Abstract:
Embodiments provide an active matrix LED pixel driving circuit and pixel layout for increased uniform illumination of LED display panels. A plurality of sub driving transistors can be located in neighbor pixels of the pixel associated with the prime driver transistor&#39;s LED. The sub driving transistors can compensate for the process variations affecting the threshold voltage variation of the prime driver transistor resulting in uniform illumination of LED&#39;s on the display panel.

Description:
RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/008,452, filed on Jun. 5, 2014. The entire teachings of the above application(s) are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    A typical active matrix light emitting diode (LED) display uses a matrix of pixels that include a capacitor charged by a driving voltage. The pixels of the active matrix LED display use the stored voltages until the scan period of the next frame. These stored voltages allow the pixel circuit to drive the current to an LED during the one frame time period. 
         [0003]    With reference to  FIG. 1 , a typical pixel in the prior art comprises a first transistor  11  and a second transistor  12 , a capacitor  13 , and an LED  14 . The gate of the first transistor  11  receives a select signal through a Select Line (SL)  16  while the source of the first transistor  11  receives a voltage data signal through a VData line  17 . The voltage data signal is transmitted to the gate of the second transistor  12  when the first transistor  11  is turned on by the select signal, and the voltage level of the data signal Vdata turns on the second transistor  12  to generate a driving current through the second transistor  12  illuminating the LED  14  during the one frame time period. A typical prior art layout of the pixels in shown in  FIG. 2 , with a plurality of pixels  201 - 215  being placed in a grid pattern with a plurality of select lines  250   a - 250   c  for the rows and a plurality of voltage data lines  260   a - 260   e  for the columns. 
         [0004]    However, the typical pixels in  FIG. 1 , and as they are placed in the layout of  FIG. 2 , suffer from drift and the second transistors&#39; threshold voltage variation, including as caused by width variation and length variation, which can result in variation of the driving current. The variations between the driving currents in the different pixels can cause non-uniform illumination on the display panel or screen. 
       SUMMARY OF THE INVENTION 
       [0005]    Embodiments of the present invention relate to an active matrix LED pixel driving circuit and pixel layout that provide increased uniform illumination of LED display panels. In accordance with one example embodiment, a light emitting diode pixel driving circuit includes a first prime driving transistor having a first gate and a first source, a select line connected to the first gate, a voltage data line connected to the first source, a second prime driving transistor having a second gate and a second drain, a light emitting diode connected to the second drain, and a plurality of sub driving transistors each having a gate and a drain wherein the gates of the plurality of sub driving transistors are connected to the second gate and the drains of the sub driving transistors are connected to the second drain. 
         [0006]    For certain embodiments, the first and second prime driving transistors are located in a first pixel and the plurality of sub driving transistors are located outside the first pixel. For certain example embodiments, the first and second prime driving transistors are located in a first pixel and each of the plurality of sub driving transistors are located in a neighbor pixel to the first pixel. For certain embodiments, the first and second prime driving transistors are P-channel transistors. For additional example embodiments, the plurality of sub driving transistors are each P-channel transistors. 
         [0007]    For certain example embodiments, the second prime driving transistor also has a second source and the example circuit further includes a capacitor connected between the second source and the second gate. For certain embodiments, the second prime driving transistor also has a second source and further included is a VDD voltage source that is connected to the second source. For additional example embodiments, the plurality of sub driving transistors also each have a source and the VDD voltage source is also connected to the sources of the plurality of sub driving transistors. For certain embodiments, the first gate is capable of receiving a select signal through the select line and the first source is capable of receiving a voltage data signal through the voltage data line. 
         [0008]    Yet for still other additional embodiments, the voltage data signal is transmitted to the second gate when the first prime driving transistor is turned on by the select signal, and the voltage level of the voltage data signal is configured to turn on the second prime driving transistor to generate a driving current through the second prime driving transistor. For certain example embodiments, the plurality of sub driving transistors are capable of compensating for a threshold variation of the second prime driving transistor resulting in an improved uniform illumination of the light emitting diode on a display panel. 
