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RT9284B15PJ5 Datasheet(PDF) 11 Page - Richtek Technology Corporation |
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RT9284B15PJ5 Datasheet(HTML) 11 Page - Richtek Technology Corporation |
11 / 14 page 11 RT9284A/B DS9284A/B-05 August 2007 www.richtek.com Figure 7. Filtered PWM Signal for LED Dimming Control of RT9284A 0 2 4 6 8 10 12 14 16 18 20 0 1020 3040 50 6070 8090 100 PWM Duty (%) Figure 8 Inductor Selection The recommended value of inductor for 3 to 5 WLEDs applications are 4.7 to 22 μH. Small size and better efficiency are the major concerns for portable device, such as RT9284A/B used for mobile phone. The inductor should have low core loss at 1.2MHz and low DCR for better efficiency. To avoid inductor saturation current rating should be considered. ) I (I (RMS) I PEAK OUT D × ≈ Capacitor Selection Input and output ceramic capacitors of 1 μF are recommended for RT9284A/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. Diode Selection Schottky diode is a good choice for RT9284A/B because of its low forward voltage drop and fast reverse recovery. Using Schottky diode can get better efficiency. The high- speed rectification is also a good characteristic of Schottky diode for high switching frequency. Current rating of the diode must meet the root mean square of the peak current and output average current multiplication as following : The diode ’ s reverse breakdown voltage should be larger than the output voltage. SS0520 is recommended Schottky diode for rectifier. Layout Guide A full GND plane without gap break. VCC to GND noise bypass –Short and wide connection for the 1 μF MLCC capacitor between Pin6 and Pin2. Minimized LX node copper area to reduce EMI. Minimized FB node copper area and keep far away from noise sources. Figure 9. Top Layer The LED current can be calculated by the following equation. PWM FB FB DC LED R2 (V Duty V ) V R3 + R I = R1 ×× − − VDD GND LX FB OVP RT9284A VIN EN 6 4 2 3 5 1 ILED 2.7V to 5.5V C1 C2 R1 D1 SS0520 L1 4.7uH to 22uH R2 PWM Signal R3 110 10k 10k 2.8V 0V RDC 3k CDC 1uF VEN > 1.5V VEN < 0.4V |
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