PATENT DOCUMENT

Publication Number: US-8575865-B2
Application Number: US-41016409-A
Country: US
Kind Code: B2

Title: Temperature based white point control in backlights

Abstract:
Systems, methods, and devices are provided for maintaining a target white point on a light emitting diode based backlight. In one embodiment, the backlight may include two or more strings of light emitting diodes, each driven at a respective driving strength. Each string may include light emitting diodes from a different color bin, and the respective driving strengths may be adjusted, for example, through pulse width modulation or amplitude modulation, to maintain the target white point. In certain embodiments, the driving strengths may be adjusted to compensate for shifts in the white point that may occur due to temperature or aging. A controller may adjust the driving strengths based on feedback from a temperature sensor, from an optical sensor, from a user input, or from calibration data included within the backlight or system.

Claims:
What is claimed is: 
     
       1. A display, comprising:
 a plurality of white light emitting diodes arranged within an electronics region and other regions of a backlight to produce an emitted white point that corresponds to a target white point, wherein the electronics region is configured to produce more heat during operation of the display than the other regions; 
 a plurality of color compensating light emitting diodes arranged only within the electronics region of the backlight and intermixed with the plurality of white light emitting diodes; 
 one or more drivers configured to drive groups of one or more color compensating light emitting diodes of the plurality of color compensating light emitting diodes at a respective driving strength; and 
 a controller configured to detect a temperature change within the electronics region and to adjust at least one of the respective driving strengths to compensate for a shift in the emitted white point of white light emitting diodes arranged within the electronics region due to the temperature change. 
 
     
     
       2. The display of  claim 1 , wherein the plurality of white light emitting diodes are selected from neighboring bins surrounding a bin encompassing the target white point. 
     
     
       3. The display of  claim 1 , wherein the controller is configured to detect the temperature change within the electronics region based on an operational state of the backlight. 
     
     
       4. The display of  claim 1 , comprising a memory configured to store a calibration curve defining driving strength adjustments based on the detected temperature change within the electronics region. 
     
     
       5. The display of  claim 1 , comprising one or more sensors disposed in the backlight and configured to detect the temperature change within the electronics region. 
     
     
       6. The display of  claim 5 , wherein the sensors comprise optical sensors configured to sense a color of the emitted white point, and wherein the controller is configured to detect the temperature change within the electronics region based on the sensed color. 
     
     
       7. The display of  claim 1 , wherein the color compensating light emitting diodes comprise color light emitting diodes. 
     
     
       8. The display of  claim 1 , wherein the one or more drivers are configured to drive groups of one or more white light emitting diodes of the plurality of white light emitting diodes at a specified driving strength, and wherein the controller is configured to maintain the specified driving strength at a constant rate. 
     
     
       9. A display, comprising:
 a plurality of white light emitting diode strings each arranged within an electronics region and other regions of a backlight to produce an emitted white point that substantially matches a target white point, wherein the electronics region is configured to produce more heat during operation of the display than the other regions; 
 a dedicated light emitting diode string disposed only in the electronics region of the backlight, wherein the dedicated light emitting diode string comprises one or more color compensating light emitting diodes, wherein only the electronics region comprises color compensating light emitting diodes; 
 a driver configured to independently drive the plurality of white light emitting diode strings of the electronics region and the other regions at specified driving strengths and to drive the dedicated light emitting diode string at a dedicated driving strength; and 
 a controller configured to detect a temperature change within the electronics region and to adjust the dedicated driving strength to compensate for a shift in the emitted white point of white light emitting diode strings arranged in the electronics region due to the temperature change. 
 
     
     
       10. The display of  claim 9 , wherein the electronics region is adjacent to circuit boards. 
     
     
       11. The display of  claim 9 , wherein the plurality of white light emitting diode strings are configured to emit a first range of chromaticity values, and the dedicated light emitting diode string is configured to emit a second range of chromaticity values outside of the first range. 
     
     
       12. The display of  claim 9 , wherein the plurality of white light emitting diode strings comprises a first string disposed in the electronics region and a second string disposed in the other regions. 
     
     
       13. The display of  claim 9 , wherein the plurality of white light emitting diode strings are disposed in a light strip and the dedicated light emitting diode string is disposed in another light strip. 
     
     
       14. The display of  claim 9 , comprising one or more sensors configured to measure temperatures within the backlight, and wherein the controller is configured to detect the temperature change based on the measured temperatures. 
     
     
       15. The display of  claim 14 , wherein the one or more sensors are configured to detect a temperature gradient within the backlight, and wherein the controller is configured to adjust the specified driving strengths and the dedicated driving strength to compensate for the temperature gradient. 
     
     
       16. A method of operating a backlight of a display, the method comprising:
 independently driving each of a plurality of white light emitting diode strings at a respective white point driving strength to produce an emitted white point that substantially matches a target white point, wherein each of the plurality of white light emitting diode strings is arranged within an electronics region and other regions of the backlight, and the electronics region produces more heat during operation of the display than the other regions; 
 driving one or more color compensating light emitting diode strings disposed only in the electronics region at a respective color compensating driving strength, wherein only the electronics region comprises color compensating light emitting diode strings; 
 detecting a temperature change within the electronics region of the backlight; 
 determining a shift in the emitted white point of each of the white light emitting diode strings arranged within the electronics region based on the detected temperature change; and 
 adjusting at least one of the respective color compensating driving strengths to compensate for the shift. 
 
     
     
       17. The method of  claim 16 , wherein detecting a temperature change within the electronics region of the backlight comprises sensing a color of light output by the white light emitting diode strings arranged within the electronics region and correlating the sensed color to the temperature change. 
     
     
       18. The method of  claim 16 , wherein detecting a temperature change within the electronics region of the backlight comprises receiving a temperature of the backlight from a temperature sensor disposed within the electronics region of the backlight. 
     
     
       19. The method of  claim 16 , wherein detecting a temperature change comprises detecting an operating period of the backlight, and wherein adjusting at least one of the respective color compensating driving strengths comprises adjusting the respective color compensating driving strength at a rate determined by a calibration curve for the detected operating period. 
     
     
       20. The method of  claim 16 , wherein adjusting at least one of the respective color compensating driving strengths comprises varying the respective color compensating driving strength at a constant rate until a temperature stabilization period is detected. 
     
     
       21. The method of  claim 16 , wherein adjusting at least one of the respective color compensating driving strengths comprises:
 determining a temperature gradient within the backlight through feedback received from one or more temperature sensors; and 
 adjusting the respective color compensating driving strengths to balance the white point across the temperature gradient. 
 
     
     
       22. The method of  claim 16 , comprising determining a temperature profile within the backlight based on the detected temperature change within the electronics region of the backlight. 
     
     
       23. The method of  claim 16 , wherein the color compensating light emitting diodes comprise yellow light emitting diodes, red light emitting diodes, or blue light emitting diodes, or a combination thereof.

