Abstract:
A system and method for reducing wavelength variations between light emitting diodes (LEDs) is provided that determines an emission wavelength of each of a plurality of LEDs at a common drive current, and drives each of the plurality of LEDs with a respective operational drive current such that wavelength variations between the plurality of LEDs, when driven at the respective operational drive currents, is less than wavelength variations between the plurality of LEDs when driven at the common drive current. This system and method of the invention minimizes wavelength variations between LEDs, thereby allowing the use of more, and in some cases all, of the LEDs that are fabricated from a single semiconductor wafer, or multiple semiconductor wafers.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to light emitting diodes and, more particularly, to a system method for driving multiple light emitting diodes so as to reduce the wavelength variation between them. 
     Light emitting diodes (LEDs) are generally manufactured in batches using standard semiconductor fabrication techniques. A single semiconductor wafer will typically yield multiple LEDs. Although the fabrication process can be controlled to obtain LEDs that emit light at a specific color, there are generally significant variations in the output wavelengths of the LEDs when they are driven with a common drive current. 
     For example, a batch of LEDs can be designed to emit green light, however, one LED could emit light at 500 nm, while another LED could emit light at 506 nm. Even with LEDs that originate from a single wafer, the output wavelengths can vary significantly. LEDs that originate from different wafers can exhibit even greater wavelength variations. 
     In some LED applications, wavelength variations between the LEDs can be undesirable. For example, automobile manufacturers often create a vehicle&#39;s unique identity, in part, through the use of “theme” wavelength for the interior trim and illumination. This illumination can include backlighting of switches, instrument cluster backlighting, and general or specific illumination applications. If LEDs are used, the theme wavelength requirements generally dictate that their output light fall within a narrow range of wavelengths. 
     Because the output wavelengths of individual LEDs can vary by more than the amount of deviation allowable for certain applications, in many cases not ail of, the LEDs from a single semiconductor wafer can be used. Because some of the LEDs from a single wafer may be rejected for a particular application, the costs associated with utilizing LEDs is increased. 
     BRIEF SUMMARY OF THE INVENTION 
     In an exemplary embodiment of the invention, a method of driving a plurality of LEDs comprises the steps of: determining an emission wavelength of each of the plurality of LEDs at a common drive current; choosing a respective operational drive current for each of the plurality of LEDs such that wavelength variations between the plurality of LEDs, when driven at their respective operational drive currents, is less than wavelength variations between the plurality of LEDs when driven at the common drive current; and driving the plurality of LEDs with the respective operational drive currents. 
     The invention also provides a lighting system comprising: a plurality of LEDs that, when driven with a common drive current, collectively emit light with initial wavelength variations; and a drive circuit for driving the plurality of LEDs with respective operational drive currents, such that the plurality of LEDs collectively emit light with operational wavelength variations that are less than the initial wavelength variations. 
     The system and method of the invention minimizes wavelength variation between LEDs, thereby allowing the use of more, and in some cases all, of the LEDs that are fabricated from a single semiconductor wafer, or multiple semiconductor wafers. This reduces the costs associated with utilizing LEDs in lighting applications that require a narrow range of output wavelengths. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an LED lighting system, in accordance with one embodiment of the present invention; 
     FIG. 2 is a block diagram of an LED lighting system, in accordance with another embodiment of the present invention; 
     FIG. 3 is a flowchart of a method of generating light with a plurality of LEDs, in accordance with one embodiment of the present invention; and 
     FIGS. 4 and 5 are flowcharts of a method of generating light with a plurality of LEDs, in accordance with another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an LED lighting system  300  in accordance with one embodiment of the present invention. The system includes a plurality of LEDs, represented by LEDs  100 ,  110 ,  120  and  130 . The system  300  also includes a drive circuit  140  that supplies drive currents to the LEDs  100 - 130  via signal lines  150 ,  160 ,  170  and  180 , respectively. 
     When driven by the drive circuit  140 , the LEDs  100 ,  110 ,  120  and  130  emit light  190 ,  200 ,  210  and  220  at respective wavelengths. The drive circuit  140  adjusts the operational drive current supplied to the LEDs  100 - 130  so that wavelength variations in the output light  190 - 220  are less than wavelength variations when the LEDs  100 - 130  are driven with a common drive current. 
