Patent Publication Number: US-2012038291-A1

Title: Color temperature tunable led light source

Description:
BACKGROUND 
     1. Field 
     The present application relates generally to light emitting diodes, and more particularly, to a color temperature tunable light emitting diode (LED) light source. 
     2. Background 
     A light emitting diode comprises a semiconductor material impregnated, or doped, with impurities. These impurities add “electrons” and “holes” to the semiconductor, which can move in the material relatively freely. Depending on the kind of impurity, a doped region of the semiconductor can have predominantly electrons or holes, and is referred to as an n-type or p-type semiconductor region, respectively. 
     In LED applications, an LED semiconductor chip includes an n-type semiconductor region and a p-type semiconductor region. A reverse electric field is created at the junction between the two regions, which cause the electrons and holes to move away from the junction to form an active region. When a forward voltage sufficient to overcome the reverse electric field is applied across the p-n junction, electrons and holes are forced into the active region and combine. When electrons combine with holes, they fall to lower energy levels and release energy in the form of light in the case of direct bandgap semiconductors such as gallium arsenide or indium phosphide. The color or wavelength of light emitted by an LED depends only on the composition of the semiconductor material. LEDs made from large bandgap semiconductors such as indium gallium nitride can convert electrical input energy to visible light, particularly blue light, with high conversion efficiency. 
     It is possible to create a white light source from one or more blue LED chips mounted typically on a ceramic or metal substrate, by encapsulating the chips with a suitable phosphor that absorb part of the blue light and fluoresce yellow since a combination of blue and yellow light appears white to the eye. Alternatively, a combination of red and green phosphors that absorb blue can be used to generate white light by a combination of red, blue and green. Furthermore, the white light source can be designed to emit white light having a particular color temperature. The color temperature of a white light source is the temperature of an ideal black-body radiator that radiates white light of comparable hue to that of the light source. The color temperature is conventionally stated in units of absolute temperature referred to as kelvin (K). 
     Typically, a white LED light source utilizes LED chips that emit blue light. Using a yellow phosphor encapsulation some of the blue light is converted to yellow light resulting in a combination which appears cool white to the eye. For example, cool white light has a color temperature of approximately 5500K. The further addition of green and red phosphors makes such a LED light source appear warm white. For example, warm white light has a color temperature of approximately 3000K. 
     Generally, people prefer a light source whose color temperature mimics that of the Sun. For example, it is desirable to have a cool color temperature light source (like the Sun at midday) to perform various detailed tasks and a warmer color temperature light source (like the Sun at dusk) for relaxing ambient lighting in the evening. A conventional incandescent light bulb exhibits these characteristics. For example, a light bulb at full power emits cool color temperature light, and when dimmed, emits warmer color temperature light. 
     Unfortunately, conventional LED light sources do not significantly change color temperature when dimmed from full power. This means that multiple LED light sources may be needed to satisfy different lighting requirements. For example, one LED light source may be needed to emit cool color temperature light during the day time and a second LED light source may be needed to emit warmer color temperature light for use in the evening. 
     Accordingly, there is a need to provide a LED light source that is color temperature tunable to provide light having warmer color temperatures when dimmed and cooler color temperatures when adjusted for full brightness. 
     SUMMARY 
     In various aspects, a color temperature tunable LED light source is provided. In one implementation, the light source emits light having warmer color temperatures when dimmed and cooler color temperatures when adjusted for full brightness. In an aspect, the color temperature tunable LED light source comprises a plurality of LED chips mounted on a substrate. The LED chips are grouped into two or more groups, where each group of chips is encapsulated with a particular encapsulation material that converts the blue light from the LEDs to white light having a specific color temperature. Each group can be referred to as an encapsulation group and is driven by a drive current so that the intensity (or lumen output) of each group can be controlled. By controlling the drive currents such that cool color temperature groups predominate when the LED light source is driven at full power and warm color temperature groups predominate when the LED light source is driven at lower power, it is possible to tune the color temperature of the resulting white light to achieve a particular color temperature characteristic. Thus, the drive currents operate to tune the color temperature of the white light emitted from the LED light source. 
     In another aspect, an LED apparatus is provided that comprises a substrate and a first group of blue LED chips mounted on the substrate that are configured with a first group of appropriate phosphors to produce white light having a first color temperature and having a first intensity value determined from a first drive current. The LED apparatus also comprises a second group of blue LED chips mounted on the substrate that are configured with a second group of appropriate phosphors to produce white light having a second color temperature and having a second intensity value determined from a second drive current. The first color temperature light and the second color temperature light combine to produce light having a resulting color temperature and a resulting intensity value. 
     In another aspect, a light emitting apparatus is provided that comprises a first light emitting means for emitting light at a first color temperature, a second light emitting means for emitting light at a second color temperature, and a drive means for driving the first and second emitting means so that the first color temperature light and the second color temperature light combine to produce light having a tunable color temperature. 
