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SP6691 Datasheet(PDF) 10 Page - Sipex Corporation |
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SP6691 Datasheet(HTML) 10 Page - Sipex Corporation |
10 / 15 page 10 Date: 5/25/04 SP6691 Micro Power Boost Regualtor, Series White LED Driver © Copyright 2004 Sipex Corporation Layout Consideration Both the input capacitor and the output capacitor should be placed as close as possible to the IC. This can reduce the copper trace resistance which directly effects the input and output ripples. The feedback resistor network should be kept close to the FB pin to minimize copper trace connections that can inject noise into the system. The ground connection for the feedback resistor network should connect directly to the GND pin or to an analog ground plane that is tied directly to the GND pin. The inductor and the schottky diode should be placed as close as possible to the switch pin to minimize the noise coupling to the other circuits, especially the feedback network. Power Efficiency For the typical application circuit, the output efficiency of the circuit is expressed by η = VOUT • IOUT VIN • IIN Where VIN , IIN, VOUT, IOUT are the input and output voltage and current respectively. While the white LED efficiency is expressed by η = (VOUT - 1.22) • IOUT VIN • IIN This equation indicates that the white LED efficiency will be much smaller than the output efficiency of the circuit when VOUT is not very large, compared to the feedback voltage (1.22V). The other power is consumed by the bias resis- tor. To reduce this power loss, two circuits can be used, as shown in Figure 23 and Figure 24. In Figure 23, a general-purpose diode (for ex- ample, 1N4148) is used to bring the voltage across the bias resistor to be around 0.7V. R1 is used to create a loop that provides around 100 µA operating current for the diode. 3% efficiency improvement can be achieved by using this method. APPLICATION INFORMAMTION: Continued Figure 23. Improve Efficiency with Diode in Feedback Loop To further improve the efficiency and reduce the effects of the ambient temperature on the diode D1 used in method 1, an op amp circuit can be used as shown in Figure 24. The gain of the op amp circuit can be calculated by: Av = R1 + R2 R1 If the voltage across the bias resistor is set to be 0.1V the current through R1 and R2 to be around 100 µA, R1 and R2 can be selected as 1K and 11.2K respectively. LMV341 can be used be- cause of its small supply current, offset voltage and minimum supply voltage. By using this method, the efficiency can be increased around 7%. VIN WLED MODULE C1 4.7uF Murata LQH32CN100K11 DS MBR0530 D1 DIODE 34.8ohm Rb C2 2.2uF R1 150Kohm L1 10uH 0.45A 1.22V 2.7-4.2V 0.7V U1 SP6691 1 2 3 4 5 SW GND FB SHDN V IN LMV341 1 3 4 + - OUT 2 6 0.1V C2 2.2uF 2.7-4.2V L1 10uH 0.45A 1.22V Vbattery U1 SP6691 1 2 3 4 5 S W GND FB SHDN V IN 5 DS MBR0530 R1 1K WLED MODULE C1 4.7uF R2 11.2K Vbattery Murata LQH32CN100K11 Rb 5.1Ω Figure 24. Improve Efficiency with Op Amp in Feedback Loop |
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