     An aspect of the present invention is the recognition that the output wavelength of light emitted by an LED can be affected by the magnitude of the drive current applied to it, and applying this phenomena to a system and method that will allow more LEDs from a single wafer, or multiple wafers, to be utilized in lighting applications with a narrow range of permissible wavelengths. 
     In the system  300  of FIG. 1, the drive circuit  140  is adapted to drive each of the LEDs  100 - 130  with a respective operational drive current that will cause the wavelength variations in the output light  190 - 220  to be less than the wavelength variations that are present when the LEDs  100 - 130  are driven at a common drive current. The LED system  300  shown in FIG. 1 can be used in any lighting application in which control of wavelength variations between LEDs is desired. For example, the lighting system  300  of FIG. 1 could be used to implement indicator lights in an automobile interior, in which certain “theme” wavelengths are desired with very little wavelength variation. 
     To determine what operational drive currents to use for the LEDs  100 - 130 , it must first be determined how the output wavelengths of the LEDs  100 - 130  vary as a function of changes in the operational drive current. As an illustrative example, assume that it is determined that the output wavelengths of LEDs  100 - 130  can be increased by approximately 2 nm by reducing the driving current by 5 mA, and that the output wavelength can be shortened by approximately 2 nm by increasing the drive current by 5 mA. Further, assume that, at a common drive current of 20 mA, LED  100  emits light  190  at 502 nm, LED  110  emits light  200  at 505 nm, LED  120  emits light  210  at 507 nm, and LED  130  emits light  220  at 508 nm. 
     If the lighting application calls for light that falls within the wavelength range of 504 nm-506 nm, then one possible solution is to drive LED  100  at 15 mA (making its output wavelength 504 nm), drive LED  110  at 20 mA (making its output wavelength 505 nm), drive LED  120  at 25 mA (making its output wavelength 505 nm), and drive LED  130  at 25 mA (making its output wavelength 506 nm). 
     In one embodiment, shown in FIG. 2, the drive circuit  140  comprises a single common voltage source  400  connected to individual resistors  430   a - 430   d  that are supplied for each of the LEDs  100 - 130 . The values of the individual resistors  430   a - 430   d  are chosen to achieve the desired operational drive current at each LED, based on the single common voltage source  400 . The positive terminal of the single common voltage source  400  is connected to each of the resistors  430   a-   430   d  via signal lines  410  and  412   a-   412   d . The negative terminal of the single common voltage source  400  is connected to each of the LEDs  100 - 130  via signal line  420 . 
     Driving the LEDs  100 - 130  with different operational drive currents will result in output light  190 - 220  of varying intensity. However, changes in intensity are generally considered insignificant when compared to variations in wavelength, particularly in the case of colors to which to human eye is most sensitive. Accordingly, the operational drive currents for the LEDs  100 - 130  are chosen so that the resulting variations in output light intensities between the LEDs  100 - 130  are within acceptable limits for the particular application. In many cases, choosing operational drive currents for the LEDs  100 - 130  that result in variations in output light intensities of less than 50 percent will be sufficient. 
     FIG. 3 is a flowchart of a method  550  of generating light with a plurality of LEDs, in accordance with one embodiment of the present invention. The method  550  starts at step  500 , where a plurality of LEDs are provided. The LEDs could originate from a single semiconductor wafer and/or multiple semiconductor wafers. 
     At step  510 , wavelength variations between the plurality of LEDs are determined at a common drive current in a manner similar to that discussed above. The method  550  then continues to step  520 , where the plurality of LEDs are driven with respective operational drive currents that are chosen to reduce the wavelength variations between the plurality of LEDs to an amount that is less than the wavelength variations when the LEDs are driven at a common drive current. As explained above, the respective operational drive currents are preferably chosen so that variations in output light intensity between the plurality of LEDs, when driven at the respective operational drive currents, are less than 50 percent. 