Description:
BACKGROUND 
     The present disclosure relates generally to backlights for displays, and more particularly to light emitting diode based backlights. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     Liquid crystal displays (LCDs) are commonly used as screens or displays for a wide variety of electronic devices, including portable and desktop computers, televisions, and handheld devices, such as cellular telephones, personal data assistants, and media players. Traditionally, LCDs have employed cold cathode fluorescent light (CCFL) light sources as backlights. However, advances in light emitting diode (LED) technology, such as improvements in brightness, energy efficiency, color range, life expectancy, durability, robustness, and continual reductions in cost, have made LED backlights a popular choice for replacing CCFL light sources. However, while a single CCFL can light an entire display; multiple LEDs are typically used to light comparable displays. 
     Numerous white LEDs may be employed within a backlight. Depending on manufacturing precision, the light produced by the individual white LEDs may have a broad color or chromaticity distribution, for example, ranging from a blue tint to a yellow tint or from a green tint to a purple tint. During manufacturing, the LEDs may be classified into bins with each bin representing a small range of chromaticity values emitted by the LEDs. To reduce color variation within a backlight, LEDs from similar bins may be mounted within a backlight. The selected bins may encompass the desired color, or target white point, of the backlight. 
     High quality displays may desire high color uniformity throughout the display, with only small deviations from the target white point. However, it may be costly to utilize LEDs from only one bin or from a small range of bins. Further, the white point of the LEDs may change over time and/or with temperature, resulting in deviations from the target white point. 
     SUMMARY 
     A summary of certain embodiments disclosed herein are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects that may not be set forth below. 
     The present disclosure generally relates to techniques for controlling the white point in LED backlights. In accordance with one disclosed embodiment, an LED backlight includes LEDs from multiple color bins. When the light output from the LEDs is mixed, the desired white point may be achieved. The LEDs from each bin may be grouped into one or more strings each driven by a separate driver or driver channel. Accordingly, the driving strength for the LEDs from different color bins may be independently adjusted to fine tune the white point to the target white point. Further, the driving strength of the LEDs may be adjusted to compensate for the shifts in the white point that may occur due to aging of the LEDs, aging of the backlight components, or temperature variations, such as localized temperature gradients within the backlight or variations in ambient temperature, among others. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which: 
         FIG. 1  is a front view of an example of an electronic device employing an LCD display with an LED backlight, in accordance with aspects of the present disclosure; 
         FIG. 2  is a block diagram of an example of components of the electronic device of  FIG. 1 , in accordance with aspects of the present disclosure; 
         FIG. 3  is an exploded view of the LCD display of  FIG. 2 , in accordance with aspects of the present disclosure; 
         FIG. 4  is a perspective view of an edge-lit LCD display that may be used in the electronic device of  FIG. 1 , in accordance with aspects of the present disclosure; 
         FIG. 5  is a block diagram of an example of components of an LCD display, in accordance with aspects of the present disclosure; 
         FIG. 6  is a diagram illustrating LED bins, in accordance with aspects of the present disclosure; 
         FIG. 7  is a front view of an LED backlight illustrating an example of an LED configuration, in accordance with aspects of the present disclosure; 
         FIG. 8  is a front view of an LED backlight illustrating another example of an LED configuration, in accordance with aspects of the present disclosure; 
         FIG. 9  is a front view of an LED backlight illustrating another example of an LED configuration, in accordance with aspects of the present disclosure; 
         FIG. 10  is a schematic diagram illustrating operation of the LED backlight of  FIG. 9 , in accordance with aspects of the present disclosure; 
         FIG. 11  is a flowchart depicting a method for operating an LED backlight, in accordance with aspects of the present disclosure; 
         FIG. 12  is a front view of an LED backlight with color compensating LEDs, in accordance with aspects of the present disclosure; 
         FIG. 13  is a schematic diagram illustrating operation of the LED backlight of  FIG. 12 , in accordance with aspects of the present disclosure; 
         FIG. 14  is a flowchart depicting a method for operating an LED backlight with color compensating LEDs, in accordance with aspects of the present disclosure; 
         FIG. 15  is a front view of an LED backlight with sensors for adjusting driving strength of the LEDs, in accordance with aspects of the present disclosure; 
         FIG. 16  is a schematic diagram illustrating operation of the LED backlight of  FIG. 15 , in accordance with aspects of the present disclosure; 
         FIG. 17  is a flowchart depicting a method for operating an LED backlight employing sensors, in accordance with aspects of the present disclosure; 
         FIG. 18  is a chart depicting the effects of aging on LED brightness, in accordance with aspects of the present disclosure; 
         FIG. 19  is a chart depicting the effects of aging on a white point, in accordance with aspects of the present disclosure; 
         FIG. 19  is a chart depicting the effects of aging on a white point, in accordance with aspects of the present disclosure; 
         FIG. 20  is a flowchart depicting a method for operating an LED backlight to compensate for aging; 
         FIG. 21  is a flowchart depicting a method for operating an LED backlight using a calibration curve, in accordance with aspects of the present disclosure; 
         FIG. 22  is a chart depicting the effects of temperature on LED chromaticity, in accordance with aspects of the present disclosure; 
         FIG. 23  is a chart depicting the change in temperature of an LCD display, in accordance with aspects of the present disclosure; 
         FIG. 24  is a front view of an LED backlight depicting the location of electronics, in accordance with aspects of the present disclosure; 
         FIG. 25  is a schematic diagram illustrating operation of the LED backlight of  FIG. 24 , in accordance with aspects of the present disclosure; 
         FIG. 26  is a flowchart depicting a method for operating an LED backlight during variations in temperature, in accordance with aspects of the present disclosure; 
         FIG. 27  is a front view of an LED backlight employing color compensating LEDs, in accordance with aspects of the present disclosure; 
         FIG. 28  is a schematic diagram illustrating operation of the LED backlight of  FIG. 27 ; 
         FIG. 29  is a front view of an LED backlight employing different LED strings to compensate for temperature, in accordance with aspects of the present disclosure; 
         FIG. 30  is a schematic diagram illustrating operation of the LED backlight of  FIG. 28 , in accordance with aspects of the present disclosure; 
         FIG. 31  is a front view an edge-lit LED backlight, in accordance with aspects of the present disclosure; 
         FIG. 32  is a front view of an LED backlight employing sensors, in accordance with aspects of the present disclosure; 
         FIG. 33  is a schematic diagram illustrating operation of the LED backlight of  FIG. 32 , in accordance with aspects of the present disclosure; 
         FIG. 34  is a flowchart depicting a method for operating an LED backlight with sensors during variations in temperature, in accordance with aspects of the present disclosure; and 
         FIG. 35  is a flowchart depicting a method for operating an LED backlight with sensors to compensate for aging effects and temperature variations, in accordance with aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     1. Introduction 
     One or more specific embodiments will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     The present disclosure is directed to techniques for dynamically controlling the white point of LED backlights. The backlights may include LEDs from multiple bins having various chromaticity values and/or brightness values. LEDs from each bin may be grouped together into one or more strings, controlled independently by separate drivers or driver channels. The independent control allows each string of LEDs to be operated at a separate driving strength to fine-tune the white point of the LED backlight. The driving strength may be adjusted by manufacturing settings, user input, and/or feedback from sensors. In certain embodiments, calibration curves may be employed to adjust the driving strength to compensate for aging and/or temperature effects. In other embodiments, sensors detecting color, brightness, and/or temperature may be employed to adjust the driving strength of the drivers or channels to maintain the desired white point. 
       FIG. 1  illustrates electronic device  10  that may make use of the white point control techniques for an LED backlight as described above. It should be noted that while the techniques will be described below in reference to illustrated electronic device  10  (which may be a laptop computer), the techniques described herein are usable with any electronic device employing an LED backlight. For example, other electronic devices may include a desktop computer, a viewable media player, a cellular phone, a personal data organizer, a workstation, or the like. In certain embodiments, the electronic device may include a model of a MacBook®, a MacBook® Pro, MacBook Air®, iMac®, Mac® mini, or Mac Pro® available from Apple Inc. of Cupertino, Calif. In other embodiments, the electronic device may include other models and/or types of electronic devices employing LED backlights, available from any manufacturer. 
     As illustrated in  FIG. 1 , electronic device  10  includes housing  12  that supports and protects interior components, such as processors, circuitry, and controllers, among others, that may be used to generate images to display on display  14 . Housing  12  also allows access to user input structures  16 , such as a keypad, track pad, and buttons, that may be used to interact with electronic device  10 . For example, user input structures  16  may be manipulated by a user to operate a graphical user interface (GUI) and/or applications running on electronic device  10 . In certain embodiments, input structures  16  may be manipulated by a user to control properties of display  14 , such as the brightness the brightness and/or color of the white point. The electronic device  10  also may include various input and output (I/O) ports  18  that allow connection of device  10  to external devices, such as a power source, printer, network, or other electronic device. In certain embodiments, an I/O port  18  may be used to receive calibration information for adjusting the brightness and/or color of the white point. 
