Abstract:
Apparatus, systems, and methods are provided for controlling the luminance of a display. One apparatus includes a pre-charge circuit configured to supply a pre-charge voltage to a column of LED pixels, a programming circuit configured to supply current to the column, and a switch configured to selectively couple the pre-charge circuit or the programming circuit to the column. A system includes an array of LED pixels arranged in a plurality of columns. A plurality of pre-charge circuits, each configured to selectively supply a pre-charge voltage to at least one column of pixels, and a plurality of current sources, each configured to selectively supply current to at least one column of pixels are also included. One method includes determining a pre-charge voltage for each of a plurality of columns based on a target luminance level selected from the plurality of luminance levels and supplying the determined pre-charge voltages to the columns.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to displays, and more particularly relates to a hybrid driver for light-emitting diode (LED) displays. 
     BACKGROUND OF THE INVENTION 
     Active matrix light emitting diode displays offer many potential advantages when compared to active matrix liquid crystal displays. Some advantages include, but are not limited to, superior image quality, thin profile, low power consumption, and lower cost. 
     Currently, two different methods are used in addressing active matrix liquid crystal displays; namely, voltage programming and current programming. A voltage programming method benefits from a large installed base of display drivers that operate in a voltage programming mode. However, voltage programmed pixel circuits suffer from the lack of ability to compensate for the variations in the pixel TFT drive currents across the surface of the display, which leads to luminance non-uniformities in the display. A current-programming method may compensate for the variations in the drive TFT performance across the display surface, which results in better display luminance and color uniformity than voltage-programmed pixels. For these reasons, current-programmed pixels are preferred over voltage-programmed pixels. 
     Notwithstanding the above-referenced preference, one drawback to current-programmed LED displays is that they exhibit longer pixel programming times than voltage-programmed pixels, particularly for lower gray levels. Longer pixel programming times are caused because current-programmed displays typically use small programming currents (e.g., 7.8 nA to 2 μA) for a typical 8-bit display driver with an 80 color groups per inch (CGPI) resolution, or even smaller currents for smaller pixel sizes in higher resolution displays. One reason for the prolonged programming time is that the data bus capacitances need to be charged before the pixel can be properly programmed, and it takes a significant amount of time to charge the data bus capacitances with these small amounts of programming current, as the data bus capacitance is significantly larger than the pixel capacitance. To alleviate this problem of slow pixel data programming times in current mode column drivers, voltage pre-charging methods have been developed as described in U.S. Pat. Nos. 7,012,378 and 7,167,406. U.S. Pat. No. 7,012,378 addresses the problem by sequentially (as the rows are scanned) applying a fixed DC pre-charge voltage to the data buses in the display during a short pre-charge interval, and then applying current programming to the pixels. The DC voltage pre-charge improves current-programmed pixel operation at low luminance (low programming currents); however, this fixed DC pre-charge voltage is useful for a very restricted range of display brightness levels (gray levels), as very low brightness levels (gray levels) require a different DC pre-charge voltage than very high brightness levels. U.S. Pat. No. 7,167,406, on the other hand, expands the pre-charge voltage&#39;s utility by providing a pre-charge voltage proportional to the desired pixel programming current; however, there are still significant shortcomings to the method described in U.S. Pat. No. 7,167,406. One shortcoming is that the use of a proportional DC pre-charge voltage does not result in sufficient display color and luminance uniformity due to the drive requirements for a red, green, and blue (R, G, B) LED pixel being different, and the pixel current feed-through effects. The pixel feed through current is a consequence of the pixel TFT switching at the end of the programming time, which may result in increasing or decreasing the current through the LED from the programmed value by ΔI P . This phenomenon produces a pixel luminance which is lower than the desired pixel luminance, and the value of ΔI P  depends upon the pixel gray level and the parasitic capacitance of the drive TFT. 
     The present invention substantially improves upon the prior art, and provides operational flexibility not provided by the prior art for achieving uniform color and gray level luminance in active matrix light emitting diode displays. The present invention integrates voltage pre-charge circuitry within the current-programmed column driver, and provides novel and practical means to optimize current-programmed pixel operation to achieve superior color and gray level luminance uniformity in the display. The present invention also provides programmable, non-proportional lookup tables to establish and define unique and optimum voltage pre-charge levels, and programming currents for each desired pixel color and luminance level (pixel gray level) by including compensation for the differences in R, G, B LED pixel drive requirements and current feed-through effects at the end of the pixel programming time. 
