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APW7136C Datasheet(PDF) 10 Page - Anpec Electronics Coropration |
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APW7136C Datasheet(HTML) 10 Page - Anpec Electronics Coropration |
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10 / 15 page  Copyright © ANPEC Electronics Corp. Rev. A.2 - Jan., 2008 APW7136A/B/C www.anpec.com.tw 10 Application Information Input Capacitor Selection The input capacitor (C IN) reduces the ripple of the input current drawn from the input supply and reduces noise injection into the IC. The reflected ripple voltage will be smaller when an input capacitor with larger capacitance is used. For reliable operation, it is recommended to select the capacitor with maximum voltage rating at least 1.2 times of the maximum input voltage. The capacitors should be placed close to the VIN and GND. Inductor Selection Selecting an inductor with low dc resistance reduces con- duction losses and achieves high efficiency. The efficiency is moderated whilst using small chip inductor which op- erates with higher inductor core losses. Therefore, it is necessary to take further consideration while choosing an adequate inductor. Mainly, the inductor value deter- mines the inductor ripple current: larger inductor value results in smaller inductor ripple current and lower con- duction losses of the converter. However, larger inductor value generates slower load transient response. A rea- sonable design rule is to set the ripple current, ∆I L, to be 30% to 50% of the maximum average inductor current, I L(AVG). The inductor value can be obtained as below, where V IN = input voltage V OUT = output voltage F SW = switching frequency in MHz I OUT = maximum output current in amp. η = Efficiency ∆I L /IL(AVG) = inductor ripple current/average current (0.3 to 0.5 typical) To avoid saturation of the inductor, the inductor should be rated at least for the maximum input current of the con- verter plus the inductor ripple current. The maximum in- put current is calculated as below: η ⋅ ⋅ = IN OUT ) MAX ( OUT ) MAX ( IN V V I I The peak inductor current is calculated as the following equation: ( ) SW OUT IN OUT IN ) MAX ( IN PEAK F L V V V V 2 1 I I ⋅ ⋅ − ⋅ ⋅ + = Output Capacitor Selection The current-mode control scheme of the APW7136 al- lows the usage of tiny ceramic capacitors. The higher capacitor value provides good load transients response. Ceramic capacitors with low ESR values have the lowest output voltage ripple and are recommended. If required, tantalum capacitors may be used as well. The output ripple is the sum of the voltages across the ESR and the ideal output capacitor. where I PEAK is the peak inductor current. Δ V OUT = Δ VESR + Δ VCOUT ⋅ − ⋅ ≈ ∆ SW OUT IN OUT OUT OUT COUT F V V V C I V ESR PEAK ESR R I V ⋅ ≈ ∆ V IN V OUT I L N-FET LX I OUT I SW C IN C OUT I IN D1 ESR I LIM I L I PEAK I IN I OUT I SW I D ∆I L ( ) ∆ η × ⋅ − × ≥ AVG L L ) MAX ( OUT SW IN OUT 2 OUT IN I I I F V V V V L |
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