         [0009]    Another example embodiment of the present invention is a method that includes receiving a select signal at a first gate of a first prime driving transistor to turn on the first prime driving transistor, transmitting a voltage data signal to a second gate of a second prime driving transistor and to each of a plurality of gates of a plurality of sub driving transistors when the first prime driving transistor is turned on by the select signal, generating a driving current through the second prime driving transistor when the voltage data signal turns on the second prime driving transistor, illuminating a light emitting diode using the driving current, and wherein the plurality of sub driving transistors also each have a drain which are connected to the second drain of the second prime driving transistor, and the plurality of sub driving transistors compensate for a threshold variation of the second prime driving transistor. For field effect transistors (FET) transistors, the threshold voltage is the minimum voltage difference between the gate and source that is needed to create a conducting path between the source and drain. 
         [0010]    For certain embodiments, the first and second prime driving transistors are located in a first pixel and the plurality of sub driving transistors are located outside the first pixel. For certain embodiments, the first and second prime driving transistors are located in a first pixel and each of the plurality of sub driving transistors are located in a neighbor pixel to the first pixel. 
         [0011]    For additional embodiments, the first and second prime driving transistors are P-channel transistors and/or the plurality of sub driving transistors are each P-channel transistors. For certain embodiments, the second prime driving transistor also has a second source and further comprises providing current to the light emitting diode from a capacitor which is connected between the second source and the second gate when the first prime driving transistor is turned off. For certain example embodiments, the second prime driving transistor also has a second source and the plurality of sub driving transistors also each have a source, and a VDD voltage source is connected to the second source and to the sources of the plurality of sub driving transistors. For some example embodiments, the select signal is received through a select line and the voltage data signal is transmitted through a voltage data line. For certain example embodiments, the plurality of sub driving transistors compensate for a threshold variation of the second prime driving transistor so as to result in an improved uniform illumination of the light emitting diode on a display panel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
           [0013]      FIG. 1  is a schematic of a prior art LED pixel driving circuit. 
           [0014]      FIG. 2  is a prior art circuit diagram of an LED pixel matrix. 
           [0015]      FIG. 3  is a schematic of a LED pixel driving circuit in accordance with an embodiment of the present invention. 
           [0016]      FIG. 4  is a circuit diagram of an LED pixel matrix in accordance with an embodiment of the present invention. 
           [0017]      FIG. 5  is a flow diagram illustrating an example embodiment of a method relating to LED pixel driving circuits in accordance with embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    A description of example embodiments of the invention follows. The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety. 
         [0019]    Active-matrix LED displays include an array of pixels that can include switching transistors and capacitors. The pixels can be arranged in a grid or matrix. To address a particular pixel, the correct row is turned on (also referred to as switched on), and then a charge is transmitted down the selected column. While this occurs, the other rows that the selected column intersects generally should be turned off so that only the capacitor for the appropriate pixel is charged. The capacitor can hold the charge until it is refreshed during the next cycle. 
         [0020]      FIG. 3  shows a LED pixel driving circuit  300  in accordance with an embodiment of the present invention. As shown in  FIG. 3 , the illustrated example embodiment includes a prime driver circuitry  305  and a plurality of sub driver circuitry  330   a - 330   d . Four sub drivers  330   a - 330   d  are shown in  FIG. 3 , but embodiments of the invention can include any practical number of sub drivers. Generally, the more sub drivers employed, the more that the threshold variations are normalized and a more uniform display is achieved. 
         [0021]    The LED pixel driving circuit  300  includes a first prime driving transistor  21  with its gate connected to a select line  310 , its source connected to a voltage data line  320 , and its drain connected to a second prime driving transistor  22 . When reference is made to connected, it is meant to refer broadly to any type of connection, including directly or indirectly, that allows for the electrical signal to be communicatively coupled. The select line is used in selecting a particular pixel driving circuit  300  and can be used to turn on the first prime driving transistor  21 . Turn on or switch on a transistor refers to applying the appropriate voltage to the gate of the transistor, including depending upon whether it is an N-channel or P-channel transistor, to cause current to flow between the source and the drain. 
         [0022]    When the first prime driving transistor  21  is turned on, the voltage data signal on the voltage data line  320  is transmitted from the source of the first prime driving transistor  21  to its drain, which is connected to the gate of second prime driving transistor  22 . This causes the second prime driving transistor  22  to turn on and generates a driving current through the second prime driving transistor  22  between its source and the drain. How hard the second prime driving transistor  22  turns on is based on the voltage data signal and can affect in which region the transistor operates and where within a region the transistor operates. The source of the second prime driving transistor  22  is connected to a voltage source, such as Vdd, and the drain of the second prime driving transistor  22  is connected to the anode of a light emitting diode (LED)  24 . The cathode of the light emitting diode (LED)  24  can be connected to a voltage source, such as Vss, or to ground depending on the design. The driving current causes the LED  24  to illuminate and provide light. 