     FIGS. 4 and 5 show a flowchart of a method  1000  of generating light with a plurality of LEDs, in accordance with another embodiment of the present invention. The method begins at step  600 , where a target wavelength is determined for the particular lighting application that the LEDs will be used for. For example, an automotive interior application may call for light at 505 nm. 
     The method then continues to step  610 , where an acceptable wavelength variation between the available LEDs is determined. The acceptable wavelength variation will vary depending on the lighting application. For example, an automotive interior application may call for light at a target wavelength of 505 nm, with a variation between LEDs of no more than 2 nm. 
     At step  620 , the number of discrete drive current values that a user is willing to use to drive the LEDs is determined. The number of discrete drive current values available to drive the LEDs may be limited, for example, by the drive circuit being used. Next, at step  630 , an available bandwidth is determined by multiplying the number of discrete drive current values, determined at step  620 , by the acceptable color variation determined at step  610 . 
     Next, at step  640 , the wavelength distribution, range and span of available LEDs is determined at a common drive current (e.g., at a drive current of 20 mA). The “wavelength span” is defined as the difference in output wavelengths, at the common drive current, between the LED with the longest output wavelength and the LED with the shortest output wavelength. The method  1000  than continues to step  650 , where it is determined whether the wavelength distribution, determined at step  640 , corresponds to a normal distribution. If the wavelength distribution corresponds to a normal distribution, the method continues to step  660 . Otherwise, the method jumps to step  680 . 
     At step  660 , an LED with an output wavelength, at the common drive current, that is closest to the middle of the wavelength distribution determined at step  640  is designated as a “reference LED”. Next, at step  670 , a first reference drive current is determined that will adjust the output wavelength of the reference LED to substantially coincide with the target wavelength. Control then continues to step  720  (FIG.  5 ). 
     At step  680 , it is determined whether the wavelength span, determined at step  640 , is less than the available bandwidth determined at step  630 . If so, control jumps to step  660 . Otherwise, control continues to step  690 . 
     At step  690 , a wavelength window is determined that has the same bandwidth as the available bandwidth, determined at step  630 , and that will encompass the output wavelength of most of the available LEDs when they are driven at the common drive current. Next, at step  700 , an LED with an output wavelength, at the common drive current, that is closest to the middle of the wavelength window is designated as the “reference LED”. 
     The method then continues to step  710 , where a first reference drive current is determined that will adjust the output wavelength of the reference LED to substantially coincide with the target wavelength. Control then continues to step  720  (FIG.  5 ). 
     At step  720 , it is determined whether the number of discrete drive currents, determined at step  620 , is an odd number. If so, controls continues to step  730 . Otherwise, control jumps to step  750 . 
     At step  730 , a first category of LEDs is established having a wavelength range with a minimum wavelength equal to the target wavelength minus one-half of the acceptable wavelength variation, and a maximum wavelength equal to the target wavelength plus one-half of the acceptable wavelength variation. Control then continues to step  740 . At step  750 , a first LED category is established having a wavelength range with a minimum wavelength equal to the target wavelength, and a maximum wavelength equal to the target wavelength plus the acceptable wavelength variation. Control then jumps to step  740 . 
     At step  740 , LEDs with output wavelengths that fall within the wavelength range of the first LED category, when driven at the first reference drive current, are grouped into the first LED category. Next, at step  760 , additional LED categories are established such that the total number of LED categories equals the number of discrete drive current values, and such that each LED category borders at least one other LED category. 
     For example, if the first LED category has a wavelength range with a minimum wavelength equal to the target wavelength minus one-half of the acceptable wavelength variation, and a maximum wavelength equal to the target wavelength plus one-half of the acceptable wavelength variation, the second LED category could have a wavelength range with a minimum wavelength equal to the maximum wavelength of the first category, and a maximum wavelength equal to the minimum wavelength plus the acceptable wavelength variation. 
     If the first LED category has a wavelength range with a minimum wavelength equal to the target wavelength, and a maximum wavelength equal to the target wavelength plus the acceptable wavelength variation, the second LED category could have a wavelength range with a maximum wavelength equal to the target wavelength, and a minimum wavelength equal to the maximum wavelength minus the acceptable wavelength variation. 