       FIG. 2  is a block diagram illustrating various components and features of device  10 . In addition to display  14 , input structures  16 , and I/O ports  18  discussed above, device  10  includes a processor  22  that may control operation of device  10 . Processor  22  may use data from storage  24  to execute the operating system, programs, GUI, and any other functions of device  10 . In certain embodiments, storage  24  may store a program enabling a user to adjust properties, such as the white point color or brightness, of display  14 . Storage  24  may include a volatile memory, such as RAM, and/or a non-volatile memory, ROM. Processor  22  also may receive data through I/O ports  18  or through network device  26 , which may represent, for example, one or more network interface cards (NIC) or a network controller. 
     Information received through network device  26  and I/O ports  18 , as well as information contained in storage  24 , may be displayed on display  14 . Display  14  may generally include LED backlight  32  that functions as a light source for LCD panel  30  within display  14 . As noted above, a user may select information to display by manipulating a GUI through user input structures  16 . In certain embodiments, a user may adjust properties of LED backlight  32 , such as the color and/or brightness of the white point, by manipulating a GUI through user input structures  16 . Input/output (I/O) controller  34  may provide the infrastructure for exchanging data between input structures  16 , I/O ports  18 , display  14 , and processor  22 . 
       FIG. 3  is an exploded view of an embodiment of display  14  employing a direct-light backlight  32 . Display  14  includes LCD panel  30  held by frame  38 . Backlight diffuser sheets  42  may be located behind LCD panel  30  to condense the light passing to LCD panel  30  from LEDs  48  within LED backlight  32 . LEDs  48  may include an array of white LEDs mounted on array tray  50 . For example, in certain embodiments, LEDs  48  may be mounted on a Metal Core Printed Circuit Board (MCPCB), or other suitable type of support. 
     The LEDs  48  may be any type of LEDs designed to emit a white light. In certain embodiments, LEDs  48  may include phosphor based white LEDs, such as single color LEDs coated with a phosphor material, or other wavelength conversion material, to convert monochromatic light to broad-spectrum white light. For example, a blue die may be coated with a yellow phosphor material. In another example, a blue die may be coated with both a red phosphor material and a green phosphor material. The monochromatic light, for example, from the blue die, may excite the phosphor material to produce a complementary colored light that yields a white light upon mixing with the monochromatic light. LEDs  48  also may include multicolored dies packaged together in a single LED device to generate white light. For example, a red die, a green die, and a blue die may be packaged together, and the light outputs may be mixed to produce a white light. 
     One or more LCD controllers  56  and LED drivers  60  may be mounted beneath backlight  32 . LCD controller  56  may generally govern operation of LCD panel  30 . LED drivers  60  may power and drive one or more strings of LEDs  48  mounted within backlight  32 . 
       FIG. 4  illustrates an embodiment of display  14  that employs an edge-lit backlight  32 . Backlight  32  may include light strip  64  inserted within frame  38 . Light strip  64  may include multiple LEDs  48 , such as side-firing LEDs, mounted on a flexible strip. LEDs  48  may direct light upwards towards LCD panel  30 , and in certain embodiments, a guide plate may be included within backlight  32  to direct the light from LEDs  48 . Although not shown in  FIG. 4 , backlight  32  may include additional components, such as a light guide plate, diffuser sheets, circuit boards, and controllers among others. Further, in other embodiments, multiple light strips  64  may be employed around the edges of display  14 . 
     2. Dynamic Mixing 
     Additional details of illustrative display  14  may be better understood through reference to  FIG. 5 , which is a block diagram illustrating various components and features of display  14 . Display  14  includes LCD panel  30 , LED backlight  32 , LCD controller  56 , and LED drivers  60 , and possibly other components. As described above with respect to  FIG. 3 , LED backlight  32  may act as a light source for LCD panel  30 . To illuminate LCD panel  30 , LEDs  48  may be powered by LED drivers  60 . Each driver  60  may drive one or more strings of LEDs  48 , with each string containing LEDs  48  that emit light of a similar color and/or brightness. 
     Specifically, LEDs  48  may include groups of LEDs selected from different bins defining properties of the LEDs, such as color or chromaticity, flux, and/or forward voltage. LEDs  48  from the same bin may generally emit light of a similar color and/or brightness. LEDs  48  from the same bin may be joined together in one or more strings, with each string being independently driven by a separate driver or driver channel. The strings may be spatially distributed throughout backlight  32  to emit a light that when mixed substantially matches the target white point. For example, an emitted white point that substantially matches the target white point may be within approximately 0 to 5 percent of the target white point, as well as all subranges therebetween. More specifically, the emitted white point may be within approximately 0 to 1 percent, 0 to 0.5 percent, or 0 to 0.1 percent of the target white point. In certain embodiments, the strings may be interlaced throughout the backlight, while, in other embodiments, certain strings may be positioned within only portions of the backlight. Further, the strings may be positioned in a patterned or random orientation. The driving strength of some or all of the strings may be adjusted to achieve a white point that substantially matches the target white point. In certain embodiments, the individualized driving strength adjustment of LED strings may allow a greater number of LED bins to be used within backlight  32 . 
     The LED strings may be driven by drivers  60 . Drivers  60  may include one or more integrated circuits that may be mounted on a printed circuit board and controlled by LED controller  70 . In certain embodiments, drivers  60  may include multiple channels for independently driving multiple strings of LEDs  48  with one driver  60 . Drivers  60  may include a current source, such as a transistor, that provides current to LEDs  48 , for example, to the cathode end of each LED string. Drivers  60  also may include voltage regulators. In certain embodiments, the voltage regulators may be switching regulators, such as pulse width modulation (PWM) regulators. 
     LED controller  70  may adjust the driving strength of drivers  60 . Specifically, LED controller  70  may send control signals to drivers  60  to vary the current and/or the duty cycle to LEDs  48 . For example, LED controller  70  may vary the amount of current passing from driver  60  to LEDs  48  to control the brightness and/or the chromaticity of the LEDs  48 , for example, using amplitude modulation (AM). In certain embodiments, the amount of current passing through strings of LEDs  48  may be adjusted to produce a white point that substantially matches the target white point. For example, if the emitted white point has a blue tint when compared to the target white point, the current through a string of yellow tinted LEDs may be increased to produce an output that substantially matches the target white point. By increasing the current through strings of LEDs  48 , the overall brightness of backlight  32  also may increase. In other embodiments, the ratio of the currents passing through LED strings may be adjusted to emit a white point that substantially matches the target white point while maintaining a relatively constant brightness. 
     The LED controller  70  also may adjust the driving strength of drivers  60  by varying the duty cycle, for example, using pulse width modulation (PWM). For example, LED controller  70  may increase the frequency of an enable signal to a current source to increase the driving strength for a string of LEDs  48  powered by that current source. The duty cycles for different LED strings may be increased and/or decreased to produce a white point that substantially matches the target white point. For example, if the emitted white point has a green tint when compared to the target white point, the duty cycle for a string of purple tinted LEDs  48  may be increased to produce light that substantially matches the target white point. 
     When adjusting the driving strength through AM, PWM, or other similar techniques, LED controller  70  may increase the driving strength of certain strings, decrease the driving strength of certain strings, or increase the driving strength of some strings and decrease the driving strength of other strings. LED controller  70  may determine the direction of the white point shift, and then increase the driving strength of strength of one or more LED strings with a color complementary to the white point shift. For example, if the white point has shifted towards a blue tint, LED controller  70  may increase the driving strength of yellow tinted strings. LED controller  70  also may decrease the driving strength of one or more LED strings with a tint similar to the direction of the white point shift. For example, if the white point has shifted towards a blue tint, the controller may decrease the driving strength of blue tinted strings. 
     LED controller  70  may govern operation of driver  60  using information stored in memory  72 . For example, memory  72  may store values defining the target white point as well as calibration curves, tables, algorithms, or the like, defining driving strength adjustments that may be made to compensate for a shift in the white point. In certain embodiments, LED controller  70  may dynamically adjust the driving strengths throughout operation of backlight  32  to maintain a light output that matches the target white point. For example, LED controller  70  may receive feedback from sensors  76  describing properties of the emitted light. Sensors  76  may be mounted within backlight  32  or within other components of display  14 . In certain embodiments, sensors  76  may be optical sensors, such as phototransistors, photodiodes, or photoresistors, among others, that sense the color and/or brightness of the light emitted by backlight  32 . In other embodiments, sensors  76  may be temperature sensors that sense the temperature of backlight  32 . Using the feedback from sensors  76 , LED controller  70  may adjust the driving strengths to maintain a light output that matches the target white point and/or brightness. 