     It is understood that other aspects of the present invention will become readily apparent to those skilled in the art from the following detailed description. As will be realized, the present invention includes other and different aspects and its several details are capable of modification in various other respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and the detailed description are to be regarded as illustrative in nature and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects described herein will become more readily apparent by reference to the following Description when taken in conjunction with the accompanying drawings wherein: 
         FIG. 1  shows top and cross-sectional views of an exemplary LED apparatus for use in aspects of a color temperature tunable LED light source; 
         FIG. 2  shows an exemplary LED apparatus for use in aspects of a color temperature tunable LED light source; 
         FIG. 3  shows an exemplary drive circuit for use in aspects of a color temperature tunable LED light source; 
         FIG. 4  shows exemplary graphs illustrating the operation of the LED apparatus shown in  FIG. 1 ; 
         FIG. 5  shows an exemplary drive current table for use in aspects of a color temperature tunable LED light source; 
         FIG. 6  shows an exemplary method for providing a color temperature tunable LED light source; and 
         FIG. 7  shows an exemplary method for providing drive currents to drive a color temperature tunable LED light source; 
         FIG. 8  shows an exemplary alternative drive circuit for use in aspects of a color temperature tunable LED light source; 
         FIG. 9  shows an exemplary alternative method for providing drive currents to drive a color temperature tunable LED light source; 
         FIG. 10  shows an exemplary LED apparatus constructed in accordance with aspects of a color temperature tunable LED light source; and 
         FIG. 11  shows an exemplary drive circuit apparatus constructed in accordance with aspects of a color temperature tunable LED light source. 
     
    
    
     DESCRIPTION 
     The present invention is described more fully hereinafter with reference to the accompanying drawings, in which various aspects of the present invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the various aspects of the present invention presented throughout this disclosure. Rather, these aspects are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. The various aspects of the present invention illustrated in the drawings may not be drawn to scale. Accordingly, the dimensions of the various features may be expanded or reduced for clarity. In addition, some of the drawings may be simplified for clarity. Thus, the drawings may not depict all of the components of a given apparatus (e.g., device) or method. 
     Various aspects of the present invention will be described herein with reference to drawings that are schematic illustrations of idealized configurations of the present invention. As such, variations from the shapes of the illustrations as a result, for example, manufacturing techniques and/or tolerances, are to be expected. Thus, the various aspects of the present invention presented throughout this disclosure should not be construed as limited to the particular shapes of elements (e.g., regions, layers, sections, substrates, etc.) illustrated and described herein but are to include deviations in shapes that result, for example, from manufacturing. By way of example, an element illustrated or described as a rectangle may have rounded or curved features and/or a gradient concentration at its edges rather than a discrete change from one element to another. Thus, the elements illustrated in the drawings are schematic in nature and their shapes are not intended to illustrate the precise shape of an element and are not intended to limit the scope of the present invention. 
     It will be understood that when an element such as a region, layer, section, substrate, or the like, is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. It will be further understood that when an element is referred to as being “formed” on another element, it can be grown, deposited, etched, attached, connected, coupled, or otherwise prepared or fabricated on the other element or an intervening element. 
     Furthermore, relative terms, such as “lower” or “bottom” and “upper” or “top,” may be used herein to describe one element&#39;s relationship to another element as illustrated in the drawings. It will be understood that relative terms are intended to encompass different orientations of an apparatus in addition to the orientation depicted in the drawings. By way of example, if an apparatus in the drawings is turned over, elements described as being on the “lower” side of other elements would then be oriented on the “upper” sides of the other elements. The term “lower”, can therefore, encompass both an orientation of “lower” and “upper,” depending of the particular orientation of the apparatus. Similarly, if an apparatus in the drawing is turned over, elements described as “below” or “beneath” other elements would then be oriented “above” the other elements. The terms “below” or “beneath” can, therefore, encompass both an orientation of above and below. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and this disclosure. 
     As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The term “and/or” includes any and all combinations of one or more of the associated listed items 
     It will be understood that although the terms “first” and “second” may be used herein to describe various regions, layers and/or sections, these regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one region, layer or section from another region, layer or section. Thus, a first region, layer or section discussed below could be termed a second region, layer or section, and similarly, a second region, layer or section may be termed a first region, layer or section without departing from the teachings of the present invention. 
       FIG. 1  shows a top view  102  and a cross-sectional view  104  of an exemplary LED apparatus  100  for use in aspects of a color temperature tunable LED light source. Referring to the top view  102 , a substrate  106  is shown that comprises a plurality of LED chips (or dies)  108  mounted thereon and which emit blue light when suitably driven by a current source. A first group of the LED chips are located on the substrate  106  within boundary  110  and a second group of the LED chips are located outside boundary  110  and within boundary  112 . The boundaries  110  and  112  form a ring or “dam” around the two groups of LEDs and are comprised of silicone or any other suitable material. 
