Abstract:
A driving circuit and method for an active matrix organic light emitting diode (AMOLED) display are provided. The driving circuit comprises a power circuit, a linear thermistor, and a pixel circuit. The power circuit provides an equivalent current. The linear thermistor coupled to the power circuit adjusts the equivalent current according to the temperature of the AMOLED display. The pixel circuit coupled to the power circuit comprises a driving transistor and a light emitting device. The driving transistor comprises a first end coupled to the power circuit, and the light emitting device coupled to a second end of the driving transistor is driven by the equivalent current to illuminate.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an active matrix OLED display, and in particular, to power circuits that compensate temperature variations when driving the display. 
   2. Description of the Related Art 
     FIG. 1  shows a pixel circuit of a conventional active matrix OLED display. A capacitor  104  is coupled to the gate of a driving transistor  106 , and an OLED  102  is coupled to the drain of the driving transistor  106 . The source of the driving transistor  106  is coupled to a terminal VDD, and the other end of the OLED  102  is coupled to a terminal VSS. The pixel circuit shown is an abstract concept, in which the driving transistor  106  may be a PMOS or an NMOS, and the OLED  102  may also be coupled to the terminal VDD and the driving transistor  106 . Thousands of variations of detailed implementations are present and known to the art. The major principle is that the capacitor  104  determines brightness of the pixel circuit, and the OLED  102  illuminates in response to the current flowing from terminal VDD to terminal VSS controlled by the driving transistor  106 . The terminal VDD and terminal VSS are provided by a power circuit (not shown). The OLED  102  and driving transistor  106  may be influenced by environmental temperature and manufacturing inaccuracy, and as a result, unstable illumination is induced in the pixel circuit. 
     FIG. 2  shows the relationships between conventional pixel brightness and temperature. The horizontal axis is temperature, and vertical axis a normalized value. The terminal VDD and terminal VSS are not influenced as temperature varies, but the brightness is proportional to the temperature. Temperature compensation is therefore desirable for driving the pixel circuit. 
   BRIEF SUMMARY OF INVENTION 
   It is an object of the present invention to provide a driving circuit for an active matrix organic light emitting diode (AMOLED) display. An exemplary embodiment of a driving circuit for an AMOLED display comprises a power circuit, a linear thermistor, and a pixel circuit. The power circuit provides an equivalent current. The linear thermistor coupled to the power circuit adjusts the equivalent current according to the temperature of the AMOLED display. The pixel circuit coupled to the power circuit comprises a driving transistor and a light emitting device. The driving transistor comprises a first end coupled to the power circuit, and the light emitting device coupled to a second end of the driving transistor is driven by the equivalent current to illuminate. 
   The pixel circuit may comprise a switch transistor, electrically coupled to a gate of the driving transistor. The pixel circuit may comprise a capacitor coupled to the gate of the driving transistor. The power circuit may comprise a first end providing the equivalent current, a second end coupled to the first end of the linear thermistor, and a third end coupled to the second end of the linear thermistor. The driving circuit may also comprise a resistor having a first end coupled to the first end of the linear thermistor, and a second end coupled to ground. The resistance of the thermistor is in reverse proportion to the temperature, and thus the equivalent current is in reverse proportion to the temperature. 
   A method for driving the AMOLED display is also provided. Temperature of the AMOLED is detected. An equivalent current is generated by the power circuit based on the temperature of the AMOLED. The light emitting device is driven by the equivalent current to illuminate. The equivalent current is in reverse proportion to the temperature. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
       FIG. 1  shows a conventional active matrix OLED display; 
       FIG. 2  is a diagram showing relationships between conventional brightness and temperature; 
       FIGS. 3   a  and  3   b  are schematic illustrations showing power units according to an embodiment of the invention; 
       FIGS. 4   a  and  4   b  are schematic illustrations showing power units according to another embodiment of the invention; 
       FIG. 5  shows the relationships between brightness and temperature according the invention; and 
       FIG. 6  is a flowchart of a driving method according to the invention. 
   

   DETAILED DESCRIPTION OF INVENTION 
   The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
     FIG. 3   a  shows an embodiment of a power unit according to the invention. A power circuit  300  comprises three terminals, in which a terminal LX and a terminal FB are coupled to a linear thermistor  302 . The terminal FB is also coupled to ground via a resistor  206 . A feedback loop is thus formed by the terminal LX and terminal FB. A node A has an electrical potential proportional to the ratio of the linear thermistor  302  to the resistor  206  based on the voltage division law. The terminal FB detects the potential on the node A as a reference for a terminal VDD, and the terminal VDD is coupled to the pixel circuit in  FIG. 1  as a power supply. In this case, the linear thermistor  302  is in reverse proportion to the temperature, thus, the potential detected by the terminal FB is proportional to the temperature. An equivalent current output from the terminal VDD of power circuit  300  is also in reverse proportion to the temperature. The light emitting device employed in the embodiment is specifically chosen to be an active matrix OLED. The terminal VDD of power circuit  300  is not necessarily coupled to the terminal VDD of the pixel circuit, and may also couple to a terminal VSS. The linear thermistor  302  coupled to the terminal LX and terminal FB is not necessarily based on the voltage division law. The pixel circuit is not restricted to be voltage driven or current driven. Any pixel circuit utilizing linear thermistor  302  to compensate temperature effect for illumination meets the goal of the invention. 
