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BCR402U Datasheet(PDF) 3 Page - Infineon Technologies AG |
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BCR402U Datasheet(HTML) 3 Page - Infineon Technologies AG |
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3 / 8 page  AN 066 Rev D 3 / 8 16-Jan-2004 Applications Note No. 066 Silicon Discretes Figure 4. Application Circuit Schematic. Switch SW1 permits comparison of BCR402R vs. “Resistor Method”. BCR402R 12 3 4 Vcc R1 D1 D2 D3 D4 SW1 R2 R3 Q1 BCR402R has a typical output current of 20mA without using external resistor R1. Current may be increased above this nominal value by using R1, and in this case, R1 was set to 82 Ω to achieve an output current of 29mA. Based on the fact that the minimum supply voltage is 9V and that there is a voltage drop of approximately 0.75V across the BCR402R at minimum supply voltage, it is possible to drive four red LEDs. On the right side of the circuit shown in the schematic (“resistor mode”), two parallel resistors (560 Ω) had to be used instead of one, so as to not exceed the maximum power dissipation of the 1208 SMD resistors while achieving a net resistance of 280 Ω. This brings up a key issue: overall system efficiency. See Figure 5. Figure 5. Comparison of Power Dissipation in BCR402R LED Bias controller vs. Resistor Biasing Method. Power Dissipation in BCR402R Power Dissipation in (R2+R3) 22mW 235mW For VS=16V, I=29mA, R2║R3 = 280Ω Compare results in Figure 5 for both sides of the application circuit (e.g. BCR402R method versus Resistor method), for the condition of VS=16V, I=29mA. When the “resistor method” is used to drive the same number of LEDs as the BCR402R, more than ten times the DC power is wasted in the resistors as is burned in the BCR402R. The difference in power dissipation between the two methods, 213mW, may seem trivial, unless one considers the effect of using large numbers of such LED circuits in a large display. If the net series resistor value is reduced, DC power wasted when using the “resistor method” could be reduced, but the already poor current regulation of the “resistor method” gets even worse. If the net series resistor value is increased to 410 Ω (two parallel 820 Ω resistors), the current stability is improved slightly, but the larger voltage drop across the larger net series resistance reduces the number of LEDs that can be driven from four to just two. For a given number of LEDs in a display, this means that the user of the resistor method would then have to add additional resistor + LED branches to the display, requiring additional current and further increasing power dissipation. In the example given here, the current consumption would have to double in order to drive a total of four red LEDs with 410 Ω series resistances. Figure 6 shows the effect on current stabilization using the BCR402R and different series resistor values (280, 340 and 410 Ω, resulting from parallel combinations of R1+R2). Note the nice, flat curve showing nearly constant current over the entire 9V to 16V supply range when the BCR402R LED Bias Controller is used, while the series resistance method shows very limited current stabilization – even with |
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Similar Description - BCR402U |
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