed as follows: 
   I=k (DATA− V   ref   −V   th ) 2   (2),           wherein k is a constant coefficient, and V th  is the threshold voltage of the transistor T 1 . Since the reference voltage V ref  imposed at the node Y is constant, the electric current I delivered to the light-emitting device  212  can be maintained constant according to the level of the data signal voltage DATA.        
       FIG. 3B  is a flowchart of a method of driving an electroluminescent display according to an embodiment of the invention. The electroluminescent display includes a plurality of light-emitting devices respectively coupled with scan and data lines SCAN, DATA. First, it is determined whether the scan signal SCAN associated with one light-emitting device is at a high voltage level, which can be logically expressed as “SCAN=1” ( 302 ). If SCAN=1, the data signal DATA coupled with the selected light-emitting device is stored ( 304 ). If SCAN=0, it is further determined whether a data signal DATA has been stored ( 306 ). If a data signal DATA is stored, an electric current is delivered to the light-emitting device according to the level of DATA while a constant voltage drop is applied to the light-emitting device ( 308 ). The light-emitting device can be thereby driven with a stable electric current.  
      Realizations in accordance with the present invention have been described in the context of particular embodiments. These embodiments are meant to be illustrative and not limiting. Many variations, modifications, additions, and improvements are possible. Accordingly, plural instances may be provided for components described herein as a single instance. Additionally, structures and functionality presented as discrete components in the exemplary configurations may be implemented as a combined structure or component. These and other variations, modifications, additions, and improvements may fall within the scope of the invention as defined in the claims that follow.