     The first group of the LED chips is encapsulated by a first encapsulation material  114  and the second group of the LED chips is encapsulated by a second encapsulation material  116 . For example, in one implementation, the first encapsulation material includes phosphor materials that are injected or otherwise introduced within the boundary  110  and operate to convert blue light emitted from the first group of the LEDs into white light having a warm color temperature. For example, warm color temperature light has a color temperature of approximately 3000K. Furthermore, the second encapsulation material includes phosphor materials that are injected or otherwise introduced between the first  110  and second  112  boundaries and operate to convert blue light emitted from the second group of the LEDs into white light having a cool color temperature. For example, cool color temperature light has a color temperature of approximately 5500K. In various implementations, the color temperature of the light emitted by the first group of LED chips is different than the color temperature of light emitted by the second group of LED chips. In an aspect, the difference in color temperature between the two groups of LED chips is at least 300K 
     In various aspects, the encapsulation groups and their associated LED chips can be arranged in virtually any arrangement to facilitate light integration to support the color temperature tuning process. For example, as shown in  FIG. 1 , the first group of LED chips is located in a region within the second group of LED chips. However, in other implementations, the encapsulation groups and/or associated LED chips may be arranged or located on the substrate in any desired configuration to facilitate light integration to support the color temperature tuning process. 
     A drive circuit  118  receives one or more control signals and a user input and outputs a first drive current (Drv 1 ) and a second drive current (Drv 2 ) that are coupled to the substrate  106  at electrically conductive pads shown generally at  120 . A return current path or ground (Gnd) is also coupled between the drive circuit  118  and the substrate  106 . A first set of conductive traces, illustrated at  132 , couple the first drive current from a first conductive pad to the first group of LED chips to allow the first drive current to control the intensity at which the first group of LEDs emits light. A second set of conductive traces, illustrate at  134 , couple the second drive current from a second conductive pad to the second group of LED chips to allow the second drive current to control the intensity at which the second group of LEDs emits light. Return currents are coupled to a third conductive pad by conductive return traces, illustrated at  136 . 
     The drive circuit  118  comprises circuitry operable to generate the first and second drive currents such that these currents are capable of driving the first and second groups of LEDs from an “off” state up to their full intensity. For example, either of the first and second drive currents may be a constant current or a pulsed current having any desired frequency or pulse rate. In various implementations, the drive circuit  118  generates the first and second drive currents based on one or more received control signals and/or user input. For example, the following is an exemplary (but not exhaustive) list of control signals that are received by the drive circuit and used to set or adjust the drive currents.
     1. Ambient Indicators—Indicate information about the ambient environment such as ambient light color temperature or intensity.   2. Device Indicators—Indicate information about a light source such as emitted light color temperature or intensity. The device indicators can be used to detect process variations or degradation associated with LED chips or their encapsulation.   3. Timing indicators—Indicate information about various timing events such as the time of day or the status of a timed event.   

     A more detailed description of the drive circuit and the control signals is provided in another section of this document. 
     Referring now to the cross-sectional view  110  derived at the cross section indicator  130 , the substrate  106  is shown. Mounted to the substrate  106  are LED chips  122  and  124  that are part of the first group and LED chips  126  and  128  that are part of the second group. The walls of the first and second boundaries  110  and  112  are also shown. Encapsulating LED chips  124  and  126  is the first encapsulation material  114  and encapsulating LED chips  122  and  128  is the second encapsulation material  116 . The first encapsulation material converts blue light from the LED chips  124  and  126  into white light having a first color temperature. The second encapsulation material converts blue light from the LED chips  122  and  128  into white light having a second color temperature. 
     During operation, the drive circuit  118  outputs the first and second drive currents to control the light emitted from the first and second groups of LEDs. For example, based on the user input and/or the control inputs, the drive circuit  118  sets the levels of the first and second drive currents. This allows color temperature tuning of the light emitted from the LED apparatus  100 . For example, when the first drive current is at its maximum and the second drive current is at its minimum then the resulting color temperature and intensity of the light emitted from the LED apparatus  100  primarily originates from the first group of LEDs and has a warm color temperature. Alternatively, when the first drive current is at its minimum and the second drive current is at its maximum then the resulting color temperature and intensity of the light emitted from the LED apparatus  100  primarily originates from the second group of LEDs and has a cool color temperature. Furthermore, if both groups are activated by the first and second drive currents, then the resulting color temperature and intensity is a combination of the light emitted from each group. 
     Thus, as the first and second drive currents are adjusted the resulting color temperature can be tuned since the resulting light emitted from the LED apparatus  100  is a combination of the color temperature and intensity of the light emitted from the first and second groups of LED chips. By adjusting the first and second drive currents, the LED apparatus  100  can provide tunable color temperatures such that warm color temperatures can be obtained by activating only the first group of LED chips, cool color temperature can be obtained by activating only the second group of LED chips, and intermediate color temperatures can be obtained by activating both the first and second groups of LED chips to emit light tuned to a desired color temperature. Therefore, the LED apparatus  100  provides for tuning the color temperature of the emitted light based on the user input and/or the control signals. It should also be noted that the LED apparatus  100  is not limited to having only two groups of LED chips, but in fact, may have any number of groups of LED chips each with a corresponding color temperature light output and the drive circuit  118  can be configured to output a corresponding number of drive currents; one for each group of LED chips. 
       FIG. 2  shows an exemplary LED apparatus  200  for use in aspects of a tunable LED light source. The LED apparatus  200  illustrates an alternative embodiment of the color temperature tunable LED light source. 
