Patent Publication Number: US-7586470-B2

Title: OLED device capable of adjusting luminance

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an organic light emitting diode (OLED) device, and more particularly to an OLED device capable of adjusting luminance. 
     2. Description of the Prior Art 
     Please refer to  FIG. 1 .  FIG. 1  illustrates a circuit diagram of controlling a conventional OLED  100 . As illustrated in  FIG. 1 , the conventional OLED device  100  includes a reference current source I REF , two transistors Q 1  and Q 2 , an OLED equivalent module  110 , a bias source VH, and a variable resistor VR 1 . The OLED equivalent module  110  includes an OLED D 1 , an equivalent capacitor C 1 , two resistors R 1  and R 2 . Luminance of the OLED D 1  is determined by current that flows through identity, therefore in order to change the luminance of the OLED D 1 , the current that flows through identity has to be changed. As illustrated in  FIG. 1 , the transistors Q 1  and Q 2  is connected as a current mirror, which duplicates the reference current I REF  to the transistor Q 2  such that the current outputted by the transistor Q 2  is the same as I REF . The current I REF  outputted by the transistor Q 2  is split into two currents I 1  and I 2  after passing through parallel circuits formed by the OLED equivalent module  110  and the variable resistor VR 1 , thus the current that flowed through the OLED equivalent module  110  is I 1 , as the addition of currents I 1  and I 2  equals to the current I REF , then the value of the current I 1  can be changed by changing the value of the current I 2 . Therefore, a resistance value of the variable resistor VR 1  can be utilized to change the value of the current I 2 , in doing so the value of the current I 1  passing through the OLED equivalent module  110  is also changed; hence the objective of changing the luminance of the OLED equivalent module  110  is reached. 
     But there are a few problems in the above-mentioned prior art in controlling the luminance of the OLED device. First, in the manufacturing process, it is not easy to fit the whole devices of the above-mentioned into a same chip as the variable resistor takes up a significant surface area. Hence this method is not suitable or compliant for the current trend of producing smaller chipsets. Second, if the variable resistor is further coupled to the external portion of the OLED in a discrete manner, the resistance value of the variable resistor may shift and fix at a predetermined value difficulty. Lastly, during manufacturing, each variable resistor that controls the OLED device needs to be adjusted so that the luminance of the OLED can be uniform, thus this method is unable to effectively control the OLED. 
     SUMMARY OF THE INVENTION 
     The claimed invention discloses an organic light emitting diode (OLED) device capable of adjusting luminance. The OLED device comprises: a control voltage generator, a plurality of transistors, a control circuit, a second switch, and an OLED equivalent module. The control voltage generator comprises a reference current source for providing a reference current, and a first transistor comprising a first end coupled to the reference current source, a second end coupled to a first voltage source, and a control end. The control voltage generator is utilized for generating a control voltage according the reference current at the control end of the first transistor. A first end of each first switch of the plurality of first switches is coupled to the control end of the first transistor. The current mirror comprises a first end and a second end. Each second transistor of the plurality of second transistors comprises a first end coupled to the first end of the current mirror, a second end coupled to the first voltage source, and a control end coupled to a second end corresponding to the first switch of the plurality of first switches. The control circuit coupled to the plurality of first switches is used for controlling switching on and off the plurality of the first switches. The second switch comprises a first end coupled to the first voltage source, and a second end. The first OLED module is coupled between the second end of the current mirror and the second end of the second switch for generating light. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a circuit diagram of controlling a conventional organic light emitting diode (OLED). 
         FIG. 2  illustrates a circuit diagram of adjusting luminance of an OLED device according to the present invention. 
         FIG. 3  illustrates a circuit diagram of adjusting luminance of an OLED device according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Certain terms are used throughout the following description and claims to refer to particular system components. As one skilled in the art will appreciate, consumer electronic equipment manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ” The terms “couple” and “couples” are intended to mean either an indirect or a direct electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. 
     Please refer to  FIG. 2 .  FIG. 2  illustrates a circuit diagram of adjusting luminance of an organic light emitting diode (OLED) device  200  according to the present invention. As illustrated in  FIG. 2 , the luminance adjustable OLED device  200  includes a reference current source I REF2 , four switches SW 1 , SW 2 , SW 3 , and SWA, six transistors QA, QB 1 , QB 2 , QB 3 , QC and QD, a current control circuit  220 (current circuit), an OLED equivalent module  210 , a bias source VH 2 , and five nodes W 1 , W 2 , W 3 , W 4 , and W 5 . The OLED equivalent module  210  includes two equivalent resistors R 3  and R 4 , an OLED D 2 , and an equivalent capacitor C 2 . The current control circuit  220  controls four switch signals S 1 , S 2 , S 3 , and SA 1  which are respectively utilized for controlling turning on or off of the switches SW 1 , SW 2 , SW 3 , and SWA. The operational theory will be explained in  FIG. 2 . 