     Accordingly, it is desirable to provide drivers, displays, and methods for controlling the luminance of the LEDs in a display by decreasing the amount of time needed to charge the data bus capacitances. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention. 
     BRIEF SUMMARY OF THE INVENTION 
     Various exemplary embodiments provide a driver for controlling the luminance of a display comprising a column of light-emitting diode (LED) pixels. The driver comprises a pre-charge circuit configured to supply a pre-charge voltage to the column of LEDs and a programming circuit configured to apply current to the column of LEDs. A switch configured to selectively couple the pre-charge circuit or the programming circuit to the column of LEDs is also included. 
     Exemplary embodiments of the invention also provide a display comprising an array of LED pixels arranged in a plurality of columns. The display also comprises a plurality of pre-charge circuits, each configured to selectively supply a pre-charge voltage based on pixel color gray level and feed-through current to at least one column of LED pixels, and a plurality of current sources, each configured to selectively supply current to at least one column of LED pixels. 
     Methods for controlling the luminance of a display comprising a plurality of columns of LED pixels characterized by a plurality of luminance levels are also provided. In one exemplary embodiment, the method comprises the steps of determining a pre-charge voltage for each of the columns of LED pixels based on a target luminance level selected from the plurality of luminance levels and supplying the determined pre-charge voltage to each of the columns of LED pixels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
         FIG. 1  is a schematic diagram of a prior art display; 
         FIG. 2  is a schematic diagram of a portion of the display of  FIG. 1 ; 
         FIG. 3  is a schematic diagram of a prior art column driver of the display of  FIG. 1 ; 
         FIG. 4  is a schematic diagram of a portion of a display in accordance with one exemplary embodiment of the invention; 
         FIG. 5  is a schematic diagram of an exemplary embodiment of a column driver; 
         FIG. 6  is a flow diagram of a method for controlling the luminance of a display in accordance with one exemplary embodiment of the invention; and 
         FIG. 7  is a graph illustrating an example of at least one of the advantages of the various embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. 
       FIG. 1  is a schematic diagram of a prior art display  100  including an array  105  of active matrix light-emitting diode (AMLED) pixels  110  arranged in a plurality of columns  107  and rows  109 . Each of the columns  107  is coupled to a different column driver  120  and each of the rows  109  is coupled to a different pair of row drivers  130 . 
     As shown in  FIG. 2 , which is a more detailed schematic diagram of a portion  200  of display  100 , each of the column drivers  120  are coupled to a display timing controller  225  that is configured to transmit video data to column drivers  120 . Furthermore, each of the column drivers  120  and each of the pairs of row drivers  130  operate in conjunction with one another to provide current to, and thus illuminate, each of the AMLED pixels  110 . The rows  109  are illuminated one row at a time during a cycle, and a period of time when each of the AMLED pixels is OFF (i.e., a blanking period) is inserted between successive cycles. 
     As  FIG. 2  also depicts, column driver  120  is coupled to each of the AMLED pixels  110  in its respective column  107  via a data bus  235 . Data bus  235  comprises a plurality of resistor-capacitor (RC) circuits  240 , each comprising a capacitive element (e.g., one or more capacitors)  244  coupled in parallel with a resistive element (e.g., one or more resistors)  247 . Each RC circuit  240  is further coupled (via a node  1112 ) to a switch (e.g., a semiconductor switch)  1102  of AMLED pixel  110 . 
     Switch  1102  is coupled to (via a node  1115 ), and switched ON/OFF by, a row driver  134  (coupled to ground) of the pair of row drivers  130  (see  FIG. 1 ). Switch  1102  is also coupled to a node  1114 , and node  1114  is coupled to a capacitor  1125  and a switch  1104 . Switch  1104  is switched ON/OFF by current supplied from capacitor  1125  and column driver  120  (via row driver  134  and switch  1102 ). Capacitor  1125  is also coupled to a node  1116 , and node  1116  is coupled between the positive terminal of a voltage source  1130  (the negative terminal being coupled to ground) and a switch  1106 . 
     Switch  1106  is coupled to, and switched ON/OFF by, a row driver  138  (coupled to ground) of the pair of row drivers  130  (see  FIG. 1 ), and is also coupled to a node  1118 . Node  1118  is coupled to switch  1104 , switch  1106 , and a switch  1108 . Switch  1108  is coupled to (via node  1115 ), and switched ON/OFF by, row driver  134 , and is also coupled to node  1112 . 