     In the LED apparatus  200 , a die encapsulation process is used so that each LED chip has it own encapsulation. For example, LED chip  202  comprises a die encapsulation with a first encapsulation material and LED chip  204  comprises a die encapsulation with a second encapsulation material. Thus, because each LED chip has its own encapsulation, the LED apparatus  200  provides more flexibility in that the LED chips may be arranged and/or organized in any desired fashion (without the use of ring boundaries or dams) while still allowing any desired encapsulation material to be used for each chip and still allowing two or more LED encapsulation groups to be defined. 
     In various aspects, LED chips from each encapsulation group can be arranged in virtually any arrangement to facilitate light integration to support the color temperature tuning process. For example, LED chip  206  has four neighbor chips where two of the neighbor chips have the same encapsulation material and two of the neighbor chips have different encapsulation material. Thus, the LED chips for all groups can be arranged using a die encapsulation process so that any particular LED chip can have at least one neighbor that is encapsulated with the same or different encapsulation material. 
       FIG. 3  shows an exemplary drive circuit  300  for use in aspects of a color temperature tunable LED light source. For example, the drive circuit  300  is suitable for use as the drive circuit  118  shown in  FIG. 1 . The drive circuit  300  comprises controller  302 , memory  304 , sensor interface  306 , and current drivers  308  all coupled to communicate over communication bus  310 . It should be noted that the drive circuit  300  is just one implementation and that other implementations are possible. 
     The memory  304  comprises RAM, ROM, EEPROM or any other type of memory device that operates to allow information to be stored and retrieved. The memory  304  is operable to store drive current tables that cross reference color temperature to drive currents at various intensity levels. The drive current tables stored in the memory  304  are accessible to the controller  302  and other modules of the drive circuit  300  using the bus  310 . In one implementation, the drive current tables are stored in the memory during device manufacture. In another implementation, the drive current tables are stored in the memory by the processor  302 , after acquiring the information from another device or through a communication link, such as a network connection. 
     The sensor interface  306  comprises one or more of a CPU, processor, gate array, hardware logic, memory elements, and/or hardware executing software. The sensor interface  306  operates to communicate with various sensors or other suitable devices to acquire various sensor information associated with the ambient environment, the light source device, or timing events. For example, the sensor interface  306  acquires timing indicators  312  such as time of day or the status of timed events. The timing indicators may be received from any suitable timing device or sensor. 
     The sensor interface  306  also acquires ambient indicators  314  that indicate parameters related to the ambient environment. For example, the ambient indicators comprise ambient light levels, ambient color temperature levels or any other parameters related to the ambient environment. The ambient indicators  314  may be obtained from one or more suitable devices sensors configured to measure the ambient environment 
     The sensor interface  306  also acquires device indicators  316  that indicate parameters relative to the light source being driven by the drive circuit  300 . For example, the device indicators  316  comprise light source color temperature, intensity, or any other parameters related to the light source. The device indicators  316  may be obtained from one or more suitable devices or sensors configured to obtain information about the light emitted from the light source device. 
     The current drivers  308  comprises hardware and/or hardware executing software that operates to output multiple drive currents (Drv x )  320  that can be used to drive corresponding encapsulation groups of a color temperature tunable LED light source to allow color temperature tuning of the emitted light. In one aspect, the drive currents  320  are set to constant currents at predetermined voltage levels. In another aspect, the drive currents have selected current amplitudes that are pulsed at a selectable pulse rate. During operation, the current drivers  308  receive drive current parameters from the controller  302  and use these parameters to generate the appropriate drive currents. A ground (Gnd)  322  or return path for the drive currents is also provided. 
     The controller  302  comprises one or more of a CPU, processor, gate array, hardware logic, memory elements, and/or hardware executing software. The controller  302  operates to control the operation of the drive circuit  300  to generate drive currents to drive a color temperature tunable LED light source. The controller  302  operates to determine drive current parameters which are passed to the current drivers  308  and used to generate the drive currents  320 . In an aspect, the controller  302  receives user input  318  which comprises parameters that are used in conjunction with other information, such as sensor information, to determine the drive current parameters. For example, the user input  318  interfaces to a keypad or other user input device. 
     During operation, the controller  302  operates to control the sensor interface  306  to acquire control signal information. Furthermore, the controller  302  operates to receive information from the user input  318 . After acquiring the control signal information and user input information the controller  302  determines the desired color temperature and intensity of the light to be emitted from the light source. The following illustrate how the controller  302  determines the desired color temperature value for the emitted light. It should be noted that the controller  302  is not limited to the operations described below and may perform any other operations utilizing the available information to determined the desired color temperature and/or intensity value of the emitted light. 
     User Input 
     In an aspect, the controller  302  receives information from the user input  318  and uses this information to determine the desired color temperature and/or intensity of the emitted light. For example, a user may indicate that the color temperature and/or intensity of the emitted light are to be increased or decreased by a selected amount. For example, the user inputs this information to the controller  302  via an input keypad. In one case the user may indicate that the color temperature and/or intensity are to be changed by a particular amount or percentage. In another case, the user may indicate that the color temperature and/or intensity are to be set to specific levels. Furthermore, the user may enter programming information that indicates the desired color temperature and/or intensity level to be set after the occurrence of selected events, such as time of day events, or ambient conditions. 