     As illustrated in  FIG. 2 , the transistor QA is utilized for receiving a reference current I REF2  and for providing gate signals to the transistors QB 1 , QB 2 , and QB 3 . Each pair of the transistors QA and QB 1 , the transistors QA and QB 2 , and the transistor QA and QB 3  respectively forms a current mirror pair. For example, when the switch SW 1  is turned on, the transistor QB 1  is turned on and a reference current I REF2  identical to the value of the reference current of the transistor QA is generated at the node W 1 . Similarly, when the switch SW 2  is turned on, the transistor QB 2  is turned on and the current I REF2  is generated at the node W 2 , and when the switch SW 3  is turned on, the transistor QB 3  is turned on and the current I REF2  is generated at the node W 3 . The amount of current flowing through the node W 4  is the total of currents at the nodes W 1 , W 2 , and W 3 . If the transistors QB 1 , QB 2  and QB 3  are turned on, then I REF2  will be respectively generated at the nodes W 1 , W 2  and W 3 , and the current at the node W 4  is three times the value of I REF2  (I REF2 +I REF2 +I REF2 ). If one of the transistors within the transistors QB 1 , QB 2  and QB 3  is turned off, and at the same time the other 2 transistors are turned on, then the current at the node W 4  will be twice the value of I REF2  (I REF2 +I REF2 ), and so forth. Therefore the value of the current at the node W 4  can be adjusted to for example, 0, 1 REF2 , twice I REF2  and 3 times I REF2 , by controlling the turning on or off of the transistors QB 1 , QB 2  and QB 3 . And the switch signals S 1 , S 2  and S 3  that controls the transistors QB 1 , QB 2  and QB 3  are transmitted from the current control circuit  220 . In this way, the current control circuit  220  can be solely operated to control the value of the current at the node W 4 . And the transistors QC and QD also form a current mirror pair, thus the value of the current flowing through the mode W 4  is also duplicated to the node W 5  of the transistor QD. If the current flowing through the node W 4  is twice I REF2 , then the current at the node W 5  is the same as twice I REF2 , therefore by controlling the value of the current flowing through the node W 4 , the value of the current flowing through the node W 5  can also be controlled. Please refer to  FIG. 2 , the OLED equivalent module  210  is coupled to the node W 5 , thus current flowing through the OLED equivalent module  210  is also identical to the value of the current flowing through the node W 4 . Therefore the current flowing through the nodes W 1 , W 2 , and W 3  can be controlled through the current control circuit  220 , so that the current flowing through the node W 4  can be further controlled, and that the current flowing through the OLED equivalent module  210  can also be controlled. Also in the characteristics of the OLED, the luminance generated directly corresponds to the current flowing through the OLED, therefore the luminance of the OLED equivalent module  210  can be controlled by controlling the current control circuit  220 . 
     In the above-mentioned, the current control circuit  220  can control the luminance of the OLED equivalent module  210 , and the current control circuit  220  further controls a switch SWA. The switch SWA is utilized for controlling the OLED equivalent module  210  to emit light or not. If the switch SWA is turned on, then the OLED equivalent module  210  emits light according to the value of the current that flows through it. If the switch SWA is turned off, then the OLED equivalent module  210  does not emit light. This is an additional function of the current control circuit  220 . 
     Although there are only three transistors in the series of transistors QB as illustrated in  FIG. 2 , the number of transistors can be expanded to N, the corresponding number of switch can also be increased to SWN, and the switch signals controlled by the current control circuit  220  can also be increased to SN for providing more current value selections for the OLED equivalent module  210  such as from 0, I REF2 , 2 times I REF2 , 3 times I REF2 , 4 times I REF2  to N times I REF2 , such that it is more flexibility for the present invention to control the OLED equivalent module  210 . 
     Please refer to  FIG. 3 .  FIG. 3  illustrates a circuit diagram of adjusting luminance of an OLED device  300  according to another embodiment of the present invention. The difference with  FIG. 2  is that  FIG. 3  further includes a set of transistor QE and an OLED equivalent module  230 . A gate of the transistor QE is coupled to a node W 7 , and an end of the OLED equivalent module  230  is coupled to the transistor QE, and another end of the OLED equivalent module  230  is coupled to the node W 6 . Similarly, luminance of the OLED equivalent module  230  can also be controlled through the current control circuit  220 , and the OLED equivalent module  230  can stop emitting light by turning off the switch SWA. 
     Although there are only two sets of OLED equivalent modules  210 ,  230  represented in  FIG. 3 , the present invention can still expand to more OLED equivalent modules such as 3, 4 or N, which is limited by the metes and bounds of the present invention. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.