     AMLED pixel  110  also includes an LED  1150 . LED  1150  is coupled to switch  1104  and coupled to a negative terminal of a voltage source  1160 , the positive terminal being coupled to ground. 
       FIG. 3  is a schematic diagram of one of the column drivers  120  (see  FIG. 1 ). Column driver  120  includes a voltage source  1210  coupled to a digital-to-analog converter (DAC)  1220 , which is configured to convert digital voltages to analog voltages. DAC  1220  is also coupled to a buffer  1230 , which is coupled to a current converter  1240 . Current converter  1240  is configured to generate current from the analog voltage signal produced by DAC  1220  (and amplified by buffer  1230 ). 
     During operation, voltage source  1210  receives video data from display timing controller  225  (see  FIG. 2 ) and generates a digital representation of the desired analog voltage, hereafter referred to as a digital voltage. The generated digital voltage varies depending on the brightness and/or color of the AMLED pixel(s)  110  to be illuminated. DAC  1220  then converts the digital voltage to an analog voltage, and the analog voltage is supplied to buffer  1230  for amplification. The amplified analog voltage is converted to current by current converter  1230 , and current converter  1230  supplies the current to data bus  235  (see  FIG. 2 ) in conjunction with current supplied from the pair of row drivers  130 . 
       FIG. 4  is a schematic diagram of a portion an exemplary embodiment of a display  400 , which comprises some components similar to display  100  discussed above. Display  400  comprises a display timing controller  425  coupled to a column driver  420  and a switch  450 . Display timing controller  425  is configured to transmit video data to column driver  420  and switch  450  based on the information to be shown on display  400 . 
     Column driver  420  comprises a programming circuit  430  and a pre-charge circuit  440 , which are each selectively coupled to AMLED pixels  110  via switch  450 . Programming circuit  430  is configured to provide current to AMLED pixels  110  (via switch  450 ) in conjunction with the pair of row drivers  130  for each respective row  109 . Pre-charge circuit  440  is configured to provide a pre-charge voltage (via switch  450 ) to data bus  235  to pre-charge each capacitor  244  prior to programming circuit  430  and row drivers  134  and  138  providing current to AMLED pixels  110 . 
       FIG. 5  is a schematic diagram of one exemplary embodiment of programming circuit  430  and pre-charge circuit  440  of column driver  420 . Programming circuit  430  comprises voltage source  1210 , DAC  1220 , buffer  1230 , and current converter  1240  configured similar to previously-discussed column driver  120  (see  FIG. 3 ). Because the configuration and operation of this circuit has already been discussed, it will not be discussed again. 
     Pre-charge circuit  440  comprises a programmable pre-charge voltage source  4410  coupled to a DAC  4420  (e.g., a voltage digital-to-analog converter (VDAC)), which is configured to convert digital voltages to analog voltages. In one embodiment, pre-charge voltage source  4410  comprises a look-up table  4412  and a memory  4414 . Look-up table  4412  is configured to store a plurality of voltages corresponding to a plurality of luminance levels for each of the AMLED pixels  110  in its respective column  107 . In another embodiment, lookup table  4412  is implemented globally (i.e., “off-board”) on a separate chip (not shown), and is in communication with each column driver  420  of the display. In yet another embodiment, look-up table  4412  is a global lookup table that downloads (e.g. at power up) into memory  4414  of each of the column drivers  420 . 
     As noted, look-up table  4412  comprises a plurality of digital voltage values that correspond to a plurality of brightness levels for AMLED pixels  110 . For example, AMLED pixels  110  are capable of being illuminated at 256 brightness levels, and look-up table  4412  stores individual digital voltages that correspond to each voltage level. That is, for brightness levels ranging from level 0 to level 255, look-up table  4412  stores 256 digital voltage values that correspond to the 256 brightness levels. In one embodiment, look-up table  4412  stores voltage values from about 0 volts to about 15 volts. Although the example specifically recites 256 levels and an associated range of voltages, the invention contemplates that look-up table  4412  may include any number of brightness levels and various ranges of voltages that vary depending on the desired brightness (luminance) of display  400  That is, the invention contemplates the use of an infinite number of voltages to produce an infinite number of colors and/or brightness levels. 