         [0023]    The drain of the first prime driving transistor  21  also is connected to the gates of the sub driving transistors  25 - 28  of the sub drivers  330   a - 330   d . Each of the drains of the sub driving transistors  25 - 28  are connected to the drain of the second prime driving transistor  22 , all of which are connected to the anode of the light emitting diode (LED)  24 . The sources of each of the sub driving transistors  25 - 28  are connected to the same voltage source, or power rail, as the source of the second prime driving transistor  22 , which is Vdd for this example embodiment. Thus, when the voltage data signal gates the second prime driving transistor  22  and the sub driving transistors  25 - 28 , driving current is supplied to the LED  24  from each of the transistors and differences due to threshold variations are mitigated, and which compensates for the process variations of the second prime driver transistor  22 . 
         [0024]    The example embodiment illustrated in  FIG. 3  uses P-channel transistors for the first and second prime driving transistors  21 ,  22  and the sub driving transistors  25 - 28 . Other example embodiments can use N-channel transistors or a combination of P-channel and N-channel transistors and can use the appropriate voltage source for the sources and drains of the chosen transistors and for what voltages are applied to the gates of the design specific transistors. 
         [0025]      FIG. 3  also illustrates the capacitor  23  that is connected between the source of the second prime driving transistor  22  and the drain of the first prime driving transistor  21 . The capacitor  23  is used to store charge so that when the transistors are turned off the driving current can be provided to the LED  24  until the next refresh cycle. 
         [0026]    For the example embodiment shown in  FIG. 3 , the sub driving transistors  25 - 28  are located in pixels that are neighbors of the pixel having the prime driving circuitry and LED  24 . Neighbor pixels can include pixels that are adjacent to the pixel with the prime driving circuitry and LED  24  or that are adjacent to one or more pixels having sub driving circuitry and are collectively adjacent to the pixel having the prime driving circuitry and LED  24  (e.g. a chain as shown in  FIG. 3  where pixels  330   a  and  330   b  are neighbors of pixel 305 and so are pixels  330   c  and  330   d ). 
         [0027]      FIG. 4  is a circuit diagram of an LED pixel matrix in accordance with an embodiment of the present invention. For each pixel, the example circuit layout utilizes a prime driver (such as  400   a - 400   e ) formed of a first and second prime driving transistors  21 ,  22 , capacitor  23 , and LED  24  as described and shown in  FIG. 3 . Each pixel also uses four sub drivers (for the pixel with prime driver  400   b , the sub drivers included with the pixel are neighbor pixels  420   a - 420   d ), which are laid out adjacent to the prime driver. The layout is skewed between pixels to account for the sub driver circuitry. The select lines  410   a - 410   c  are routed so that they can gate the first prime driving transistors. Similarly, the voltage data lines  415   a - 415   e  are routed so that they are connected to the source of the first prime driving transistors. Of course, a wide variety of layouts can be employed in accordance with the present disclosure. For example, sub driver circuitry could be located within the space for the layout of an adjacent pixel or on another layer. 
         [0028]      FIG. 5  is a flow diagram illustrating an example embodiment of a method relating to LED pixel driving circuits in accordance with embodiments of the present invention. The layout area illustrated in  FIG. 5  provides for (generates) a uniform illumination of LEDs/pixels on a display panel or other screen. In accordance with this example method, first a select signal is received  510  at a first gate of a first prime driving transistor to turn on the first prime driving transistor. Then, a voltage data signal is transmitted  520  to a second gate of a second prime driving transistor and to each of a plurality of gates of a plurality of sub driving transistors when the first prime driving transistor is turned on by the select signal. The plurality of sub driving transistors also each have a drain which are connected to the second drain of the second prime driving transistor. This provides for the plurality of sub driving transistors to compensate for a threshold variation of the second prime driving transistor. A driving current is then generated  530  through the second prime driving transistor when the voltage data signal turns on the second prime driving transistor. The driving current is used to illuminate  540  a light emitting diode. 
         [0029]    While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.