     Timing Indicators 
     In an aspect, the controller  302  receives the timing indicators  312  and uses this information to determine the desired color temperature and/or intensity of the emitted light. For example, a particular time of day or the completion of a measured time interval may indicate that the color temperature and/or intensity of the emitted light are to be increased or decreased by a selected amount. For example, the user may input the color temperature to be used at specific times during the day. The controller  302  determines whether those times have occurred from the timing indicators and sets the color temperature and/or intensity of the emitted light accordingly. 
     Ambient Indicators 
     In an aspect, the controller  302  receives the ambient indicators  314  and uses this information to determine the desired color temperature and/or intensity of the emitted light. For example, a particular time of day the color temperature and/or intensity of the ambient light may reach a specified level. The user may indicate through the user input  318  what these levels are. Once these levels are reached, the controller  302  operates to set the color temperature and/or intensity of the emitted light to predetermined levels. 
     Device Indicators 
     In an aspect, the controller  302  receives the device indicators  316  and uses this information to determine the desired color temperature and/or intensity of the emitted light. For example, the device indicators  316  indicate the color temperature and intensity of the light currently being emitted by the light source. This information functions as a feedback for the drive circuit  300  in that the controller  302  can use this information to verify that light having the desired color temperature and intensity is being emitted from the light source. The device indicators can be use to compensate for process variations during manufacture with regards to the LED chips used in the light source or variations in the phosphor encapsulation material. 
     In an aspect, to achieve consistent light output from all manufactured light sources, the controller  302  can use the device indicators to determine whether the color temperature and/or intensity of the emitted light needs to be changed to maintain a particular light output. For example, if the light source is to emit light having a color temperature of 4500K and the device indicators indicate that the emitted light is actually 4800K due to process variation, then the controller  302  can adjust the color temperature of the light output to maintain the correct value. 
     In another aspect, to compensate for degradation of the LED chips or the phosphor encapsulation material, the controller  302  can use the device indicators to determine whether the color temperature and/or intensity of the emitted light needs to be changed to maintain a particular light output. For example, if the light source is to emit light having a color temperature of 4500K and the device indicators indicate that the emitted light is actually 4800K due to degradation of the LEDs, or phosphor encapsulation, then the controller  302  can adjust the color temperature of the light output to maintain the correct value. 
     Once the controller  302  determines what the color temperature and/or intensity of the emitted light should be, the controller  302  accesses the memory  304  with color temperature/intensity information to determine the appropriate drive currents. For example, the controller  302  accesses the drive current tables in the memory  304  to determine the drive currents necessary to achieve a desired light output. The controller  302  may also directly compute the drive currents as described in another section of this document. 
     Once the controller  302  has determined the appropriate drive currents the controller  302  generates drive current parameters that are passed to the current drivers  308 , which uses these parameters to generate the appropriate drive currents  320  to obtain the desired light output. Thus, the controller  302  operates to receive user input and various control signals to determine the desired color temperature and/or intensity of the light source output. This information is then used to cross reference the drive current tables in the memory  304  to determine the appropriate drive current values. The drive current values are passed to the current drivers  308  so that drive currents can be generated to drive the light source to emit light having the desired color temperature and/or intensity. 
     In various implementations, the drive circuit  300  comprises a computer program product having one or more program instructions (“instructions”) or sets of “codes” stored or embodied on a computer-readable medium. When the codes are executed by at least one processor, for instance, a processor at the controller  302 , their execution results in the functions of the drive circuit  300  described herein. For example, the computer-readable medium comprises a floppy disk, CDROM, memory card, FLASH memory device, RAM, ROM, or any other type of memory device or computer-readable medium that interfaces to the drive circuit  300 . In another aspect, the sets of codes may be downloaded into the drive circuit  300  from an external device or communication network resource. The sets of codes, when executed, operate to provide aspects of the color temperature tunable light source as described herein. 
       FIG. 4  shows exemplary graphs  400  illustrating the operation of the LED apparatus  100  shown in  FIG. 1 . The graph  402  shows plot line  404  that illustrates the resulting color temperature and intensity of light emitted from the LED apparatus  100  during operation. The graph  406  shows plot lines  408  and  410  that illustrate the amplitude of the first (Drv 1 ) and second (Drv 2 ) drive currents. 
     As the amplitude of the first drive current increases (as shown at  408 ) the intensity of the emitted warm color temperature white light increase while the color temperature remains constant, as shown in the graph  404 . As the amplitude of the second drive current increases (as shown at  410 ), the resulting intensity of the emitted light increases while the resulting color temperature shifts to the second color temperature, as shown in the graph  404 . 
     In one implementation, the first drive current is maintained at a fixed value while the second drive current is adjusted from its minimum value to its maximum value. Thus, initially the emitted light has a warm color temperature and intensity determined from the first group of LED chips. As the second drive current increases, the emitted light has a color temperature and intensity determined from a combination of the first and second groups of LED chips. As the second drive current continues to increase to its maximum value, the emitted light has a cool color temperature and intensity determined primarily from the second group of LED chips. Thus, the graph  400  illustrates how the LED apparatus  100  provides a tunable color temperature light output that provides an approximately linear relationship between color temperature and lumen output. 
