Abstract:
An improved electrical circuit design and method to drive a plurality of LEDs in an LCD backlight in order to produce a uniform color distribution across the entire viewable surface of the display. The embodiments disclosed have features that permit a predetermined reduction in the amount of current provided to the LEDs positioned along the edge of the display region. This results in color uniformity, and consequently, an improved picture quality.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a non-provisional patent application and claims no priority. 
   TECHNICAL FIELD 
   An exemplary embodiment relates in general to light emitting diode (LED) control circuits, and more particularly, to an electrical circuit that can solve the problems, which may be caused by the arrangement of such LEDs in an LED backlight panel. 
   BACKGROUND OF THE ART 
   Liquid Crystal Displays (LCDs) often incorporate backlight panels to permit viewing in poor lighting conditions. A cold cathode fluorescent lamp (CCFL) is widely used as a light source of a conventional backlight for an LCD. Since the CCFL uses mercury gas, it may cause environmental pollution. Furthermore, the CCFL has a relatively slow response time and a relatively low color reproduction. In addition, the CCFL is not proper to reduce the weight, thickness, and overall volume of an LCD panel to which it is applied. 
   The use of LEDs (Light Emitting Diodes) is also known for the purpose of illuminating such LCD displays. LEDs are eco-oriented and have a response time of several nanoseconds, thereby being effective for a video signal stream and enabling impulsive driving. Furthermore, the LEDs have 100% color reproduction and can properly vary luminance and color temperature by adjusting a quantity of light emitted from red, green and blue LEDs. In addition, the LEDs are proper to reduce the weight, thickness and overall volume of the LCD panel. Therefore, in recent years, they have been widely used as a light source of a backlight unit for the LCD. 
   The LCD backlight employing the LEDs can be classified into an edge type backlight and a direct type backlight according to positions of the light source. In the edge type backlight, the light source is positioned at a side and emits light toward a front surface of the LCD panel using a light guide plate. In the direct type backlight, the light source is a surface light source placed under the LCD panel and having a surface area almost identical to that of the LCD panel and directly emits light toward the front surface of the LCD panel. 
   For direct type LED backlighting of LCD displays, it is desirable to use color (red, green, blue) LEDs to achieve the best color presentation through the LCD glass. The high brightness color LEDs are arranged in a pattern behind the LCD glass, and for many applications the surface area available for LEDs is no larger than the area of the LCD glass. This results in a pattern that will be non uniform along the edges of the LCD. For example, the top edge of this pattern may have too much red and green light, and the bottom edge of the pattern may have too much blue light. 
   There is an unmet need in the art for a system that produces color uniformity along the edges of LED or OLED displays. 
   SUMMARY 
   It is possible to attenuate the bright LED regions with mechanical transmission filters. However, an exemplary embodiment of the present invention solves the problem of color uniformity with an innovative electrical circuit. Accordingly, at least one embodiment is directed to a surface light source that substantially obviates one or more problems due to limitations and disadvantages of the related art. 
   At least one embodiment is an electrical circuit that will drive a plurality of LEDs to produce a uniform color distribution across the entire viewable surface of an LCD display. 
   To achieve these advantages and in accordance with exemplary embodiments of the invention, there is provided an electrical circuit design with features that permit a reduction in the amount of current flowing to the LEDs positioned along the edge of the display region. In at least one embodiment, the LEDs are arranged in series configuration, divided between the “center LEDs” and the “edge LEDs”. At the electrical node between the center and edge LEDs a shunt tap is adapted to divert a portion of the total current away from the edge LEDs, thereby attenuating the light emitted from the edge positioned LEDs. This results in an improved picture quality. 
   Also disclosed are exemplary methods for achieving color uniformity in an LCD display with an LED backlight. In at least one exemplary method a plurality of LEDs is arranged along an electrical circuit in series. The edge LEDs are then divided from the center LEDs by way of an electrical node. A predetermined amount of current is diverted away from the edge LEDs through a shunt tap placed at the node. This attenuates the light emitted by the edge LEDs. 
   Additional advantages and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. It is to be understood that both the foregoing general description and the following detailed description of the at least one embodiment are exemplary and explanatory. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of an exemplary embodiment will be obtained from a reading of the following detailed description and the accompanying drawings wherein identical reference characters refer to identical parts and in which: 
       FIG. 1  is an explanatory schematic illustrating the color uniformity difficulties that arise when multi-color LEDs are used for backlighting an LCD display. 