     In other embodiments, LED controller  70  may receive feedback from other sources instead of, or in addition to, sensors  76 . For example, LED controller  70  may receive user feedback through input structure  16  ( FIG. 2 ) of electronic device  10 . Electronic device  10  may include hardware and/or software components allowing user adjustment of the white point emitted by backlight  32 . In certain embodiments, display  14  may include a color temperature control that allows a user to select the color temperature (for example, from a small set of fixed values) of the light emitted when display  14  receives an electrical signal corresponding to a white light. LED controller  70  also may receive feedback from device  10  or from backlight  32 . For example, backlight  32  may include a clock that tracks total operating hours of backlight  32 . In certain embodiments, LED controller  70  may compare the operating hours to a calibration curve or table stored in memory  72  to determine a driving strength adjustment. In other embodiments, LED controller  70  may receive feedback from LCD controller  56  or processor  22  ( FIG. 2 ). The feedback may include data describing an operating state of backlight  32  or of electronic device  10 . For example, the feedback may specify the amount of time since backlight  32  or electronic device  10  has been powered on. 
     Based on the feedback received from sensors  76 , device  10 , or backlight  32 , LED controller  70  may adjust the driving strength of LEDs  48 . In certain embodiments, LED controller  70  may determine which strings should be adjusted. The determination may be made based on the color of the LEDs in the string, or the location of the string within backlight  32 , among other factors. 
     In certain embodiments, the backlight may include color compensating LEDs  78 , in addition to white LEDs  48 . The color compensating LEDs may be LEDs of any color and may be selected based on the white point shift generally seen within backlight  32 . In a backlight  32  employing phosphor based white LEDs, the white point may shift towards the color of the LED die as the LED ages. For example, as a blue die coated with a yellow phosphor ages, the blue spectrum emitted by the die may decrease. However, the excited spectrum emitted by the yellow phosphor that mixes with the blue spectrum to produce white light may decrease at a higher rate than the blue spectrum. Therefore, the light emitted may shift towards a blue tint. To compensate for this shift, color compensating LEDs  78  may have a yellow color or tint. In another example, a blue die coated with red and green phosphor materials may shift towards a blue tint, as the red and green excitement spectrums decrease at a faster rate than the blue spectrum. In this example, color compensating LEDs  78  may include intermixed red and green LEDs to compensate for the shift. 
     Color compensating LEDs  78  may be positioned at various locations throughout backlight  32 . In certain embodiments, LED controller  70  may only adjust the driving strength of color compensating LEDs  78  while maintaining the driving strength of white LEDs  48  at a constant rate. However, in other embodiments, color compensating LEDs  78  may be adjusted along with adjustment of white LEDs  48 . 
     As described above with respect to  FIG. 5 , LEDs  48  may be selected from multiple bins, with each bin defining color and/or brightness properties of the LEDs, such as color, brightness, forward voltage, flux, and tint, among others.  FIG. 6  illustrates a representative LED bin chart  80 , such as from a commercial LED manufacturer, that may be used to group LEDs into bins, with each bin of LEDs exhibiting a different white point. Bin chart  80  may generally plot chromaticity values, describing color as seen by a standard observer, on x and y axes  82  and  84 . For example, bin chart  80  may use chromaticity coordinates corresponding to the CIE 1931 chromaticity diagram developed by the International Commission on Illumination (CIE). In certain embodiments, the CIE D series of standard illuminates may be employed, with D65 representing standard D65 representing standard daylight and corresponding to a color temperature of 6,500 K. On bin chart  80 , x-axis  82  may plot the x chromaticity coordinates, which may generally progress from blue to red along x-axis  82 , and y-axis  84  may plot the y chromaticity values, which may generally progress from blue to green along y-axis  84 . 
     Each LED backlight  32  may have a reference or target white point, represented by a set of chromaticity coordinates, tristimulus values, or the like. For example, in certain embodiments, the CIE D series of standard illuminants may be used to select the target white point. LEDs for each backlight  32  may be selected so that when the light from each of the LEDs  48  is mixed, the emitted light may closely match the target white point. In certain embodiments, LEDs  48  also may be positioned within an LED backlight to reduce local variations in the color of the light emitted by backlight  32 . 
     LEDs  48  with a light output close to the target white point may be selected to assemble LED backlight  32  with a light output that substantially matches the target white point. For example, as shown on chart  80 , bin W may encompass the target white point. A backlight employing all bin W LEDs may substantially match the target white point. However, manufacturing costs may be reduced if a larger number of bins are used within a backlight. Accordingly, LEDs from neighboring bins N 1-12 , for example, may be employed within the backlight. The LEDs from the neighboring bins N 1-12  may be selectively positioned, interlaced, or randomly mixed within a backlight to produce an output close to the target white point. The LEDs from the same bin may be joined on separate strings, so that the driving strength of LEDs from different bins may be independently adjusted, for example through AM or PWM, to more closely align the emitted light with the target white point. 
     In certain embodiments, LEDs from two or more neighboring bins N 1-12  may be selected and mixed within an LED backlight. For example, a backlight may employ LEDs from complementary bins N 9  and N 4 ; complementary bins N 3  and N 8 ; complementary bins N 12  and N 6 ; or complementary bins N 9 , N 7 , and N 2 . Moreover, LEDs from the target white point bin W and from the neighboring bins N 1-12  may be mixed to yield the desired white point. For example, a backlight may employ LEDs from bins W, N 7 , and N 2 ; bins W, N 11 , and N 5 ; or bins W, N 1 , and N 6 . Further, color compensating LEDs  78  may be included with white LEDs  48 . Of course, any suitable combination of bins may be employed within a backlight. Further, a wider range of bins that is shown may be employed. 
       FIGS. 7-9  illustrate embodiments of LED arrangements that may be employed within backlights  32 .  FIG. 7  depicts an embodiment of backlight  32  that includes two light strips  64 A and  64 B. LEDs from different bins may be employed within each light strip  64 A and  64 B. Specifically, upper light strip  64 A includes LEDs from bins N 4  and N 9 , while lower light strip  64 B includes LEDs from bins N 9 , N 4 , and W. The LEDs from each bin may be grouped into separate strings so the driving strength may be independently adjusted for each bin to fine tune backlight  32  to the desired white point. In other embodiments, the LED bins employed may vary. 
       FIGS. 8 and 9  illustrate embodiments of backlight  32  with LEDs  48  mounted in array tray  50 . In  FIG. 8 , LEDs from bins W, N 1 , and N 7  are arranged in backlight  32 . Bins N 1 , and N 7  may represent complementary bins selected from opposite sides of white point bin W. In  FIG. 9 , white point bin W is not present. However, LEDs from complementary neighboring bins N 3  and N 8  have been positioned throughout backlight  32 . In other embodiments, multiple patterns or random orders of LEDs from LEDs from any number of neighboring bins N 1-12  may be included within backlight  32 . Further, the number of different bins N 1-12 , and W employed may vary. 
       FIG. 10  is a schematic diagram illustrating operation of LED backlight  32  shown in  FIG. 9 . The LEDs from each bin N 3  and N 8  are organized into separate strings, each driven by a separate driver  60 A or  60 B. Specifically, the string of bin N 8  LEDs is connected to driver  60 A and the string of bin N 3  LEDs is connected to driver  60 B. Each driver  60 A and  60 B is communicatively coupled to LED controller  70 . In certain embodiments, LED controller  70  may transmit control signals to vary the driving strength of each driver. For example, to adjust the white point, LED controller  70  may send signals to drivers  60 A and  60 B to vary PWM duty cycles  88  and  90 . As shown, driver  60  currently energizes the bin N 8  LEDs at PWM duty cycle  88  that has about half the frequency of PWM duty cycle  90  applied by driver  60 B to the bin N 3  LEDs. However, if LED controller  70  determines that a white point adjustment should be made, LED controller  70  may vary one or both of duty cycles  88  and  90  to adjust the white point to match the target white point. 
     In certain embodiments, control signals corresponding to the white point adjustments may be stored within memory  72 . During operation of the backlight, LED controller  70  may make continuous or period adjustments to duty cycles  88  and  90  to maintain a light output that substantially matches the target white point. The independent driving strengths for LEDs from each bin N 3  and N 8  may allow more precise mixing of the light output from each bin of LEDs to achieve the target white point. Further, although the adjustments are shown in the context of PWM duty cycles, in other embodiments, LED controller  70  may adjust the level of the current applied to drivers  60 A and  60 B instead of, or in addition to varying duty cycles  88  and  90 . 
       FIG. 11  depicts a flowchart of a method  92  for dynamically driving LEDs within a backlight. The method may begin by determining (block  94 ) a driving strength for LEDs selected from a first bin, such as bin N 8  shown in  FIG. 10 . For example, LED controller  70  ( FIG. 10 ) may set the driving strength based on data, such as manufacturer settings, calibration curves, tables, or the like, stored in memory  72 . In certain embodiments, LED controller  70  may determine the driving strength based on feedback received from one or more sensors  76  ( FIG. 5 ). In other embodiments, a user may enter the driving strength through the GUI, for example, through input structure  16 , of device  10 . In these embodiments, I/O controller  34  ( FIG. 2 ) may transmit driving strength information from processor  22  ( FIG. 2 ) to display  14 . Further, in yet other embodiments, LED controller  70  may retrieve the driving strength from processor  22  ( FIG. 2 ). For example, electronic device  10  may execute hardware and/or software programs to determine the driving strength based on user input, feedback received from sensors  76 , external inputs received from other electronic devices, or combinations thereof. 
     After determining the driving strength, LED controller  70  may adjust (block  96 ) the driver for the LEDs from the first bin. For example, as shown in  FIG. 10 , LED controller  70  may send a control signal to driver  60 A to adjust the driving strength of the LEDs from bin N 8 . In certain embodiments, the control signal may adjust the level of the current or the duty cycle of the current passing from driver  60  to the LEDs. 
