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RT9285BGQW Datasheet(PDF) 9 Page - Richtek Technology Corporation |
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RT9285BGQW Datasheet(HTML) 9 Page - Richtek Technology Corporation |
9 / 12 page RT9285A/B Preliminary 9 DS9285A/B-02 August 2007 www.richtek.com Application Information LED Current Control The RT9285A/B regulates the LED current by setting the current sense resistor (RSET) connecting to feedback and ground. The RT9284A/B feedback voltage (VFB) is 0.25V. The LED current (ILED) can be set by a resistor RSET. ILED = 0.25/RSET In order to have an accurate LED current, a precision resistor is preferred (1% is recommended). Inductor Selection The recommended value of inductor for 4 to 5WLEDs applications are 10 µH to 22µH. For 3WLEDs, the recommended value of inductor is 4.7 µH to 22µH. Small size and better efficiency are the major concerns for portable device, such as RT9285A/B used for mobile phone. The inductor should have low core loss at 1MHz and low DCR for better efficiency. LX GND VDD FB VOUT RT9285A/B EN C1 1uF R SET L1 C2 Chip Enable V IN 10uH to 22uH 2.7V to 5.5V 0.22uF to 1uF 12.5 LX GND VDD FB VOUT RT9285A/B EN C1 1uF R SET L1 C2 Chip Enable V IN 10uH to 22uH 2.7V to 5.5V 0.22uF to 1uF 12.5 Figure 3. Application for Driving 4 Series WLEDs Figure 4. Application for Driving 5 Series WLEDs The inductor saturation current rating should be considered to cover the inductor peak current. Capacitor Selection Input and output ceramic capacitors of 1 µF are recommend- ed for RT9285A/B applications. For better voltage filtering, ceramic capacitors with low ESR are recommended. X5R and X7R types are suitable because of their wider voltage and temperature ranges. Output Voltage Control The output voltage of R9285 can be adjusted by the divider circuit on FB pin. Figure 6 shows a 2-level voltage control circuit for OLED application. The output voltage can be calculated by the following equations in Figure 6. LX GND VDD FB VOUT RT9285A/B EN C1 1uF R2 L1 C2 Chip Enable V IN 10uH to 22uH 2.7V to 5.5V 0.22uF to 1uF 10k R1 590k V OUT 15V Figure 5. Application for Constant Output Voltage 10k R2 ; R2 R2 R1 0.25 VOUT > + × = Figure 6. Application Circuit for Output Voltage Control and Related Equations VDD EN VOUT GND FB RT9285A/B LX R GPIO V IN GPIO OLED R A R B V EN |
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