     It should also be noted that it is possible to adjust the drive currents to achieve the same color temperature light with different intensity levels. For example, if the intensity is increase but the same ratio of light from the two groups of LED chips is maintained, only the intensity of the light will increase but the color temperature will remain the same. The information presented in the graphs  400  is quantified in the exemplary drive current table provided in  FIG. 5 . 
       FIG. 5  shows an exemplary drive current table  500  illustrating the relationship between color temperature and drive currents. For example, the drive current table  500  may be stored in the memory  304  for use during operation of the drive circuit  300 . 
     The drive current table  500  comprises a color temperature column  502 , and two intensity levels  504  and  506  that relate color temperature to drive current according to the relationships illustrated in  FIG. 4 . In each of the first and second intensity levels  504 ,  506 , drive currents are shown associated with each color temperature. Thus, for any particular color temperature, drive currents can be determined that will result in emitted light having that color temperature at the desired intensity. 
     Mathematical Computation of Drive Currents 
     Typically the light output of a white LED, measured in lumens, is proportional to its drive current, with the proportionality constant dependent on the color temperature assuming all other factors being equal. For example, a white LED source that can be driven with current up to one amp may produce light at the rate of 100 lumens per amp when configured as a 6000K cool-white source, but when configured as a 3000K warm-white source may only produce light at the rate of 70 lumens per amp. 
     Color Temperature Tuning Example 
     The following is an example that illustrates how the first and second drive currents can be mathematically computed to produce light having a desired intensity and color temperature. For example, the controller  302  is operable to perform the following calculation to determined necessary drive currents. 
     It will be assumed that the first group of LED chips are encapsulated with the first encapsulation material and emit a warm white light having a color temperature of T w  Kelvin. Then the intensity of the warm white light that is emitted in lumens (L w ) can be determined from the following expression; 
         L   w   =W*I   w    (1)
 
     where L w  is the warm-white light intensity in lumens produced by the first group of LED chips when driven by the first drive current (Drv 1 ) of I w  amps, with W representing a constant of efficacy in lumens per amp of the first group of LED chips. 
     Similarly, it will also be assume that the second group of LED chips are encapsulated with the second encapsulation material and emit a cool white light having a color temperature of T c  Kelvin. Then the intensity of the cool white light that is emitted in lumens (L c ) can be determined from the following expression; 
         L   c   =C*I   c    (2)
 
     where L c  is the cool-white intensity in lumens produced by the second group of LED chips when driven by the second drive current (Drv 2 ) of I c  amps, with C representing a constant of efficacy in lumens per amp of the second group of LED chips. 
     Then the total intensity of light in lumens (L T ) that is produced can be determined from the following expression; 
         L   T   =L   c   +L   w   =C*I   c   +W*I   w    (3)
 
     Furthermore, the perceived average color temperature (T avg ) of the light produced when combining the light emitted from both groups of LED chips can be determined by superposition according to the following expression; 
         T   avg =( L   c   *T   c   +L   w   *T   w )/( L   c   +L   w )   (4)
 
     Therefore, using algebraic manipulations it can be shown that the values of the two drive currents (Drv 1 =I w  and Drv 2 =I c ) that are needed for the two groups of LED chips to produce a total light output of L T  lumens at a average color temperature T avg  Kelvin can be determined from the following expressions; 
         I   w   =L/W *[( T   c   −T )/( T   c   −T   w )]  (5)
 
         I   c   =L/C *[( T−T   w )/( T   c   −T   w )]  (6)
 
     Using the above equations, it is possible for the controller  302  to determine the current drive values to complete the table  500 . For example, the controller  302  can determine the values of drive currents that would be used to produce a range of color temperatures for the two intensity levels of total light output. It should be noted that although two intensity levels are provided in  FIG. 5 , the drive current table  500  may include any number of intensity levels and the controller  302  may also directly compute the drive currents to produce the desired color temperature and any desired intensity level. 
       FIG. 6  shows an exemplary method  600  for providing a color temperature tunable LED light source. 
     At block  602 , a substrate size and material is determined. For example, the size and material of the substrate  106  shown in  FIG. 1  is determined. 
     At block  604 , the number of encapsulation groups is determined. For example, various embodiments of the invention are suitable for use with any number of encapsulation groups. Each encapsulation group will comprise one or more LEDs encapsulated with a particular encapsulation material that output light having a particular color temperature. 
     At block  606 , encapsulation material for each group is identified. For example, a first group can have an encapsulation material the converts blue LED output to a warm white color temperature and a second group can have an encapsulation material the converts blue LED output to a cool white color temperature. 
     At block  608 , the number of LED chips in each group is determined. For example, the number of LED chips in each group affects the intensity of light emitted by that group which in turn affects how light emitted from each group combines with other groups to produce a resulting light output. 
     At block  610 , the LEDs for each group are mounted on the substrate. In an aspect, the LEDs are mounted in any arrangement or are organized in any fashion to allow encapsulation with the appropriate material and to allow light emitted from each group to combine with other groups to be perceived as an integrated light source. 