     LED controller  70  may then determine (block  98 ) the driving strength for LEDs selected from a second bin, such as bin N 3  shown in  FIG. 10 . LED controller  70  may determine the driving strength based on data stored in memory  72 , data retrieved from processor  22 , data input by a user, and/or feedback received from sensors  76  (FIG.  5 ) among others. The LED controller may then adjust (block  100 ) the driver for the LEDs from the second bin. For example, as shown in  FIG. 10 , LED controller  70  may send a control signal to driver  60 B to adjust the driving strength of the LEDs from bin N 3 , for example by using AM or PWM. 
     The drivers  60 A and  60 B may then continue to drive the LEDs from the first and second bins at independent driving strengths until LED controller  70  receives (block  102 ) feedback. For example, LED controller  70  may receive feedback from sensors  76  ( FIG. 5 ) indicating that the white point has shifted from the target white point. In another example, LED controller  70  may receive feedback from a user, through the GUI of electronic device  10 . In yet another embodiment, LED controller  70  may receive feedback from processor  22  ( FIG. 2 ) indicating an operating state of device  10 . For example, a clock within device  10  may provide feedback that a specified time has elapsed, and LED controller  70  may adjust the drivers accordingly. In other embodiments, LED controller  70  may receive feedback indicating an operating state of device  10  from a device, such as a clock, indicated within LED controller  70 . 
     In response to the feedback, LED controller  70  may again determine (block  94 ) the driving strength of the LEDs from the first bin. The method  92  may continue until all driving strengths have been adjusted. Moreover, in other embodiments, LED controller  70  may adjust the driving strengths for any number of LED bins. For example, LED controller  70  may adjust the driving strength for LEDs from one, two, three, four, five, or more bins. The independent driving strength adjustments may be made using individual drivers or separate channels within the same driver. In certain embodiments, LED controller  70  may adjust the driving strength of only some of the LED strings, while other LED strings remain driven at a constant rate. Further, in certain Further, in certain embodiments, LEDs from the same bin may be grouped into more than one string, with each string being individually adjusted. 
       FIG. 12  illustrates an embodiment of LED backlight  32  that may employ color compensating LEDs  78  to achieve the desired white point. The color compensating LEDs  78  may be intermixed between white LEDs  48  and may be grouped together into one or more strings. The strings of color compensating LEDs  78  may be separate from the strings of white LEDs  49  to allow the driving strength of color compensating LEDs  78  to be adjusted independently from the driving strength of white LEDs  48 . In other embodiments, the orientation of color compensating LEDs  78  may vary. Further, any number of color compensating LEDs  78  may be used and dispersed throughout backlight  32  or located within various regions of backlight  32 . 
     The color compensating LEDs  78  may include LEDs selected from a bin C. As described above with respect to  FIG. 5 , bin C for color compensating LEDs  78  may represent a color designed to compensate for a white point shift. In certain embodiments, bin C may be selected based on white point shifts experienced by LEDs within backlight  32 . For example, certain backlights may experience a white point shift towards a blue tint. In these backlights, color compensating LEDs  78  may be selected from a yellow color spectrum to allow compensation for the blue shift. 
       FIG. 13  is a schematic diagram illustrating operation of the LED backlight of  FIG. 12 . Color compensating LEDs  78  are joined together in a string driven by one driver  60 B. White LEDs  48  are joined together in another string driven by another driver  60 A. However, in other embodiments, white LEDs  48  and color compensating LEDs  78  may be driven by separate channels of the same driver. Moreover, in certain Moreover, in certain embodiments, white LEDs  48  may be driven at separate driving strengths, using individual drivers or channels. 
     As shown, driver  60 A may drive white LEDs  48  at a constant driving strength; while driver  60 B varies the driving strength of color compensating LEDs  48  maintain the target white point. In certain embodiments, LED controller  70  may continuously vary or periodically vary the driving strength of driver  60 B to maintain the target white point. Further, in certain embodiments, driver  60 B may not drive color compensating LEDs  78  until white point compensation is desired. 
       FIG. 14  is a flowchart depicting a method  104  for employing color compensating LEDs  78  to achieve the target white point. The method may begin by setting (block  106 ) the driving strength of the white LEDs. For example, as shown in  FIG. 13 , LED controller  70  may set driver  60 A to a desired driving strength to drive the white LEDs from bins N 8  and N 4  at a constant rate. Each string of white LEDs may be driven at the same or different rates. After setting the white LED driving strength, LED controller  70  may determine (block  108 ) the driving strength of color compensating LEDs  78 . The driving strength may be determined based on user input, information stored in memory  72  ( FIG. 13 ), feedback from sensors  76  ( FIG. 5 ), and/or information received from device  10 , as described above with respect to  FIG. 11 . In certain embodiments, LED controller  70  may use the input or information to determine the direction and/or amount of deviation from the target white point. Based on the deviation, LED controller  70  may then determine a driving strength that may compensate for the deviation. 
     The controller may then adjust (block  110 ) the color compensating LED LED driver to the determined driving strength. For example, as shown in  FIG. 13 , LED controller  70  may adjust driver  60 B to the determined driving strength. Drivers  60 A and  60 B may then drive LEDs  48  and  78  at their respective driving strengths until additional feedback is received (block  112 ). The feedback may include information from sensors  76  ( FIG. 5 ), processor  22  ( FIG. 2 ), a user input, or the like, that indicates that a white point adjustment is needed. For example, sensors  76  may transmit information, such as color or temperature values, to LED controller  70  to indicate a white point shift. After receiving (block  112 ) feedback, LED controller  70  may again determine (block  108 ) a driving strength for the color compensating LEDs. 
     In certain embodiments, methods  92  and  104 , shown in  FIGS. 11 and 14 , may be combined to allow dynamic adjustment of both the driving strengths of color compensating LEDs  78  and white LEDs  48 . For example, in certain situations, a driving strength adjustment of the color compensating LEDs may not fully compensate for the white point deviation. In these situations, the driving strength of white LEDs  48  also may be adjusted to achieve the target white point. Moreover, in certain embodiments, methods  92  and  94  may be employed during different operational states or periods of device  10 . For example, if the white point deviation is caused by aging of the backlight components, the driving strength of the color compensating LEDs may be used to compensate for the deviation as illustrated in  FIG. 14 . However, if the white point deviation is high ambient temperature, the driving strength of white LEDs  48  may be adjusted to compensate for the deviation as illustrated in  FIG. 11 . In another example, backlight  32  may experience white point deviation during startup of LEDs  48 . The driving strength of white LEDs  48 , color compensating LEDs  78 , or a combination thereof, may be adjusted during the startup period. In other embodiments, the method  92   92  or  94  selected may depend on the operational hours backlight  32  has experienced, the magnitude of the deviation from the white point, or the direction of the deviation from the white point, among others. As will be appreciated, the operating states and periods are provided by way of example only, and are not intended to be limiting. The methods  92  and  94  may be used in conjunction with each other or independently in a variety of operational states or periods. 
       FIG. 15  depicts an embodiment of backlight  32  that incorporates sensors  76 . Sensors  76  may include optical sensors, temperature sensors, or combinations thereof. For example, in certain embodiments, sensors  76  may include phototransistors that generate signals whose magnitude is related to the brightness of the LEDs. In other embodiments, the sensors may include photo diodes, photo resistors, or other optical sensors that detect the color and/or brightness of the light emitted by LEDs  48  and  78 . In another example, sensors  76  may include temperature sensors that sense the temperature of backlight  32 . In these embodiments, LED controller  70  may use the temperature data to determine a white point adjustment. Any number and arrangement of sensors  76  may be included within backlight  32 . Further, in certain embodiments, sensors  76  may be located in other locations of backlight  32 , such as the back of array tray  50  ( FIG. 3 ) or frame  38  ( FIG. 3 ), among others. 
       FIG. 16  is a schematic diagram illustrating operation of backlight  32  shown in  FIG. 15 . Sensors  76  may be communicatively coupled to LED controller  70  to provide feedback to LED controller  70  for adjusting the driving strength of drivers  60 A and  60 B. For example, sensors  76  may detect chromaticity values of the light emitted by LEDs  48  and  78  and may send signals corresponding to these values to LED controller  70 . LED controller  70  may use these signals to determine a driving strength adjustment adjustment for drivers  60 A and  60 B, and may, in turn, transmit control signals to drivers  60 A and  60 B to vary their driving strength. 
     The backlight  32  of  FIGS. 15 and 16  includes white LEDs from bins N 5  and N 11 , and includes color compensating LEDs  78  from two different bins C 1  and C 2 . The LEDs from each bin are joined together into strings, with each string being independently driven by a channel of one of the drivers  60 A or  60 B. Bins C 1  and C 2  may include colored LEDs designed to compensate for a white point shift. For example, in a backlight employing phosphor based LEDs with red and green phosphor materials, bin C 1  may encompass a red spectrum, and bin C 2  may encompass a green spectrum. 
     In response to receiving feedback from sensors  76 , LED controller  70  may determine a driving strength adjustment. For example, LED controller  70  may receive chromaticity values or temperature values from sensors  76 , and may compare these values to compensation information  118  stored within memory  72 . The compensation information  118  may include calibration curves, algorithms, tables, or the like that LED controller  70  may use to determine a driving strength adjustment based on the feedback received from sensors  76 . In certain embodiments, compensation information  118  may include algorithms for determining the direction and amount of deviation from the target white point. Compensation information  118  also may specify the amount of driving strength adjustment as well as which strings of LEDs  48  and  78  should be adjusted based on the white point deviation. 
