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EUP2571 Datasheet(PDF) 10 Page - Eutech Microelectronics Inc |
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EUP2571 Datasheet(HTML) 10 Page - Eutech Microelectronics Inc |
10 / 14 page ![]() EUP2571 DS2571 Ver1.1 May. 2007 10 Application Information LED Current Control The EUP2571 regulates the LED current by setting the current sense resistor (R2) connecting to feedback and ground. The internal feedback reference voltage is 0.25V. The LED current can be set from following equation easily. LED I 0.25V R2 = --------------------------------------(1) In order to have an accurate LED current, precision resistors are preferred (1% is recommended). The table for R2 selection is shown below. R2 Resistor Value selection ILED (mA) R2 (Ω) 5 49.9 10 24.9 12 21 15 16.5 20 12.4 Inductor Selection The recommended value of inductor for 2 to 6 WLEDs applications are 4.7 to 22µH. Small size and better efficiency are the major concerns for portable device, such as EUP2571 used for mobile phone. The inductor should have low core loss at 1.1MHz and low DCR for better efficiency. To avoid inductor saturation current rating should be considered. Dimming Control a. Using a PWM Signal to EN Pin For controlling the LED brightness, the EUP2571 can perform the dimming control by applying a PWM signal to EN pin. The average LED current is proportional to the PWM signal duty cycle. The magnitude of the PWM signal should be higher than the maximum enable voltage of EN pin, in order to let the dimming control perform correctly. Figure 4. PWM Dimming Control Using the EN Pin b. Using a DC Voltage Using a variable DC voltage to adjust the brightness is a popular method in some applications. The dimming control using a DC voltage circuit is shown in Figure 5. According to the Superposition Theorem, as the DC voltage increases, the voltage contributed to VFB increases and the voltage drop on R2 decreases, i.e. the LED current decreases. For example, if the VDC range is from 0V to 2.8V, the selection of resistors in Figure 5 sets dimming control of LED current from 20mA to 0mA. Figure 5. Dimming Control Using a DC Voltage Figure 6. Recommended Soft-Start Circuit |
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