     At block  612 , each encapsulation group is encapsulated with the appropriate encapsulation material. For example, each LED in a particular group is encapsulated with the encapsulation material identified for that group. In one implementation, multiple LED chips are encapsulated together by surrounding them with a boundary material and injecting the encapsulation material to cover all LED chips within the boundary. In another implementation, each LED chip in a group is encapsulated with the appropriate encapsulation material using a die encapsulation technique. 
     At block  614 , the LED chips of each group are coupled to receive a drive current for each group, respectively. For example, if there are three encapsulation groups, then there are three drive currents; one for each group. 
     At block  616 , each group&#39;s drive current is adjusted so that the device emits a resulting light output having a particular color temperature and intensity. For example, the drive circuit  118  operates to adjust the first and second drive currents based on received control signals and/or user input as described above. 
     Therefore, the method  600  operates to providing a color temperature tunable LED light source in accordance with aspects of the present invention. It should be noted that the operations of the method  600  may be rearranged or otherwise modified within the scope of the various aspects. Thus, other implementations are possible with the scope of the various aspects described herein. 
       FIG. 7  shows an exemplary method  700  for driving a color temperature tunable light source having multiple encapsulation groups. For example, the method is suitable for use with the drive circuit  300  shown in  FIG. 3 . 
     At block  702 , default drive current tables are set up in a memory. For example, the default drive current table maybe the drive current table  500  shown in  FIG. 5 . In one implementation, the default drive current table is stored in the memory  304  during device manufacture or installation. 
     At block  704 , sensor inputs are received. For example, the timing indicators  312 , ambient indicators  314 , and device indicators  316  are received by the sensor interface  306  and passed to the controller  302 . 
     At block  706 , color temperature, intensity, and timing events associated with a light source are determined from the sensor inputs. For example, the controller  302  processes the timing indicators  312 , ambient indicators  314 , and device indicators  316  to determine various parameters associated with the operation of a color temperature tunable light source. 
     At block  708 , user parameters are received. For example, the controller  302  receives user parameters from the user input  318 . 
     At block  710 , a desired color temperature and intensity of a color tunable LED light source is determined. The controller  302  determines the desired color temperature and intensity of the color temperature tunable light source based on the received sensor inputs and user inputs. For example, at a particular time of day a particular color temperature light is desired. The controller  302  may also determine that due to process variation or degradation the light being emitted has drifted from the desired color temperature. Thus, the controller  302  may determine a desired color temperature and/or intensity by processing the sensor information and/or user input as described above. 
     At block  712 , a determination is made as to whether the color temperature or intensity of the LED light source needs to be adjusted. For example, the controller  302  stores information about the current color temperature and intensity of light being emitted from the light source. This information is compared to a desired color temperature determined from the sensor inputs and/or the user input. If the desired color temperature or intensity are different from the current color temperature or intensity, then the controller  302  determines that a color temperature or intensity adjust is necessary. If adjustment is necessary, the method proceeds to block  714 . If adjustment is not necessary, the method returns to block  704   
     At block  714 , drive current tables are accessed to determine drive current necessary to achieve the desired light output. For example, the controller  302  accesses the drive current tables in the memory  304  to determine the drive currents necessary to obtained the desired light output. The controller  302  cross references the drive tables with the desired color temperature at the desired intensity to determine the required drive currents. In another implementation, the controller  302  determined the drive currents through direct computation as described above. 
     At block  716 , the drive currents for each encapsulation group of the LED light source are adjust to the appropriate level as determined from the drive current tables. For example, the controller  302  pass the drive current parameters to the current drivers  308  which in turn adjusts the drive currents to the appropriate levels to obtain emitted light having the desired color temperature and intensity. 
     Therefore, the method  700  operates to provide drive a color temperature tunable LED light source in accordance with aspects of the present invention. It should be noted that the operations of the method  700  may be rearranged or otherwise modified within the scope of the various aspects. Thus, other implementations are possible with the scope of the various aspects described herein. 
       FIG. 8  shows an exemplary alternative drive circuit  800  for use in aspects of a color temperature tunable LED light source. For example, the drive circuit  800  is suitable for use as the drive circuit  118  shown in  FIG. 1 . The drive circuit  800  comprises dimmer  802 , first current driver  804 , and second current driver  806 . It should be noted that the drive circuit  800  is just one implementation and that other implementations are possible. 
     The drive circuit  800  is coupled to drive a color temperature tunable LED light source  810  that is part of a device  808 . For example, the color temperature tunable light source  810  may comprise the LED apparatus  100  shown in  FIG. 1 . 
     The first current driver  804  comprises discrete hardware and/or hardware executing software that operates to receive AC power  808  and generate a first drive current (Drv 1 )  812  that is coupled to drive a corresponding encapsulation group of the color temperature tunable LED light source  810 . For example, the first drive current  812  is coupled to drive a first group of LED chips of the light source  810  to generate warm color temperature light. In one implementation, the first drive current  812  is set to drive the first group of LED chips at their maximum intensity. 