     The memory  72  also may include limits  120  that specify maximum values, minimum values, ratios, or ranges for the driving strengths. Before making the driving strength adjustments, LED controller  70  may ensure that the new driving strengths fall within limits  120 . For example, limits  120  may ensure that only a small difference exists between the driving strengths to prevent visible artifacts on LCD panel  30  ( FIG. 2 ). 
       FIG. 17  depicts a flowchart of a method  122  for employing sensors to maintain a target white point. Method  122  may begin by receiving (block  124 ) sensor feedback. For example, as shown in  FIG. 16 , LED controller  70  may receive feedback from sensors  76 . The feedback may be in the form of electrical signals representing the brightness, chromaticity values, temperature, or other data that LED controller  70  may use to determine the white point emitted by backlight  32 . LED controller  70  may then determine (block  126 ) the deviation from the target white point, for example, using algorithms, tables, calibration curves, routines, or the like, stored within memory  72 . For example, LED controller  70  may receive chromaticity values from sensors  76 . Based on the chromaticity values, LED controller  70  may determine the white point deviation. For example, LED controller  70  may compare the chromaticity values to target white point values stored within memory  72  to determine whether the emitted light is too blue or yellow when compared to the target white point. 
     After determining the white point deviation, LED controller  70  may then determine (block  128 ) the white point compensation. In certain embodiments, based on the direction of the white point deviation, LED controller  70  may determine which strings of LEDs should receive driving strength adjustments. For example, if the white point deviation reveals that the emitted light is too purple, LED controller  70  may determine a driving strength adjustment for driving LEDs from a green bin at an increased driving strength. In one example, as shown in  FIG. 16 , the color compensating LEDs from bin C 2  may emit a green spectrum, while the color compensating LEDs from compensating LEDs from C 1  may emit a red spectrum. If the light emitted is too purple, the LED controller may 1) drive the C 2  LEDs at a higher driving strength, 2) drive the C 1  LEDs and a lower driving strength, or 3) may adjust the ratio of the C 1  and C 2  driving strengths. As described above with respect to FIGS.  5  and  10 - 11 , LED controller  70  may employ AM, PWM, or other suitable techniques to vary the driving strength. 
     Once the new driving strengths have been determined, LED controller  70  may determine (block  130 ) whether the adjustments are within limits. For example, as shown in  FIG. 16 , LED controller  70  may determine whether the new driving strengths for drivers  60 A and  60 B fall within limits  120  stored within memory  72 . In certain embodiments, limits  120  may improve consistency across backlight  32  and LCD panel  30 , and may reduce visible artifacts. 
     If the determined compensation is not within the limits, LED controller  70  may again determine the compensation (block  128 ). For example, LED controller  70  may determine different driving strength values or ratios that still compensate for the white point deviation. Once the compensation is within the limits, LED controller  70  may then adjust (block  132 ) the drivers to the determined driving strengths. Of course, in certain embodiments, limits  120  may not be included, and block  130  may be omitted. 
     The driving strength adjustments described in  FIGS. 5-17  may be used with a variety of backlights including white point LEDs  48 , color compensating LEDs  78 , or combinations thereof. Further, the adjustments may be used with backlights incorporating LEDs from any number of bins. The adjustments may be made periodically or continuously throughout operation of the backlight. However, in certain embodiments, the driving strength adjustments may be particularly useful in compensating for white compensating for white point deviation that occurs over time due to aging of LEDs  48  and  78  and other backlight or display components. For example, over time the brightness and/or color output of LEDs may change. 
     3. Aging Compensation 
       FIG. 18  is a chart illustrating how the luminance of backlight  32  may shift over time. Y-axis  138  indicates the luminance of the backlight in Nits, and the x-axis  140  indicates the operational life of the backlight, measured here in hours. Curve  142  illustrates how luminance  138  may decrease as operational time  140  increases. As noted above, a change in the luminance of backlight  32  may cause the white point to shift. 
       FIG. 19  depicts chart  144 , which illustrates how the chromaticity of a backlight may shift over time as LEDs  48  and  78  and other components age. Specifically, chart  144  illustrates the change in chromaticity for a backlight that includes yellow phosphor LEDs. Y-axis  146  shows the chromaticity values, and x-axis  148  shows the operational life of the backlight in hours. The x chromaticity values are shown by curve  150 , and the y chromaticity values are shown by curve  152 . As shown by curve  150 , the x values may generally shift from red to blue with age. As shown by curve  152 , the y values may generally shift from yellow to blue with age. Overall, the white point of the backlight may shift towards a bluish tint. Therefore, to maintain the desired white point, the driving strength of strings of LEDs with a yellow and/or red tint may increased over time to compensate for the white point shift. 
       FIG. 20  is a flowchart depicting a method  158  for maintaining a target white point as a display ages. Method  158  may begin by detecting (block  160 ) aging of display  14  ( FIG. 2 ). For example, a clock within display  14  ( FIG. 2 ), backlight  32  ( FIG. 2 ), or device  10  ( FIG. 2 ) may track operation times of the backlight. When a certain operating time is exceeded, the clock may provide feedback to LED controller  70  indicating that aging has occurred. The clock may track operating time for backlight  32 , operating time for individual components within the backlight, such as LEDs  48 , or operating time for display  14 , among others. In other embodiments, the clock may continuously provide operating times to LED controller  70 , and LED controller  70  may determine when a threshold operating time has been exceeded. 
     Aging also may be detected by sensors included within the backlight  32 . For example, sensors  76 , shown in  FIG. 15 , may provide feedback to LED controller  70  that indicates aging. In certain embodiments, sensors  76  may detect the color or brightness of the light emitted by backlight  32 . LED controller  70  may then use the feedback from sensors  76  to determine that aging has occurred. For example, LED controller  70  may compare the feedback from sensors  76  to brightness or color thresholds stored within memory  72 . In certain embodiments, LED controller  70  may detect that aging has occurred when the feedback from sensors  76  indicate that the emitted white point has shifted by a specified amount from the target white point. 
     Upon detecting aging, LED controller  70  may determine the shift in the white point due to aging. LED controller  70  may use tables, algorithms, calibration curves, or the like to determine the white point deviation. In certain embodiments, LED controller  70  may use the brightness and/or color values from sensors  76  to determine how much the emitted light has deviated from the target white point. For example, LED controller  70  may compare color values from sensors  76  to target white point values stored within memory  72  to determine the white point shift. In other embodiments, LED LED controller  70  may use the operating time provided by the clock to determine the white point deviation. For example, LED controller  70  may compare the operating time to a calibration curve stored in memory  72  that correlates operating time to white point shifts. 
     Based on the white point shift, the controller may then determine (block  164 ) the white point compensation. In certain embodiments, the white point compensation may compensate for a reduction in brightness, as generally illustrated by  FIG. 18 . For example, if LED controller  70  determines that the brightness has decreased, LED controller  70  may increase the driving strength of each driver to achieve a target brightness level. In certain embodiments, a target brightness level may be stored within memory  72  ( FIG. 5 ) of the backlight  32 . 
     LED controller  70  also may determine individual driving strengths adjustments for the white point compensation. The individual driving strength adjustments may compensate for a shift in the color or chromaticity values of the emitted light, as generally illustrated in  FIG. 19 . As described above with respect to  FIG. 17 , LED controller  70  may determine which strings of LEDs should receive driving strength adjustments based on the white point deviation. For example, if the emitted white point is too blue, LED controller  70  may increase the driving strength of a string of yellow tinted LEDs. LED controller  70  may select strings of white LEDs  48  and/or strings of color compensating LEDs  78  to receive driving strength adjustments. 
     The amount of the driving strength adjustment may depend on the magnitude of the white point deviation. Moreover, in certain embodiments, LED controller  70  may be configured to continuously increase specific driving strengths at a specified rate upon detecting aging. For example, rates of driving strength increases may be stored within memory  72 . Further, in certain embodiments, LED controller  70  may ensure that the adjustments fall within limits  120  ( FIGS. 16-17 ) stored within memory  72 . 
     LED controller  70  also may account for the brightness of the backlight when determining the driving strength adjustments. For example, LED controller  70  may adjust the ratio between driving strengths while increasing the overall driving strength of each string to achieve both the target brightness and target white point. 
     After determining the white point compensation, LED controller  70  may adjust (block  166 ) the driving strengths to the determined levels. LED controller  70  may then detect (block  160 ) further aging, and method  158  may begin again. In certain embodiments, LED controller  70  may continuously receive feedback from sensors  76  to detect aging. However, in other embodiments, LED controller  70  may periodically check for aging. Moreover, in other embodiments, LED controller  70  may check for aging when device  10  receives a user input indicating that a check should be performed. 