       FIG. 2  shows an exemplary embodiment of an LED backlight shunting system. 
       FIG. 3  is a circuit diagram illustrating an embodiment of the shunting process. 
       FIG. 4  is a circuit diagram illustrating an embodiment of the shunting process with pulse width modulation included. 
       FIG. 5  is a circuit diagram illustrating an embodiment utilizing an exemplary shunt control mechanism that may be employed. 
       FIG. 6  is an example LED control circuit. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   With reference to  FIG. 1 , a schematic showing one possible arrangement of multi-color LEDs in an LCD backlight for a display  75 . The display area has discrete edges; a right edge  12 , a bottom edge  15 , a left edge  9  and a top edge  6 . The color LEDs are placed in color groups  33  comprising Red (R) LEDs  3 , Green (G) LEDs  5 , and Blue (B) LEDs  4 . More than 3 color varieties may be present for some LED backlights. As shown, each LED color group  33  may be uniformly arranged to provide backlighting for an electronic display  75 . Display  75  represents a typical display area which generally is square or rectangular in shape. Other shapes are possible and the actual number of LEDs needed for the backlight will depend on the size of the display  75 , the luminous flux of each LED, and the required brightness of the display. 
   As may be understood from  FIG. 1 , color uniformity issues arise when multi-color LEDs are arranged to provide the backlighting for an LCD screen. Although most of the LCD display will have a uniform mixture of Red, Green, and Blue light, the edge portions of the display will tend to emit an overabundance of the particular colored light from the specific LEDs that are concentrated along the given edge. As is apparent, color uniformity problems will occur no matter how the LEDs color groups  33  are arranged if the color groups are arranged uniformly. Furthermore, non-uniform arrangements of the LED color groups  33  will only shift the color uniformity issues toward the center of the display  75 . 
   In the example shown if  FIG. 1 , the bottom edge  15  of the display  75  shown will tend to have an overabundance of blue light because there are more blue LEDs  4  located in that region. Similarly, the left edge  9  will appear overly red because of the position of the red LEDs  3 . Likewise, the right edge  12  of the display will look overly Green because there is a concentration of green LEDs  5 . Similar problems will exist at each of the display edges. The effect from these unevenly mixed regions of color LEDs is an undesirable picture quality. 
     FIG. 2  shows an exemplary LED active shunt current control system for controlling current flow through an LED backlight network. The schematic includes a plurality of LEDs arranged in series along a circuit (D 1 -D 5 ). The LEDs at the edge position  94  (“edge LEDs”) in the display are divided from center LEDs  63  in the display by an electrical node  23 . The term “edge LEDs” refers to all those LEDs that line the peripheral edges of the display  75  (e.g., Red LED  3 , Blue LED  4 , and Green LED  5  in  FIG. 1 ). The term “center LEDs” refers to all other LEDs making up the backlight panel. A shunt tap  101  is located at the electrical node  23  between the center LEDs  63  and the edge LEDs  94 . 
   In operation, current passes through the center LEDs, “Icenter”. However, before the current reaches the edge LEDs  94 , the shunt tap  101  may divert a predetermined amount of that current, “I-shunt” away from the edge LEDs  94 . Only the remaining current “ledge” is available to drive edge LEDs  94 . In this arrangement, ledge may be determined by the equation: ledge=Icenter−Ishunt. In this way, the overabundance of colored light produced by edge LEDs may be attenuated to improve the picture quality. As may be appreciated by one of skill in the art, there are many possible ways to regulate the Ishunt value and thus determine the extent to which the edge LED light emission is attenuated. 
     FIG. 3  shows one exemplary system that may be used to shunt current away from edge LEDs  94 . In this embodiment, two analog inputs are provided by an analog output generator, microprocessor  288 . The outputs comprise the shunt setting  420  and the edge setting  402 . These voltages set the reference currents for the edge control circuit  206  and the shunt control circuit  306 . In the embodiment shown, the regulated currents, ledge and Ishunt, may be proportional to the two output voltages, edge setting  402  and shunt setting  420 , respectively. However, the two outputs, shunt setting  420  and edge setting  402 , are independent of one another. Note that the two outputs may be adjusted as necessary to achieve the desired attenuation of the edge LEDs light emissions. 