     FIG. 3   b  is an embodiment according to  FIG. 3   a . The terminal VDD is coupled to the pixel circuit as shown in  FIG. 1 . A capacitor  104  is coupled to the gate of a driving transistor  106 , and an OLED  102  is coupled to the drain of the driving transistor  106 . The source of driving transistor  106  is coupled to the terminal VDD, and the other terminal of the OLED  102  is coupled to the terminal VSS. The pixel circuit shown is an abstract concept, in which the driving transistor  106  may be a PMOS or an NMOS, and the OLED  102  may also be coupled to the terminal VDD and the driving transistor  106 . Thousands of variations of detailed implementations are present and known to the art. The major principle is that the capacitor  104  determines brightness of the pixel circuit, and the OLED  102  illuminates in response to the current flowing from the terminal VDD to the terminal VSS controlled by the driving transistor  106 . 
     FIG. 4   a  shows a power unit according to another embodiment of the invention. Similarly, the power circuit  300  comprises three terminals. A resistor  206  is coupled to a terminal LX and a terminal FB, and the terminal FB is also coupled to ground via a linear thermistor  302 . A feedback loop is thus formed between the terminal LX and terminal FB. A node A has an electrical potential proportional to the ratio of linear thermistor  302  to resistor  206  based on the voltage division law. The terminal FB detects the potential on the node A as a reference for a terminal VDD, and the terminal VDD is coupled to the pixel circuit in  FIG. 1  as a power supply. In this case, the linear thermistor  302  is proportional to the temperature, thus the potential detected by the terminal FB is in reverse proportion to the temperature. An equivalent current output from the terminal VDD of the power circuit  300  is also in reverse proportion to the temperature. The light emitting device employed in the embodiment is specifically chosen to be an OLED. The terminal VDD of the power circuit  300  is not necessarily coupled to the terminal VDD of the pixel circuit, and may also couple to a terminal VSS. The linear thermistor  302  coupled to the terminal LX and terminal FB is not necessarily. based on the voltage division law. The pixel circuit is not restricted to be voltage driven or current driven. Any pixel circuit utilizing the linear thermistor  302  to compensate temperature effect for illumination meets the goal of the invention. 
     FIG. 4   b  is an embodiment according to  FIG. 4   a . The terminal VDD is coupled to the pixel circuit as shown in  FIG. 1 . A capacitor  104  is coupled to the gate of a driving transistor  106 , and an OLED  102  is coupled to the drain of the driving transistor  106 . The source of the driving transistor  106  is coupled to the terminal VDD, and the other terminal of OLED  102  is coupled to the terminal VSS. The pixel circuit shown is an abstract concept, in which the driving transistor  106  may be a PMOS or an NMOS, and the OLED  102  may also be coupled to the terminal VDD and the driving transistor  106 . Thousands of variations of detailed implementations are present and known to the art. The major principle is that the capacitor  104  determines brightness of the pixel circuit, and the OLED  102  illuminates in response to the current flowing from the terminal VDD to the terminal VSS controlled by the driving transistor  106 . 
     FIG. 5  shows a relationship between brightness and temperature according to the invention. The terminal VDD of the power circuit  300  is in reverse proportion to the temperature. The linear thermistor  302  varies with temperature to compensate the terminal VDD, such that brightness is kept consistent. As the pixel circuit implementation varies, the power circuit  300  may provide a terminal VDD proportional or reverse proportional to the temperature through the linear thermistor  302 , and the terminal VDD may be coupled to the terminal VDD terminal or terminal VSS terminal of the pixel circuit. The major goal of the invention is to provide a linear thermistor to compensate the temperature variation, such that the AMOLED illuminates with consistency. 
     FIG. 6  is a flowchart of the driving method according to the invention. In step  602 , the temperature of the active matrix OLED display is detected. In step  604 , the equivalent current of the power circuit is adjusted through the linear thermistor according to the temperature of the active matrix OLED display. In step  606 , the light emitting device is driven by the equivalent current to illuminate. The equivalent current output from the terminal VDD of the power circuit is in reverse proportion to the temperature, thus the brightness of the light emitting device remains constant as temperature varies. 
   While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.