     At step  765 , LEDs that were not grouped into the first LED category are grouped into the additional LED categories established at step  760 , if their output wavelength at the first reference drive current fall within the wavelength range of any of the additional LED categories. Then, at step  770 , an LED in each LED category with an output wavelength, at the first reference drive current, that is closest to the middle of the wavelength range of the respective category is designated as a “second reference LED”. Next, at step  780 , a respective operational drive current is determined, for each additional LED category, that will adjust the output wavelength of the second reference LED in each additional category to substantially coincide with the target wavelength. The method then proceeds to step  790 , where the LEDs in the first LED category are driven with an operational drive current that is equal to the first reference drive current, and the additional LED categories are driven with the respective operational drive currents determined at step  780 . 
     The following is an illustrative example of how the method  1000  of FIGS. 4 and 5 can be applied to a specific lighting application. At step  600 , it is determined that the lighting application requires light with a target wavelength of 506 nm. At step  610 , it is determined that a wavelength variation of no more than 6 nm is needed for the lighting application. 
     Then, at step  620 , it is determined that a maximum of three discrete drive current values can be used to drive the available LEDs. Next, at step  630 , the number of discrete drive current values (three) is multiplied by the acceptable wavelength variation (6 nm) to yield an available bandwidth of 18 nm. Then, at step  640 , the available LEDs are driven at a common drive current (20 mA in this example) and their wavelength distribution, range and span, at that common drive current, is determined. 
     As discussed above, the wavelength span is defined as the difference in output wavelengths, at the common drive current, between the LED with the longest output wavelength and the LED with the shortest output wavelength. For this example, it is assumed that the available LEDs, when driven at the common drive current of 20 mA, emit light with a wavelength range of 500 nm-505 nm. This would make the wavelength span of the available LEDs when driven at the common drive current 5 nm (505 nm-500 nm). 
     At step  660 , it is determined whether the wavelength distribution of the available LEDs, when driven at the common drive current, corresponds to a normal distribution. If so, an LED with an output wavelength, at the common drive current, that is closest to the middle of the wavelength distribution is designated as a reference LED. For this example, it is assumed that one of the available LEDs emits light at 503 nm when driven at the common drive current of 20 mA, which falls in the middle of the wavelength distribution of the available LEDs. The 503 nm LED is designated as a reference LED. 
     At step  670 , a first reference drive current that will adjust the output wavelength of the 503 nm reference LED to substantially coincide with the target wavelength of 506 nm is determined. For this example, it is assumed that the output wavelength of the reference LED can be increased by 2 nm for every 5 mA decrease in drive current, and decreased by 2 nm for every 5 mA increase in drive current. Thus, decreasing the drive current from the common drive current of 20 mA to 12.5 mA will adjust the output wavelength of the reference LED to the target wavelength of 506 nm. Thus, in this example, the first reference drive current is 12.5 mA. 
     If the wavelength distribution of the available LEDs, when driven at the common drive current, is not a normal distribution, it is determined at step  680  whether the wavelength span of the available LEDs is less than the available bandwidth (calculated at step  630 ). If the wavelength span is less than the available bandwidth, the reference LED and first reference drive current are determined as discussed above in connection with a normal wavelength distribution. 
     If the wavelength span is not less than the available bandwidth, a “wavelength window” is determined that has the same bandwidth as the available bandwidth determined at step  630 , and that will encompass most of the available LEDs. Specifically, the minimum and maximum wavelengths of the wavelength window are chosen so that the output wavelengths of as many of the available LEDs as possible, when driven at the common drive current of 27 mA, fall within the wavelength window. As an example, assume that the available LEDs have output wavelengths at the common drive current that range from 480 nm to 510 nm. This would correspond to a wavelength range of 30 nm. For this example, a wavelength window having a bandwidth of 18 nm, which is the same as the available bandwidth determined at step  630 , will be defined with minimum and maximum wavelength values chosen to encompass as many of the available LEDs as possible. For this example, assume that a wavelength window with a minimum wavelength of 490 nm and a maximum wavelength of 508 nm will encompass the output wavelengths of most of the available LEDs. 
