Application Note 11 Balancing White Color I. Preface Light’s three primary colors are red, green and blue. Controlling these 3 colors’ luminous intensity of LED can produce various colors. This note explains how to reproduce white color with Nichia LED. CIE 1931 Chromaticity Diagram II. Color Reproduction The triangle in Figure 1 shows the color 0.9 reproduction range of color television and LED. NTSC(National Television System Committee) is the 0.7 Color Television video signal system(method) which is adopted in Japan, the United States, and Canada. 0.6 y coordinate Light Emitting Diode NTSC LED Color Television 0.8 The wider the triangle becomes, the better color reproduction. As you see in Figure 1, 0.5 0.4 color reproduction of LED is closer to the NTSC’s than Color TV. 0.3 0.2 III. White Balance Reproducing white without close control may become 0.1 0 reddish white, bluish white and etc. To produce the white color by LEDs, one must adjust 0 0.1 0.2 the luminous intensity of blue, green and red. This adjustment is called “White balance.” 0.3 0.4 0.5 x coordinate 0.6 0.7 Figure 1 In general, to produce white color (x=0.33 y=0.33), the luminous intensity ratio is R:G:B=3:7:1. (It depends on the color coordinate of each Red, Green and Blue.) IV. Balancing White Color of Full Color LED i) Nichia’s full color surface mount LED is composed of red, green, and blue elements in 1 package. The following is an example of full color surface mount LED. Blue Green Red Unit Min. Typ. Max. Min. Typ. Max. Min. Typ. Max. Item Symbol Condition Forward Voltage VF B,G IF=10[mA] R IF=20[mA] - 3.6 4.0 - 3.5 4.0 - 1.9 2.4 A Reverse Current IR VR=5V - - 50 - - 50 - - 50 µA Luminous Intensity Iv B,G IF=10[mA] 52 R IF=20[mA] 72 144 215 300 600 92 125 260 mcd Figure 2 Initial Electricity / Optical Characteristic of NECM325C. Information in this document is tentative and subject to change without notice for improvement. 1/2 (STS-KSE4255) August 1st, 2004 0.8 Application Note 22 ii) Luminous Intensity Suppose: The luminous intensity of Red: 125mcd.(Typical rating in Figure 2) The luminous intensity of Green and Blue is derived from the ratio: R:G:B = 3:7:1 More specifically,Green=125 x 7/3 = 291.6[mcd] 3.5 Blue =125 x 1/3 = 41.6[mcd] Therefore, to reproduce white color (x=0.33 y=0.33), Red=125mcd Green=291mcd Blue=41mcd iii) Forward Current Refer to Figure 2， 2.5 2.0 1.5 Blue Green Red 1.0 0.5 Suppose 300mcd is as 1. 291 / 300=0.97[times] 0.0 0 The forward current is about 9.8mA.(See Figure 3) c)Blue iv) Power Dissipation When you use full-color LED, total power dissipation must be within the absolute maximum rating. Figure 4 shows “Ambient temperature” vs. “Power Dissipation” of NECM325C. Power Dissipation is derived as follows. PD[mW]=IF[mW] x VF Power Dissipation for ii) and iii) PD[mW]=20[mA]x1.9[v]+9.8[mA]x3.5[v]+4.8[mA]x3.6[v] =89.58[mW] 10 20 30 IF (mA) 40 50 60 Figure 3 The same way of calculation result as 4.8mA 70 60 Power Dissipation (mW) Light Emitting Diode a)Red=125mcd The current value is 20mA. b)Green Relative Luminousity (a,u) the required luminous intensity is as follows respectively. 3.0 And 89.5mW exceeds absolute maximum rating in Figure 4. You must lower the forward current of each die. 50 40 30 20 10 0 0 20 40 60 80 o Ambient Temperture ( C) 100 Figure 4 Luminous intensity of Red : 60mcd, Calculating required luminous intensity of blue and green. Derived form (Refer to ii). Green = 140mcd Blue = 20mcd The approximate current values are derived form Figure 3 at the above luminous intensity. Red=11mA Green=3.5mA Blue=1.8mA PD[mW]=11[mA] x 1.9[v]+3.5[mA] x 3.5[v]+1.8[mA] x 3.6[v]=39.63[mW] Total power dissipation of 39.63mW should be operated within 65 oC from Figure 4. It is important that the total power dissipation must be within the absolute maximum rating. Information in this document is tentative and subject to change without notice for improvement. 2/2 (STS-KSE4255) August 1st, 2004