Patent Publication Number: US-7719494-B2

Title: Brightness adjustment circuit and electroluminescent display using the same

Description:
RELATED APPLICATIONS 
   The present application is based on, and claims priority from, Taiwan Patent Application Serial Number 95113490, filed Apr. 14, 2006, the disclosure of which is hereby incorporated by reference herein in its entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The invention relates to a brightness adjustment circuit, and in particular, to a brightness adjustment circuit for an electroluminescent display. 
   2. Related Art 
   In the industry of flat-panel displays, organic light-emitting diode (OLED) displays have received a lot of attention in recent years due to its self-luminescence, high brightness, super-wide viewing angle, high response speed, low working voltage, and light weight. However, the electroluminescent displays only have a limited market to date. Various display manufacturers have been trying to find an optimal manufacturing method for improving the yield product properties in order to make them more popular. 
   The OLED display is a self-luminescent flat-panel display. Its light source is produced by converting from a current flowing through the OLED pixels. By adjusting the current, we can determine the maximum brightness of the panel. Different gray levels are defined by the coupled thin film transistors (TFTs). The display thus achieves full colors and multiple color levels. 
   The working voltage (V dd  or V ss ) of each pixel unit of the OLED is supplied by an external source. The power consumption required by all the light-emitting units in the OLED panel is provided by this single power supply system. In the circuit design, all the working voltage input terminals V dd  of all the pixels are connected in parallel. Likewise, the terminals V ss  are all coupled in parallel, before individually guiding them to the outermost edge of the panel and to the external power supply system via a flexible print circuit (FPC) or wires. 
   The external power supply is designed as a stable voltage source system. That is, it is designed to have a fixed voltage across its output terminals. This ensures that the output voltage does not vary due to an unstable input voltage or noise interference. The ordinary stable voltage source system uses a voltage feedback control means.  FIG. 1  is a schematic view of a conventional feedback stable voltage power supply unit. The power supply unit  100  has a power supply  102  and a feedback circuit. The power supply  102  has a voltage output terminal V out  and a feedback terminal  108 . The voltage output terminal V out  provides the work voltage (V dd  or V ss ) required by the pixel units (not shown). The feedback circuit includes two resistors  104 ,  106  and is connected to the pixel units in parallel. One end of the resistor  104  is electrically coupled to the voltage output terminal V out , and one end of the resistor  106  is electrically coupled to a reference voltage source. The feedback terminal  108  is electrically coupled between the resistor  104  and the resistor  106 . The resistors  104 ,  106  have the effect of dividing the potential difference. The purpose is to produce a reference voltage (generally 1.25V) set for the system and feed it to the feedback terminal  108 , monitoring the stability of the output voltage. When the reference voltage increases, the feedback terminal  108  sends out a signal to reduce the output voltage of the voltage output terminal V out , bringing the reference voltage back to 1.25V. On the other hand, when the reference voltage decreases, the feedback terminal  108  sends out a signal to increase the output of the output voltage terminal V out . In either case, the reference voltage is brought back to 1.25V. 
   However, the prior art shown in  FIG. 1  only controls the stability in the brightness of the pixel units instead of adjusting the brightness. For solving problems in lifetime and residual images, some operating mode has to be provided to elongate the lifetime of the material and panel. For example, it is better not to have images with fixed positions and not to use high-brightness images for a long time. High brightness and continuous ON of the OLED are the primary reasons for shortening the panel lifetime. In view of the two consequences mentioned above, one can adopt the scheme of intermittent light-ups, e.g. 10 or 20 seconds. Afterwards, the brightness of the screen is reduced to one half or ⅓. This method can increase the panel lifetime without sacrificing its practical uses. 