     Next, at step  700 , an LED with an output wavelength, at the common drive current, that is closest to the middle of the wavelength window is designated as a reference LED. In this example, assume that an LED with an output wavelength at 499 nm at the common drive current exists. The 499 nm LED would be designated as the reference LED, because it falls in the middle of the wavelength window. 
     Next, at step  710 , a first reference drive current that will adjust the output wavelength of the reference LED to substantially coincide with the target wavelength is determined. For this example, assume that a first reference drive current of 2.5 mA will adjust the output wavelength of the reference LED to coincide with the 506 nm target wavelength (a 5 mA decrease in drive current for every 2 nm increase in output wavelength). 
     Once the first reference drive current has been determined, using either of the methods discussed above, it is determined whether the number of available discrete drive currents is an odd number (step  720 ). If the number of discrete drive currents is an odd number, a first LED category is established (step  730 ) having a wavelength range with a minimum wavelength equal to the target wavelength minus one-half the acceptable wavelength variation and a maximum wavelength equal to the target wavelength plus one-half of the acceptable wavelength variation. In this example, there are three discrete drive currents (an odd number) that can be used. Thus, the first LED category will have a minimum wavelength equal to 503 nm (506 nm -3 nm), and a maximum wavelength of 509 nm (506 nm−3 nm). 
     At step  740 , LEDs that fall within a wavelength range of the first LED category, when driven at the first reference drive current, are grouped into the first LED category. Next, at step  760 , additional LED categories are established such the total number of LED categories equals the number of discrete current drive values, and such that each LED category borders at least one other LED category. In the present example, two additional LED categories are established for a total of three LED categories (there are three available discrete drive current values). In this example, the second LED category will have a minimum wavelength of 509 nm and a maximum wavelength of 515 nm and the third LED category will have a minimum wavelength of 497 nm and a maximum wavelength of 503 nm. 
     At step  765 , LEDs that fall within the wavelength range of either of the two additional LED categories are grouped into that LED category. Then, at step  770 , for each of the three categories, an LED with an output wavelength (at the reference drive current) that is closest to the middle of the wavelength range of its respective category is designated as a second reference LED. In this example, an LED with an output wavelength of 506 nm at the first reference drive current is designated as a second reference LED in the first LED category, an LED with an output wavelength of 512 nm at the first reference drive current is designated as the second reference LED in the second LED category, and an LED with an output wavelength of 500 nm at the first reference drive current is designated as the second reference LED in the third LED category. 
     Next, at step  780 , respective operational drive currents are determined for each additional LED category that will adjust the output wavelength of the second reference LED in each additional LED category to substantially coincide with the target wavelength. The second reference LED in the first LED category has an output wavelength of 506 nm at the reference drive current. Thus, the operational drive current for the first LED category will be the same as the first reference drive current. The reference LED in the second LED category has an output wavelength of 512 nm at the first reference drive current. Assuming that the output wavelength can be shortened by approximately 2 nm by increasing the drive current by 5 mA, then the operational drive current for the second LED category will be 15 mA more than the first reference drive current. The second reference LED in the third LED category has an output wavelength of 500 nm at the first reference drive current. Thus, assuming the same output wavelength dependence on drive current, the operational drive current for the third LED category will be 15 mA less than the first reference drive current. 
     At step  790 , the LEDs in the first LED category are driven with an operational drive current that is equal to the first reference drive current, and the LEDs in the second and third LED categories are driven with the respective operational drive currents determined at step  770 . 
     Although the LEDs&#39; wavelength dependence on drive current has been described as an inverse dependence in the examples discussed above, e.g., an increase in current will shorten the output wavelength and vice versa, it should be appreciated that some LEDs exhibit a direct wavelength dependence on changes in drive current, e.g., increasing the drive current will lengthen the wavelength and vice versa. The system and method of the present invention can be used with LEDs that exhibit a direct wavelength dependence and an inverse wavelength dependence on changes in drive current. 
     While the foregoing description includes many details and specificities, it should be understood that these have been included for purposes of explanation only, and are not to be interpreted as limitations of the present invention. Many modifications to the embodiments described above can be made without departing from the spirit and scope of the invention, as is intended to be encompassed by the following claims and their legal equivalents.