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RT9218GS Datasheet(PDF) 10 Page - Richtek Technology Corporation |
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RT9218GS Datasheet(HTML) 10 Page - Richtek Technology Corporation |
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10 / 18 page  RT9218 10 DS9218-08 March 2007 www.richtek.com Application Information Inductor Selection The selection of output inductor is based on the considerations of efficiency, output power and operating frequency. Low inductance value has smaller size, but results in low efficiency, large ripple current and high output ripple voltage. Generally, an inductor that limits the ripple current ( ΔIL) between 20% and 50% of output current is appropriate. Figure 1 shows the typical topology of synchronous step-down converter and its related waveforms. V L V IN - VOUT - V OUT i L I L = IOUT Δ I L i S1 i S2 T S T ON TOFF V g1 V g2 Figure 1. The waveforms of synchronous step-down converter OUT L IN OUT IN OUT IN OUT IN L V ΔI D VV L ; Δt; D Δtfs V V L(V V ) Vfs ΔI −= = = =− × ×× According to Figure 1 the ripple current of inductor can be calculated as follows : (1) Where : VIN = Maximum input voltage VOUT = Output Voltage Δt = S1 turn on time ΔIL = Inductor current ripple fS = Switching frequency D = Duty Cycle rC = Equivalent series resistor of output capacitor Output Capacitor The selection of output capacitor depends on the output ripple voltage requirement. Practically, the output ripple voltage is a function of both capacitance value and the equivalent series resistance (ESR) rC. Figure 2 shows the related waveforms of output capacitor. L dt = dt L V OUT = V OR i L i C di L Δ I L 1/2 0 0 Δ I L x rc V OC t1 t2 Δ V OC Δ I L V IN-VOUT T S I OUT di L Figure 2. The related waveforms of output capacitor S1 S2 V IN i S1 i S2 I OUT V OUT + - R L r C C OUT i C V OR + - V OC + - V L + - L I L |
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