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APW7079-30DI-TRL Datasheet(PDF) 9 Page - Anpec Electronics Coropration |
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APW7079-30DI-TRL Datasheet(HTML) 9 Page - Anpec Electronics Coropration |
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9 / 15 page  Copyright © ANPEC Electronics Corp. Rev. A.3 - Oct., 2008 APW7079 www.anpec.com.tw 9 Application Information Input Capacitor Selection The input capacitor is chosen based on the voltage rating and the RMS current rating. For reliable operation, it is recommended to select the capacitor voltage rating at least 1.3 times higher than the maximum input voltage. The maximum RMS current rating of the input capacitor is calculated as the following equation: Output Capacitor Selection L T V 3 1 I ON IN RMS ⋅ ⋅ = where T ON = main switch max. on-time (4µ s typical) V IN = input voltage L = inductor value in µH The capacitors should be placed close to the inductor and the GND. An output capacitor is required to filter the output and sup- ply the load transient current. The output ripple is the sum of the voltages across the ESR and the ideal output capacitor. The peak-to-peak voltage of the ESR is calcu- lated as the following equations: LIM ON IN IN OUT OUT PEAK I L 2 T V V I V I ≤ ⋅ ⋅ + η ⋅ ⋅ = ESR x I V PEAK ESR = ∆ Where I PEAK = peak current of inductor in amp η = efficiency (0.85 typical) The peak-to-peak voltage of the ideal output capacitor is calculated as the following equation: For the applications using tantalum capacitors, the ∆V COUT is much smaller than the V ESR and c an b e i g n o re d . Therefore, the AC peak-to-peak output voltage ( ∆V OUT) is shown as below: OUT ON OUT COUT C T I V × = ∆ ESR x I V PEAK OUT = ∆ Since the output ripple is the product of the peak inductor current and the output capacitor ESR, using low-ESR tan- talum capacitors for the best performance or connecting two or more filter capacitors in parallel is recommended. Inductor Selection η ⋅ × − − = L 2 V V T I V V I IN OUT OFF LIM OUT IN ) MAX ( OUT where T OFF = main switch min. off-time (0.9µs typical) Therefore, to consider the balance of the efficiency and component size, an inductor value of 22 µH to 47µH is recommended in most applications. V IN V OUT I LX N-FET P-FET LX I OUT ESR I SWN I SWP C IN C OUT I IN The inductor value determines the inductor ripple current and affects the load transient response. It is recom- mended to select the boost inductor in order to keep the maximum peak inductor current below the current limit threshold of the power switch. For example, the current limit threshold of the APW7079’s switch is 600mA. For choosing an inductor which has peak current passed, firstly, it is necessary to consider the output load (I OUT), input (V IN), and output voltage (VOUT). Secondly, the de- sired current ripple in the inductor also needed to be taken into account. The current was calculated in “Output Capacitor Selection”. Since the output ripple is the prod- uct of the peak inductor current and the output capacitor ESR, the larger inductor value reduces the inductor cur- rent ripple and output voltage ripple but typically offers a larger physical size. The inductor value also slightly affects the maximum out- put current. The maximum output current can be calcu- lated as below: |
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