   Therefore, a circuit that can adjust the brightness of the pixel units has been proposed in the prior art. Suppose the pixel units are required to switch among four different brightness modes, the feedback stable power supply unit has to provide a switch for four voltage outputs. A conventional method is to provide four different feedback resistor sets along with a channel switch for producing different output voltages. As shown in  FIG. 2 , the conventional power supply unit  200  that can adjust the brightness of the pixel units has a power supply  202  and a feedback circuit. The power supply  202  has a voltage output terminal V out  and a feedback terminal  204 . The voltage output terminal V out  provides the working voltage (V dd  or V ss ) required by the pixel units (not shown). In particular, V dd  is used on the pixel units driven by P-type metal oxide semiconductor (MOS) transistors, whereas V ss  is used on the pixel units driven by N-type MOS transistors. The feedback circuit includes four sets of serial resistors connected in parallel. Each set has two resistors in series and is connected with the pixel unit in parallel. The four sets of serial resistors include resistors  208 ,  210 , resistors  214 ,  216 , resistors  220 ,  222 , and resistors  226 ,  228 . One end of each of the resistors  208 ,  214 ,  220 ,  226  is electrically coupled to the voltage output terminal V out , and one end of each of the resistors  210 ,  216 ,  222 ,  228  is electrically coupled to a reference voltage. The four sets of serial resistors  208  and  210 ,  214  and  216 ,  220  and  222 , and  226  and  228  have connection points  212 ,  218 ,  224 , and  230 , respectively. The switch  206  and the feedback terminal  204  are electrically connected. The switch  206  can be electrically connected to the connection points  212 ,  218 ,  224 , and  230 . The four sets of serial resistors  208  and  210 ,  214  and  216 ,  220  and  222 , and  226  and  228  can be designed to have different resistance values, so that the switch  206  provides different feedback voltages on the feedback terminal  204  when it is electrically connected to the connection points  212 ,  218 ,  224 , and  230 . The output of V out  of the power supply  202  is adjusted to change the brightness of the pixel units. However, the conventional design of the brightness adjustment circuit occupies a substantial area in the entire circuit. Therefore, how to effectively reduce its area is an important issue in the field. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, the invention provides a brightness adjustment circuit for an OLED that can greatly reduce the area occupied by the brightness adjustment circuit by at least 50%. 
   In a preferred embodiment of the invention, the OLED brightness adjustment circuit includes an electroluminescent panel and a power supply unit. The power supply unit provides a working voltage to the electroluminescent panel in response to a feedback voltage. The brightness adjustment circuit at least includes: an integrated circuit module and a switch. The integrated circuit module, having a variable resistor, is electrically coupled to the electroluminescent panel and the power supply unit. The integrated circuit module includes: an internal circuit unit and an external circuit unit, wherein the internal circuit unit electrically coupled to the power supply unit and one end of the external circuit unit is electrically coupled to a common potential, such as a reference voltage unit. The switch modulates the resistance of the circuit module in response to a control signal, thereby adjusting the feedback voltage. The method of modulating the resistance of the circuit module changes the switch among different connection points to connect different parts of the external circuit with a part of the internal circuit in parallel. In another embodiment, the connection is serial. The feedback voltage is extracted from the voltage at a specific point in the circuit module. The operation of the switch changes the partial voltage at this specific point inside the circuit module, thereby changing the feedback voltage. Besides, the brightness adjustment circuit provided by the invention can be installed on an electroluminescent panel. In some embodiments, a brightness adjustment circuit for use in an electroluminescent display having an electroluminescent panel and a power supply unit, the power supply unit provides a working voltage to the electroluminescent panel in response to a feedback voltage provided from the brightness adjustment circuit. The brightness adjustment circuit comprises a circuit module electrically coupled to the power supply unit and the electroluminescent panel, and a voltage supply unit to provide a control voltage to one end of the circuit module to adjust the feedback voltage, the control voltage is variable. 
   In another embodiment of the invention, the electroluminescent display includes an electroluminescent panel and a power supply unit. The power supply unit provides a working voltage to the electroluminescent display in response to the feedback voltage. The brightness adjustment circuit includes at least: a circuit module, which is electrically coupled to the power supply unit and the electroluminescent panel; and a voltage supply unit, which sends a control voltage into one end of the circuit module and the feedback voltage is adjusted by the variable control voltage. The voltage supply unit can be a digital-to-analog converter (DAC) or a power, for example. 
   The disclosed brightness adjustment circuit does not need to modify or compute the output data of the system. It may only modify the serial or parallel connection between a part of the internal circuit and a part of the external circuit. Alternatively, a control voltage is used to achieve the enhancement or reduction of panel brightness. Besides, the invention can be used to increase the brightness of the electroluminescent display. The circuit units inside the circuit module are not limited to only two sets. There may be more than two sets of circuit units to achieve multiple-step brightness modulation. 
   Another aspect of the invention is to disclose an electroluminescent display using the above-mentioned brightness adjustment circuits. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the invention will become apparent by reference to the following description and accompanying drawings which are given by way of illustration only, and thus are not limitative of the invention, and wherein: 
       FIG. 1  is a schematic view of the conventional feedback stable voltage power supply unit; 
       FIG. 2  is a schematic view of the conventional feedback stable voltage power supply unit that can adjust the brightness of pixel units; 
       FIG. 3  shows a brightness adjustment circuit for an electroluminescent display according to a first embodiment of the invention; 
       FIG. 4  shows a brightness adjustment circuit for an electroluminescent display according to a second embodiment of the invention; and 
       FIG. 5  shows a brightness adjustment circuit for an electroluminescent display according to a third embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention will be apparent from the following detailed description, which proceeds with reference to the accompanying drawings, wherein the same references relate to the same elements. 