     After aging compensation has occurred, further adjustments may be made to fine tune the emitted white point to the target white point.  FIG. 21  is a flowchart depicting a method  168  for fine-tuning the emitted white point. Method  168  may begin by detecting (block  170 ) aging. For example, as described with respect to  FIG. 21 , the controller may detect aging based on feedback from a clock or from sensors. LED controller  70  may then determine (block  172 ) the white point compensation based on the aging. For example, LED controller  70  may use compensation information  118  (FIG. compensation information  118  ( FIG. 16 ), such as a calibration curve, table, algorithm, or the like, that correlates a driving strength or driving strength adjustment to operational hours, color values, brightness values, or the like. Compensation information  118  also may specify the drivers or channels that should receive the driving strength adjustment. After determining the white point compensation, LED controller  70  may adjust (block  174 ) the drivers to the determined driving strength. The adjustment may restore the light output to an emitted white point that substantially matches the target white point. 
     The controller may then determine (block  176 ) a fine adjustment that may allow the emitted white point to more closely match the target white point. For example, device  10  may include a software application for receiving a fine adjustment input from a user. The user may provide the input through the GUI using, for example, one of the user input structures  16  ( FIG. 1 ). In certain embodiments, a user may compare the white point of the display to a calibration curve or chart to determine the fine adjustment input. In other embodiments, LED controller  70  may receive a fine adjustment input from another electronic device connected, for example, through network device  26  ( FIG. 2 ) or through I/O port  18  ( FIG. 2 ). Based on the input, controller  70  may determine a fine adjustment to bring the emitted white point even closer to the target white point. 
     In another example, LED controller  70  may determine the fine adjustment based on feedback received from one or more sensors included within device  10 . For example, sensors  76  may provide feedback to LED controller  70  for fine-tuning the drivers. For example, LED controller  70  receive feedback from sensors  76  ( FIG. 16 ) and determine the fine adjustment in a manner similar to that described with respect to  FIG. 17 . 
     After determining (block  176 ) the fine adjustment, LED controller  70  may adjust (block  178 ) the drivers. However, in certain embodiments, the fine adjustment may be combined with adjusting (block  174 ) the drivers to compensate for the white point shift. In these embodiments, the fine adjustment may be determined along with the white point compensation determination. After the drivers have been adjusted, LED controller  70  may again determine (block  170 ) the time elapsed, and method  168  may begin again. 
     4. Temperature Compensation 
     In addition to shifting over time due to aging, the emitted white point of backlight  32  may shift due to temperature. In general, as temperature increases, brightness decreases due to reduced optical retardation. The change in brightness may cause a white point shift. Further, certain sections of backlight  32  may experience different temperatures, which may create color and/or brightness variations throughout backlight  32 . 
       FIG. 22  depicts chart  184 , which illustrates how the brightness of different colored LEDs may change with temperature. Y-axis  186  indicates the relative flux of the light emitting diodes, and the x-axis  188  indicates the temperature in degrees Celsius. In general, the flux may be the relative percentage of the total amount of light from an LED. Separate lines  190 ,  192 , and  194 , each correspond to different color LEDs, normalized to 25 degrees Celsius. Specifically, line  190  represents the change in flux for a red LED, line  192  represents the change in flux for a green LED, and line  194  represents the change in flux for a blue LED. The flux generally decreases as the temperature increases, and the rate of decrease varies between different color LEDs. The differing rates of change may cause a shift in the white point. For example, in backlights employing white LEDs  48  that mix light from individual colored LEDs, the white point may shift because the relative flux of the LEDs within white LEDs  48  may change. The increased temperature also may cause a white point shift for phosphor based LEDs. 
       FIG. 23  depicts chart  206 , which illustrates how the temperature of a backlight may change over time. Y-axis  208  indicates temperature, and x-axis  210  indicates time. Curve  212  generally indicates how temperature  208  may increase and then stabilize after the backlight is turned on. After the backlight is turned on, the temperature may increase until stabilization time  214 , generally indicated by the dashed line. After stabilization time  214 , the temperature may remain constant. Stabilization time  214  may vary depending on the specific features of backlight  32  ( FIG. 2 ), LDC panel  30  ( FIG. 2 ), and electronic device  10  ( FIG. 2 ). Moreover, in other embodiments, the temperature profile may increase, stabilize, or decrease any number of times at various rates. 
     The temperature of backlight  32  also may vary between different sections of the backlight. For example, certain sections of the backlight may experience higher temperatures due to proximity to electronic components that give off heat. As shown in  FIG. 24 , electronics  218  may be located within one section of backlight  32 . Electronics  218  may produce heat creating a localized temperature gradient within backlight  32 . In certain embodiments, electronics  218  may include LCD controller  56  and LED drivers  60  as shown in  FIG. 3 . LEDs  48  located near electronics  218  may experience increased temperatures when compared to other LEDs  48  within the backlight, which may result in variation in the emitted white point and/or brightness across backlight  32 . Moreover, the temperature variation may change with time, as illustrated in illustrated in  FIG. 23 . For example, upon initial operation of the backlight, LEDs  48  within the backlight may be exposed to approximately the same temperature. However, after backlight  32  has been turned on, the temperature of backlight  32  near electronics  32  may increase as shown in  FIG. 23 , until stabilization period  214 . After stabilization period  214 , LEDs  48  near electronics  218  may be exposed to a higher temperature than LEDs  48  disposed throughout the rest of backlight  32 . In other embodiments, the location of electronics  218  may vary. Further, temperature gradients may be created due to other factors, such as the proximity of other components of electronic device  10 , the location of other devices, walls, or features, and the location of a heat sink, among others. 
       FIG. 25  is a schematic diagram illustrating operation of backlight  32  shown in  FIG. 24 . The LEDs from different bins N 2  and N 9  may be joined together on strings, each driven by a separate driver  60 A and  60 B. Each string may be driven at a different driving strength to produce a white point in backlight  32  that substantially matches the target white point. The driving strength of each string also may vary over time to compensate for the white point shift produced by a temperature change within backlight  32 . For example, the temperature of backlight  32  may increase upon startup, as shown in  FIG. 23 . To account for the increase in temperature, the driving strength of each string may vary with time. For example, LED controller  70  may transmit control signals to drivers  60 A and  60 B to vary duty cycles  220  and  222 . Before stabilization period  214 , drivers  60 A and  60 B may have a lower driving strength, indicated by duty cycles  220 A and  222 A. After stabilization period  214 , LED controller  70  may increase the frequency of the duty cycles, as represented by duty cycles  220 B and  222 B. Further, in other embodiments, LED controller  70  may vary the amount of current provided to LEDs  48 , for example using AM, instead of, or in addition to using PWM. 
     In certain embodiments, the changes in driving strength may be stored within memory  72 , and a clock within LED controller  70  may track the operating time. Based on the operating time, LED controller  70  may detect stabilization period  214  and vary the driving strength. LED controller  70  may vary the driving strength to account for temperature changes at various times throughout operation of the backlight. In certain embodiments, the driving strength may be varied based on an operational state of backlight  32 . For example, processor  22  may provide information to LED controller  70  indicating the type of media, for example a movie, sports program, or the like, being shown on display  14  ( FIG. 2 ). 
       FIG. 26  is a flowchart a flowchart depicting a method  228  for maintaining a target white point during temperature changes. The method may begin by detecting (block  230 ) a temperature change. For example, LED controller  70  may detect that a temperature change is occurring based on an operational state of the backlight. For example, LED controller  70  may detect a temperature change upon sensing that backlight  32  has been turned on. In certain embodiments, a clock within electronic device  10  may track operational hours of the backlight. Based on the operational hours, electronic device  10  may detect a temperature change, for example, by using table or calibration curves stored within memory  72 . 
     Upon detecting a temperature change, LED controller  70  may adjust (block  232 ) the drivers to temperature compensation driving strength. For example, as shown in  FIG. 25 , LED controller  70  may adjust drivers  60 A and  60 B to employ duty cycles  220 A and  222 A. In certain embodiments, the compensation driving strengths may be stored within memory  72  ( FIG. 25 ). During the periods of changing temperature, the drivers may be driven at the same driving strengths, or the driving strength may be adjusted throughout the period of changing temperature. For example, in certain embodiments, after initially detecting a temperature change, such as by sensing startup of the backlight, LED controller  70  may enter a temperature compensation period where the driving strengths are determined by compensation information  118  ( FIG. 16 ) such as calibration curves, tables, or the like. Compensation information  118  may provide varying driving strengths corresponding to specific times within the temperature compensation period. However, in other embodiments, LED controller  70  may adjust the drivers in response to each detected temperature change. Accordingly, LED controller  70  may continuously vary or periodically vary the driving strengths during the temperature compensation period to maintain the target white point. 
     The LED controller  70  may continue to operate drivers  60  at the compensation driving strengths until LED controller  70  detects (block  234 ) a temperature stabilization period. For example, a clock within device  10  may indicate that the temperature has stabilized. LED controller  70  may then adjust (block  236 ) the drivers to a temperature stabilization driving strength. For example, as shown in  FIG. 25 , LED controller  70  may adjust drivers  60 A and  60 B to duty cycles  220 B and  222 B. In certain embodiments, the stabilization driving strengths may be stored within memory  72 . 