   Although a microprocessor  288  is a preferred way of accomplishing the output voltages, the microprocessor  288  is not required. Only the EDGE setting  402  and Shunt setting  420  outputs are needed. Resistive dividers (not shown) may also be used to provide these outputs. 
   An edge LED control circuit  206  may receive the edge setting output  402 . The edge control circuit  206  senses ledge through an edge current feedback signal  95  because of the placement of resistor RE 1   231 . The circuit then produces an LED voltage  605  at the anode of D 1  to maintain the edge current as specified by the edge setting output  402 . 
   A shunt control circuit  306  is utilized to determine the shunt transistor (Q 3 ) control current  131 . The shunt control circuit  306  receives the shunt setting output  420 . The shunt control circuit  306  also receives a shunt current feedback signal  90  because of the placement of resistor RS 1   320 . With the shunt current feedback signal  90 , the shunt control circuit may then control transistor Q 3  base current  131  to maintain the Ishunt specified by the shunt setting  420 . 
   In operation, the LEDs are arranged in a series configuration with a shunt current tap at the node between the center LEDs  63  and the edge LEDs  94 . At the electrical node  23  between D 3  and D 4 , a portion of the center LED current is diverted away from the Edge LEDs through transistor Q 3   315  and resistor RS 1   320  under control of the shunt setting output of the Microprocessor  288  control. The amount of Ishunt depends on the base current reaching transistor Q 3  ( 315 ). Transistor Q 3   315  operates in analog mode to determine the Ishunt current. 
     FIG. 4  shows a diagram of an embodiment incorporating pulse width modulation (PWM). In this case there are at least three inputs to the LED Active Shunt current control system. As with the last embodiment there is an EDGE setting  402  that sets the edge LED current, ledge, and a shunt setting  420  to set the shunt current, Ishunt. However, in this embodiment, a pulse width modulation is provided. PWM is a common method of LED brightness dimming. PWM dimming is not required for the LED Active shunt operation, but is included here for illustration. In the embodiment diagramed in  FIG. 4 , there are again two control circuits which operate as previously described in  FIG. 3 . However, with the addition of pulse width modulation the shunt control circuit  306  may shut off transistor Q 3   315  during PWM inactive for dimming purposes. Furthermore, transistor Q 2   703  operates in a digital mode to turn off edge LED current during PWM inactive for dimming purposes. 
     FIG. 5  illustrates another exemplary shunt control circuit  306  to actively shunt current away from the edge LEDs. As with  FIG. 4 , this embodiment also includes PWM. The LED edge control circuit  206  is a standard circuit that senses LED current at RE 1   231 . An edge current feedback signal  95  is sent to the edge control circuit  206 . The edge control circuit  206  may then modify the LED voltage  605  applied at the anode node of D 1 . An N-channel field effect transistor (N-FET) Q 2   703  provides dimming control via the PWM (pulse width modulation) signal. The edge control circuit  206  sets the edge LED current under control of the “EDGE setting” output  402  of the Microprocessor  288  control. 
   At the node between D 3  and D 4  a portion of the Center LED current is again diverted through transistor Q 3   315  and resistor RS 1   320  under control the “Shunt setting” output  420  of the Microprocessor  288  control. For dimming control, Q 1   807  sets the Ishunt to zero during PWM inactive. The Ishunt sensed by RS 1   320  is input to operational amplifier “A”  613  with an arbitrary gain. The output OUTA from operational amplifier “A”  613  is used as a feedback input to operational amplifier “B”  619 . Operational Amplifier “B”  619  produces a voltage output on OUTB  67  such that the differential input voltage between “−INB” and “+INB” is zero. The voltage output on OUTB then provides the base current for transistor Q 3   315 . This determines the shunt current, Ishunt. Capacitors C 1 - 540 , C 2 - 541 , C 3 - 542 , and C 5 - 544  modify the AC behavior of the circuit to control loop stability and response time. 
     FIG. 6  provides an example LED control circuit which may be used with certain embodiments disclosed herein. As may be appreciated by one skilled in the art, the LED control circuit shown is one of many possible LED control circuits that may be used to determine LED voltage  605 . The example shown here is for illustration. 
   Having shown and described exemplary embodiments of the invention, those skilled in the art will realize that many variations and modifications may be made to affect the described invention and still be within the scope of the claimed invention. Thus, many of the elements indicated above may be altered or replaced by different elements which will provide the same result and fall within the spirit of the claimed invention. It is the intention, therefore, to limit the invention only as indicated by the scope of the claims.