   In order for the brightness adjustment circuit of an electroluminescent display to be clearly illustrated, the embodiments in this specification use a circuit module with two circuit units as an example. The invention is certainly not limited to using the circuit module with only two circuit units. The circuit modules with more than two circuit units can be used to achieve multiple-step brightness modulation as well. Moreover, P-type MOS transistors are used to drive the electroluminescent display or pixel units in these embodiments. 
   Embodiment 1 
   With reference to  FIG. 3 , the power supply device  300  can provide a stable working voltage V dd  or V ss  to an electroluminescent display or pixel unit (not shown). The power supply device  300  has a power supply  302  with a power supply unit (not shown) and a reference voltage unit (not shown). The power supply unit and the reference voltage unit are electrically coupled. The power supply unit is coupled to the voltage output terminal V out , and the reference voltage unit is coupled to the feedback terminal  304 . This is the structure of an power supply  302  and will not be repeated below. The power supply  302  has a voltage output terminal V out  and a feedback terminal  304 . The voltage output terminal V out  provides the working voltage V dd  or V ss  for the pixel unit (not shown). The feedback circuit module  305  includes an internal circuit  301 , an external circuit  303 , and a switch  306 . The internal circuit  301  includes resistors  308  and  310 , which are connected to each other and have one end electrically coupled to the voltage output terminal V out . The feedback terminal  304  is electrically coupled to the other end of the resistor  308 . The other end of the resistor  310  is electrically coupled to the switch  306 . The switch  306  modulates the resistance of the circuit module  305  in response to a control signal, thereby adjusting the feedback voltage. 
   The external circuit  303  includes resistors  312 ,  316  and  320 . One end of each of the resistors  312 ,  316  and  320  is connected to a common potential, e.g., the ground. The other end of each of the resistors  312 ,  316  and  320  are connected to the connection points  314 ,  318  and  322 . The switch  306  connects the resistors  312 ,  316  and  320  to the internal circuit  301  in series via the connection points  314 ,  318  and  322 , respectively. Since the resistors  312 ,  316  and  320  have different resistance values, the feedback voltage fed by the feedback circuit module  305  to the feedback terminal  304  varies as the switch  306  changes among the resistors  312 ,  316  and  320 . 
   Take some numbers as an explicit example. Suppose the reference voltage unit of the power supply  302  provides a 0.6V reference voltage, and the resistors  308 ,  310 ,  312 ,  316  and  320  have the resistance values of 50 kΩ, 10 kΩ, 10 kΩ, 50 kΩ and 100 kΩ, respectively. When the switch  306  connects to the connection point  314  to connect the resistors  308 ,  310  and  312  in series, the output of the voltage output terminal V out  is 2.1V. When the switch  306  connects to the connection point  318  to connect the resistors  308 ,  310  and  316  in series, the output of the voltage output terminal V out  is 1.1V. When the switch  306  connects to the connection point  322  to connect the resistors  308 ,  310  and  320  in series, the output of the voltage output terminal V out  is 0.87V. Therefore, the output voltage at the voltage output terminal V out  drops from 2.1V to 0.87V. The working voltage V dd  or V ss  provided to the electroluminescent display or pixel unit also drops from 2.1V to 0.87V, thereby lowering its brightness. Beside, the brightness adjustment circuit can be installed on or outside the electroluminescent panel. 
   Embodiment 2 
   With reference to  FIG. 4 , the power supply device  400  can provide a stable working voltage V dd  or V ss  to an electroluminescent display or pixel unit (not shown). The power supply device  400  has a power supply  402  with a power supply unit (not shown) and a reference voltage unit (not shown). The power supply unit and the reference voltage unit are electrically coupled. The power supply unit is coupled to the voltage output terminal V out , and the reference voltage unit is coupled to the feedback terminal  404 . This is the structure of an ordinary power supply  402  and will not be repeated below. The power supply  402  has a voltage output terminal V out  and a feedback terminal  404 . The voltage output terminal V out  provides the working voltage V dd  or V ss  for the pixel unit (not shown). The feedback circuit module  405  includes an internal circuit  401 , an external circuit  403 , and a switch  406 . The internal circuit  401  includes resistors  408  and  410 , which are connected in series and have one end electrically coupled to the voltage output terminal V out . The feedback terminal  404  is electrically coupled to the other end of the resistor  408 . The other end of the resistor  410  is electrically coupled to a common potential (e.g., the ground) and the switch  406 . The switch  406  modulates the resistance of the circuit module  405  in response to a control signal, thereby adjusting the feedback voltage. 