     In certain embodiments, a dedicated string of LEDs may be used to compensate for temperature changes. For example, as shown in  FIG. 27 , color compensating LEDs  78  from a bin C 3  may be placed near electronics  218  of backlight  32 .  32 . In certain embodiments, bin C3 may be selected based on the white point shift generally exhibited due to temperature changes. For example, in LED backlight  32  that includes yellow phosphor LEDs, the white point may shift towards a blue tint as temperature increases. Therefore, bin C 3  may encompass a yellow spectrum to compensate for the blue shift. Color compensating LEDs  78  may be disposed near electronics  218  within backlight  32  to allow compensation for localized white point shifts. However, in other embodiments, color compensating LEDs  78  may be dispersed throughout backlight  32  to allow compensation for temperature changes affecting other regions of backlight  32  or entire backlight  32 . 
       FIG. 28  schematically illustrates operation of backlight  32  shown in  FIG. 27 . Color compensating LEDs  78  may be driven by one driver  60 A while white LEDs  48  are driven by another driver  60 B. The separate drivers  60 A and  60 B may allow the driving strength of color compensating LEDs  78  to be adjusted independently from the driving strength of white LEDs  48 . As temperature changes occur within backlight  32 , LED controller  70  may adjust the driving strength of driver  60  to compensate for a white point shift that may occur due to temperature. For example, during increased temperatures, LED controller  70  may drive color compensating LEDs  78  at a higher rate to maintain the target white point. In certain embodiments, LED controller  70  may adjust the driving strength of driver  60 A during a temperature compensation period as described with respect to  FIG. 26 . 
       FIG. 29  illustrates another embodiment of backlight  32  that may compensate for temperature changes. Instead of, or in addition to color compensating LEDs  78 , dedicated string  240  of white LEDs  48  may be located near electronics  218  to account for temperature variations. As shown, string  240  includes LEDs from bin W. However, in other embodiments, the string may include LEDs from neighboring bins, such as bins N 1-12 . 
     As illustrated in  FIG. 30 , dedicated string  240  may be driven by one driver  60 A, while other LEDs  48  are driven by another driver  60 B. In certain embodiments, the other driver  60 B may include multiple channels for independently driving LEDs from separate bins N 1  and N 6 . The separate channels may allow the relative driving strengths for each bin to be varied to achieve the desired white point as described with respect to  FIGS. 5-17 . 
     The LED controller  70  may adjust the driving strength of driver  60 A to reduce white point variation throughout backlight  32 . For example, the white point emitted near electronics  218  may vary from the white point emitted throughout the rest of the board due to a temperature gradient that may occur near electronics  218 . LED controller  70  may adjust the driving strength for dedicated string  240  to maintain the target white point near electronics  218 . LED controller  70  also may vary the driving strength of dedicated string  240  during temperature compensation periods as described with respect to  FIG. 26 . 
       FIG. 31  illustrates an edge-lit embodiment of backlight  32  that may adjust driving strengths to compensate for temperature changes. Backlight  32  includes two light strips  64 A and  64 B, with each light strip  64 A and  64 B employing LEDs from different bins N 2  and N 7 . The driving strength of each light strip  64 A and  64 B may be adjusted independently to maintain the target white point during temperature changes. Further, the driving strength of upper light strip  64 A may be adjusted to account for the increased temperatures that may be generated by electronics  218 . In other embodiments, multiple strings of LEDs from various bins may be included within each light strip  64 A and  64 B. In certain embodiments, the separate strings of LEDs may be adjusted independently to compensate for temperature changes as described with respect to  FIG. 26 . 
       FIG. 32  illustrates another embodiment of backlight  32  that includes sensors  76 . Any number of sensors  76  may be disposed in various arrangements throughout backlight  32 . As described above with respect to  FIG. 5 , sensors  76  may sense temperatures of backlight  32  and provide feedback to LED controller  70  ( FIG. 5 ). For example, sensors  76  may be used to detect a temperature compensation period as described in  FIG. 26 . Sensors  76  also may be used to detect local variations in temperature within backlight  32 . For example, sensors  76  may provide feedback indicating the extent of the temperature gradient near electronics  218 . In other embodiments, sensors  76  may detect a color of the light output by LEDs  48 . LED controller  70  may use the feedback to adjust the driving strength to maintain the target white point. 
       FIG. 33  schematically illustrates operation of the backlight of  FIG. 32 . Sensors  76  may provide feedback to LED controller  70  that LED controller  70  may use to detect temperature compensation periods and/or local temperature variations. LED controller  70  may use the feedback to determine driving strengths for drivers  60 A and  60 B to achieve the target white point. For example, LED controller  70  may compare the feedback to compensation information  118  stored within memory  72  to determine the driving strengths. If, for example, the sensors indicate a high temperature period, LED controller  70  may decrease the driving strength of color compensating LEDs  78  to maintain the target white point. In another example, LED controller  70  may vary the relative driving strengths of the LEDs from bins N 9  and N 2  to achieve the target white point during temperature variations. 
       FIG. 34  is a flowchart illustrating a method  248  for using sensors to maintain a target white point during temperature variations. The method may begin by detecting (block  250 ) a temperature change based on sensor feedback. For example, as shown in  FIG. 33 , sensors  76  may detect changes in the white point, for example by sensing temperature and/or chromaticity values, and provide feedback to LED controller  70 . Using the feedback, LED controller  70  may determine the temperature profile (block  252 ) of the backlight  32 . For example, LED controller  70  may determine whether the temperature profile includes local variation, for example, near electronics  218 . LED controller  70  also may determine whether the temperature has increased across backlight  32  as a whole. 
     The LED controller  70  may then determine (block  254 ) the compensation driving strengths. In certain embodiments, LED controller  70  may compare the temperature profile determine in block  252  to compensation information  118  ( FIG. 33 ) to determine which drivers to adjust. For example, as shown in  FIGS. 32 and 33 , if sensors  76  detect an increase in temperature only near electronics  218 , LED controller  70  may adjust the driving strength of driver  60 B to drive the color compensating LEDs from bin C 3  at an increased strength. However, if sensors  76  detect a temperature increase throughout backlight  32 , for example due to an increase in ambient temperature, LED controller  70  may increase the driving strengths of both drivers  60 A and  60 B. In certain embodiments, the driving strengths may be adjusted to compensate for both a localized temperature profile and an overall temperature change. After determining (block  254 ) the compensation driving strengths, LED controller  70  may may adjust (block  256 ) the drivers to the compensation driving strengths. 
     Sensors  76  also may be used maintain the target white point during shifts due to both aging and temperature. For example, if both the sensors  76  detect a color and/or brightness of the light, sensors  76  may provide feedback for adjusting the white point, regardless of whether the shift is due to temperature, aging, or any other factor. In another example, sensors  76  may include optical sensors to detect shifts due to aging and temperature sensors to detect shifts due to temperature. Further, in other embodiments, sensors  76  may include temperature sensors to detect white point shifts due to temperature changes, and compensation information  118  ( FIG. 20 ), such as calibration curves, may be employed to compensate for white point shifts due to aging. 
       FIG. 35  is a flowchart illustrating a method for compensating for white point shifts due to aging and temperature variations. Method  258  may begin by receiving (block  260 ) sensor feedback. For example, LED controller  70  may receive feedback from sensors  76 , shown in  FIG. 33 . Based on the feedback, LED controller  70  may determine (block  262 ) white point variation. For example, sensors  76  may indicate localized temperature variation near electronics  218  ( FIG. 32 ). In another example, sensors  76  may indicate local white point variations due to an aging LED string. LED controller  70  may then determine (block  264 ) local white point compensation. For example, LED controller  70  may adjust the driving strength of an individual string of LEDs, to reduce variation in the white point throughout backlight  32 . 
     After determining compensation driving strengths to reduce variation throughout backlight  32 , LED controller  70  may then determine (block  266 ) the deviation from the target white point. For example, LED controller  70  may use feedback from feedback from sensors  76  to detect a shift in the white point due to aging of backlight  32  or due to a change in ambient temperature. The controller may determine (block  268 ) the white point compensation driving strengths for achieving the target white point. For example, if the emitted white point has a blue tint when compared to that target white point, LED controller  70  may increase the driving strength of yellow tinted LEDs. LED controller  70  may adjust the driving strengths as described above with respect to  FIGS. 11-17 . After determining the driving strengths, LED controller  70  may adjust (block  270 ) the drivers to determine driving strengths. 
     The specific embodiments described above have been shown by way of example, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and scope of this disclosure.

Metadata:
Filing Date: 20090324
Publication Date: 20131105
Grant Date: 20131105
Priority Date: 20090324
Inventors: CHEN CHENG
QI JUN
ZHONG JOHN Z.
YIN VICTOR H.
CHEN WEI
Assignee: APPLE INC
CPC Classifications: [{"code": "G02F1/133603", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/133603", "inventive": true, "first": true, "tree": "[]"}, {"code": "G09G3/3413", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/3426", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2320/041", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/048", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0626", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0666", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2360/145", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/048", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2360/145", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/3413", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/3426", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2320/041", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0626", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0666", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 42783288