     In accordance with one exemplary embodiment, look-up table  4412  is a non-proportional look-up table. That is, look-up table  4412  comprises voltage values to compensate for non-ideal display operating characteristics (e.g., delta current feed through) related to the color and circuit design of AMLED pixel  110 , in addition to the pre-charge voltage needed for gray level. Specifically, when AMLED pixel  110  is programmed to a desired current, and is then commanded to operate in hold mode, the current through AMLED pixel  110  changes from its programmed current value by an amount equal to the delta current feed through. Parasitic capacitances between the transistor gates and the transistor source and drain connections of AMLED pixel  110  cause bias voltage shifts when the transistors are enabled and disabled. These voltage shifts, in turn, produce changes in the programmed current values. 
     With respect to color produced by AMLED pixel  110 , each color is produced by a diode (e.g., diode  1150 ) with unique electrical properties because the dielectric constant may be unique for any given emitter material. The forward voltages of diode  1150  may also be unique, and the conductive properties of each diode  1150  will vary. The degree to which any of these characteristics adversely affects programming of AMLED pixel  110  may be characterized, and a particular compensation voltage applied by lookup table  4412  based on these factors. Specifically, look-up table  4412  provides compensation for gray level, the circuit design of AMLED pixel  110 , and the color of AMLED pixel  110  when the programming current and pre-charge voltage are determined and applied to display  400 . 
     In another embodiment, the pre-charge voltage is one of a plurality of pre-determined voltages based on an associated gray level of the image to be displayed. That is, pre-charge voltage source  4410  is configured to modify the amount of pre-charge voltage it supplies to DAC  4420  based on the gray level of each respective image to be displayed on display  400 . 
     During operation, display timing controller  425  commands switch  450  to couple pre-charge circuit  440  to data bus  235 . Display timing controller  425  also provides video data to pre-charge circuit  440 . In response to the video data, pre-charge circuit  440  utilizes look-up table  4412  to determine the amount of voltage needed to charge capacitive elements  244  for the particular image to be displayed on display  400 . Once the proper pre-charge voltage is determined, pre-charge voltage source  4410  supplies the voltage to DAC  4420 , which converts the digital voltage to an analog voltage. The analog voltage is amplified by buffer  4430  and applied to the capacitive elements  244  on data bus  235  via switch  450 . 
     Once the capacitive elements are appropriately pre-charged, display timing controller  425  commands switch  450  to connect data bus  235  to programming circuit  430 . Programming circuit  430  and row drivers  134  and  138  then provide current to each AMLED pixel  110  so that individual pixels in array  105 , are illuminated with the appropriate color(s) and/or brightness(es). 
       FIG. 6  is a flow diagram of one exemplary embodiment of a method  600  for controlling the luminance of a display (e.g., display  400 ). Method  600  begins by one or more column drivers (e.g., column drivers  420 ) receiving video data to be displayed on display  400  from a display timing controller (e.g., display timing controller  425  of  FIG. 4 ) (step  605 ). The video data includes the color and/or brightness level of at least one column  107  of AMLED pixels  110  of display  400 . 
     Column driver  420  then determines the pre-charge voltage needed for the capacitances (e.g., capacitive elements  244 ) on the data bus (e.g., data bus  235 ) (step  610 ). The pre-charge voltages vary depending on the color, delta feed-through current, and/or brightness required for each AMLED pixel  110 . That is, the image (as dictated by the video data) to be displayed on display  400  determines the amount of voltage needed to pre-charge capacitive elements  244  prior to current being supplied from column driver  420  (via programming circuit  430 ). In one embodiment, column driver  420  matches the color and/or brightness level of each AMLED pixel  110  in the video data to the corresponding voltage representing that particular color and/or brightness level in a look-up table (e.g., look-up table  4412 ). 
     Once the pre-charge voltage is determined, column driver  420  provides the pre-charge voltage determined from look-up table  4412  to data bus  235  to pre-charge the capacitive elements  244  on data bus  235  (step  615 ). After the capacitive elements  244  have been pre-charged, column drivers  420  provide current (e.g., programming current) to each column  107  of AMLED pixels  110  in conjunction with each pair of row drivers  130  (step  620 ). 
       FIG. 7  is a graph  700  illustrating an example of at least one of the advantages of the various embodiments of the invention. Graph  700  depicts a curve  702  representing the programming time of AMLED pixel  110  utilizing a conventional column driver (e.g., column driver  120 ), and a curve  704  representing the programming time of AMLED pixel  110  utilizing the various embodiments of column driver  420 . 
     As illustrated, the programming time of AMLED pixel  110  is significantly less utilizing column driver  420 . Furthermore, column driver  420  enables AMLED pixel  110  to be programmed with very small amounts of current, which allows AMLED pixel  110  to have a greater range of colors and/or a greater number luminance levels. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.