     The second current driver  806  comprises discrete hardware and/or hardware executing software that operates to receive adjust AC power  818  and generate a second drive current (Drv 2 )  814  that is coupled to drive a corresponding encapsulation group of the color temperature tunable LED light source  810 . For example, the second drive current  814  is coupled to drive a second group of LED chips of the light source  810  to generate cool color temperature light. In one implementation, the second drive current  814  is adjustable from a fully “off” state to its maximum value based the adjusted AC power  818 . 
     The dimmer  802  comprises one or more of a CPU, processor, gate array, state machine, hardware logic, discrete circuitry, memory elements, and/or hardware executing software. The dimmer  802  operates to receive user parameters  816  and the AC power  808  to generate the adjusted power  818  that is input to the second current driver  806 . 
     In one implementation, the dimmer  802  generates the adjusted AC power  818  by adjusting the AC power input  808  in response to the user parameters  816 . For example, the dimmer  802  may reduce the AC power  808  to produce the adjusted AC power  818 , which results in a reduced second drive current  814 . For example, the dimmer  802  may be a rheostat, potentiometer, or other user operated device which a user can operate to change the adjusted AC power  818  and thereby set the second drive current to obtain a desired color temperature light emitted from the light source  810 . For example, when the second drive current  814  is minimized the light output is generated from the first group of LED chips and has a warm color temperature. When the second drive current  814  is increased, the light output is generated by both groups of LED chips and a resulting cool color temperature light is emitted. Thus, in one implementation, the dimmer  802  allows a user to change the intensity and color temperature of the light emitted from the light source  810 . 
     Therefore the drive circuit  800  operates to adjust the drive currents provided to a color tunable LED light source so that the intensity and color temperature can be adjusted. 
       FIG. 9  shows an exemplary method  900  for driving a color temperature tunable light source having multiple encapsulation groups. For example, the method is suitable for use with the drive circuit  300  shown in  FIG. 3 . 
     At block  902 , first and second drive currents are activated. For example, the first current driver  804  and the second current driver  806  generate the first  812  and second  814  drive currents that are coupled to a color temperature tunable light source  810 . 
     At block  904 , user parameters are received. For example, the dimmer  302  receives user parameters from the user input  816  and uses these parameters to generate the adjusted AC power  818 . 
     At block  906 , the second drive current is adjusted based on the user parameters to set the color temperature and/or intensity of the light source. For example, the second current driver  806  adjusts the second drive current  814  based on the adjusted AC power  818  so as to adjust the color temperature and/or the intensity of the light emitted from the light source  810 . 
     Therefore, the method  900  operates to adjust the color temperature and/or intensity of a tunable LED light source in accordance with aspects of the present invention. It should be noted that the operations of the method  900  may be rearranged or otherwise modified within the scope of the various aspects. Thus, other implementations are possible with the scope of the various aspects described herein. 
       FIG. 10  shows an exemplary color temperature tunable LED apparatus  1000  constructed in accordance with aspects of a color temperature tunable LED light source. 
     The apparatus  1000  comprises a first light emitting means for emitting light at a first color temperature. For example, the first light emitting means may be the first group of LED chips within the boundary  110  and encapsulated with the first encapsulation material. 
     The apparatus  1000  also comprises a second light emitting means for emitting light at a second color temperature. For example, the second light emitting means may be the second group of LED chips between the boundaries  110  and  112  encapsulated with the second encapsulation material. 
     The apparatus  1000  also comprises a drive means for driving the first and second light emitting means to produce a tunable color temperature light output. For example, in one implementation, the drive means comprises the conductive mounting pads  120  and associated electrical connections to the first and second groups of LED chips shown in  FIG. 1 . Thus, the apparatus  1000  operates to provide a color temperature tunable white light source. 
       FIG. 11  shows an exemplary drive circuit apparatus  1100  constructed in accordance with aspects of a color temperature tunable LED light source. 
     The apparatus  1100  comprises means ( 1102 ) for outputting a first drive current to drive a first group of LED chips of the light source to emit first color temperature light, which in an aspect comprises the first current driver  804 . 
     The apparatus  1100  comprises means ( 1104 ) to output a second drive current to drive a second group of LED chips of the light source to emit second color temperature light, which in an aspect comprises the second current driver  806 . 
     The apparatus  1100  also comprises means ( 1106 ) for controlling the first and second drive currents so that the first color temperature light and the second color temperature light combine to produce a resulting light having a selected color temperature and a selected intensity value, which in an aspect comprises the dimmer  802 . 
     The various aspects of this disclosure are provided to enable one of ordinary skill in the art to practice the present invention. Various modifications to aspects presented throughout this disclosure will be readily apparent to those skilled in the art, and the concepts disclosed herein may be extended to other applications. Thus, the claims are not intended to be limited to the various aspects of this disclosure, but are to be accorded the full scope consistent with the language of the claims. All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. 
     Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. §112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.” 
     Accordingly, while aspects of an efficient LED array have been illustrated and described herein, it will be appreciated that various changes can be made to the aspects without departing from their spirit or essential characteristics. Therefore, the disclosures and descriptions herein are intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.