   The external circuit  403  includes resistors  412 ,  416  and  420 , one end of each of which is connected to a common potential (e.g., the ground). The other end of each of resistors  412 ,  416  and  420  is connected to the connection points  414 ,  418  and  422 , respectively. The switch  406  can connect to the connection points  414 ,  418  or  422  so as to connect the resistor  412 ,  416  or  420  with the resistor  410  of the internal circuit  401  in parallel. Since the resistors  412 ,  416  and  420  have different resistance values, the feedback voltage fed by the feedback circuit module  405  to the feedback terminal  404  varies as the switch  406  changes among the resistors  412 ,  416  and  420 . 
   Take some numbers as an explicit example. Suppose the reference voltage unit of the power supply  402  provides a 0.6V reference voltage, and the resistors  408 ,  410 ,  412 ,  416  and  420  have the resistance values of 50 kΩ, 50 kΩ, 1000 kΩ, 50 kΩ and 40 kΩ, respectively. When the switch  406  connects to the connection point  414  to connect the resistors  410  and  412  in parallel, the output of the voltage output terminal V out  is 1.23V. When the switch  406  connects to the connection point  418  to connect the resistors  410  and  416  in parallel, the output of the voltage output terminal V out  is 1.8V. When the switch  406  connects to the connection point  422  to connect the resistors  410  and  420  in parallel, the output of the voltage output terminal V out  is 1.95V. Therefore, the output voltage at the voltage output terminal V out  increases from 1.23V to 1.95V. The working voltage V dd  or V ss  provided to the electroluminescent display or pixel unit also increases from 1.23V to 1.95V, thereby increasing its brightness. Beside, the brightness adjustment circuit can be installed on or outside the electroluminescent panel. 
   Embodiment 3 
   With reference to  FIG. 5 , the power supply device  500  can provide a stable working voltage V dd  or V ss  to an electroluminescent display or pixel unit (not shown). The power supply device  500  has a power supply  502  with a power supply unit (not shown) and a reference voltage unit (not shown). The power supply unit and the reference voltage unit are electrically coupled. The power supply unit is coupled to the voltage output terminal V out , and the reference voltage unit is coupled to the feedback terminal  504 . This is the structure of an power supply  502  and will not be repeated below. The power supply  502  has a voltage output terminal V out  and a feedback terminal  504 . The voltage output terminal V out  provides the working voltage V dd  or V ss  for the pixel unit (not shown). The feedback circuit module includes a circuit  501  and an input terminal  506 . The circuit  501  includes resistors  508  and  510 , which are connected to each other and have one end electrically coupled to the voltage output terminal V out . The feedback terminal  504  is electrically coupled to the other end of the resistor  508 . The other end of the resistor  510  is electrically coupled to the input terminal  506 . 
   A voltage supply unit (not shown) automatically sends in a control voltage V m  via the input terminal  506 , the control voltage V m  is variable. With the variation of the control voltage V m , the voltage changes between the resistors  508  and  510  also occur. Therefore, the feedback voltage to the feedback terminal  504  changes accordingly. 
   Take some numbers as an explicit example. Suppose the reference voltage unit of the power supply  402  provides a 0.6V reference voltage, and the resistors  508  and  510  have the resistance values of 10 kΩ and 50 kΩ, respectively. When a control voltage Vm=0.1V is sent from the voltage supply unit and into the input terminal  506 , the output of the voltage output terminal Vout is 3.1V. When a control voltage Vm=0.3V is sent from the voltage supply unit to the input terminal  506 , the output of the voltage output terminal Vout is 2.1V. When a control voltage Vm=0.5V is sent from the voltage supply unit and into the input terminal  506 , the output of the voltage output terminal Vout is 1.1V. Therefore, the output voltage at the voltage output terminal Vout drops from 3.1V to 1.1V. The working voltage Vdd or Vss provided to the electroluminescent display or pixel unit also drops from 3.1V to 1.1V, thereby reducing the brightness of the electroluminescent display. Beside, the brightness adjustment circuit can be installed on or outside the electroluminescent panel. In this embodiment, the voltage supply unit can be a DAC or a power. 
   The disclosed brightness adjustment circuit may not need to modify or compute the output data of the system. By changing the serial/parallel connections between a part of the internal circuit and a part of the external circuit or by inputting a control voltage, the invention can achieve the effects of increasing or reducing the brightness of the panel. Moreover, the invention can be used to increase the brightness of an OLED. The external circuit inside the circuit module is not limited to two or three sets. There can be more sets, depending upon the desired multiple-step brightness modulation. 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.