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EUP2518 Datasheet(PDF) 6 Page - Eutech Microelectronics Inc |
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EUP2518 Datasheet(HTML) 6 Page - Eutech Microelectronics Inc |
6 / 8 page EUP2518 DS2518 Ver1.0 Mar. 2008 6 Application Information LED Current Control The EUP2518 regulates the LED current by setting the current sense resistor (RFB) connecting to feedback and ground. The internal feedback reference voltage is 104mV. The LED current can be set from following equation easily. LED I 104mV FB R = -------------------------------------(1) In order to have an accurate LED current, precision resistors are preferred (1% is recommended). The table for RFB selection is shown below. RFB Resistor Value selection ILED (mA) RFB (Ω) 5 21 10 10.4 15 6.93 20 5.2 CAP and Inductor Selection The recommended value of inductor for 2 to 6 WLEDs applications are 10µH to 47µH. Small size and better efficiency are the major concerns for portable device. A 22µH inductor with low DCR (Inductor resistance) is recommended to improve efficiency. To avoid inductor saturation current rating should be considered. A 1µF ceramic capacitor is recommended for the input capacitance in the real system, and a larger capacitor will get smaller input voltage ripple. A 0.22uF output ceramic capacitor is sufficient for most applications. Dimming Control a. Using a PWM Signal to EN Pin For controlling the LED brightness, the EUP2518 can perform the dimming control by applying a PWM signal to EN pin. The internal soft-start and wide range dimming frequency from 100Hz to 100KHz can insignificantly reduce audio noise when dimming. 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 3. 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 4. According to the Superposition Theorem, as the DC voltage increases, the voltage contributed to VFB increases and the voltage drop on RFB decreases, i.e. the LED current decreases. For example, if the VDC range is from 0V to 2V, the selection of resistors in Figure 4 sets dimming control of LED current from 20mA to 0mA. Figure 4. Dimming Control Using a DC Voltage c. Using a Filtered PWM signal Another common application is using a filtered PWM signal as an adjustable DC voltage for LED dimming control. A filtered PWM signal acts as the DC voltage to regulate the output current. In this circuit, the output ripple depends on the frequency of PWM signal. For smaller output voltage ripple (<100mV), the recommended frequency of 2V PWM signal should be above 2kHz. To fix the frequency of PWM signal and change the duty cycle of PWM signal can get different output current. According to the application circuit of Figure 5, output current is from 20.5mA to 5.5mA by adjusting the PWM duty cycle from 10% to 90%. Figure 5. Filtered PWM Signal for LED Dimming Control |
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