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APW717810QBI-TRG Datasheet(PDF) 10 Page - Anpec Electronics Coropration |
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APW717810QBI-TRG Datasheet(HTML) 10 Page - Anpec Electronics Coropration |
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10 / 16 page  Copyright © ANPEC Electronics Corp. Rev. A.2 - Jan., 2011 APW7178 www.anpec.com.tw 10 Application Information Input Capacitor Selection Because buck converters have a pulsating input current, a low ESR input capacitor is required. This results in the best input voltage filtering, minimizing the interference with other circuits caused by high input voltage spikes. Also, the input capacitor must be sufficiently large to sta- bilize the input voltage during heavy load transients. For good input voltage filtering, usually a 4.7 µF input capaci- tor is sufficient. It can be increased without any limit for better input-voltage filtering. Ceramic capacitors show better performance because of the low ESR value, and they are less sensitive against voltage transients and spikes compared to tantalum capacitors. Place the input capacitor as close as possible to the input and GND pin of the device for better performance. Inductor Selection For high efficiencies, the inductor should have a low DC resistance to minimize conduction losses. Especially at high-switching frequencies the core material has a higher impact on efficiency. When using small chip inductors, the efficiency is reduced mainly due to higher inductor core losses. This needs to be considered when select- ing the appropriate inductor. The inductor value deter- mines the inductor ripple current. The larger the inductor value, the smaller the inductor ripple current and the lower the conduction losses of the converter. Conversely, larger inductor values cause a slower load transient response. A reasonable starting point for setting ripple current, ∆I L, is 40% of maximum output current. The recommended inductor value can be calculated as below: L SW IN OUT OUT I F V V 1 V L ∆ ⋅ − ≥ I L(MAX) = IOUT(MAX) + 1/2 x ∆IL To avoid the saturation of the inductor, the inductor should be rated at least for the maximum output current of the converter plus the inductor ripple current. Output Voltage Setting In the adjustable version, the output voltage is set by a resistive divider. The external resistive divider is con- nected to the output, allowing remote voltage sensing as + ⋅ = + ⋅ = 2 R 1 R 1 6 . 0 2 R 1 R 1 V V REF OUT shown in “Typical Application Circuits”. A suggestion of maximum value of R2 is 200k Ω to keep the minimum current that provides enough noise rejection ability through the resistor divider. The output voltage can be calculated as below: Output Capacitor Selection The current-mode control scheme of the APW7178 al- lows the use of tiny ceramic capacitors. The higher ca- pacitor value provides the 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. ⋅ ⋅ + ⋅ ⋅ − ⋅ ≅ ∆ OUT SW SW IN OUT OUT OUT C F 8 1 ESR L F V V 1 V V When choosing the input and output ceramic capacitors, choose the X5R or X7R dielectric formulations. These dielectrics have the best temperature and voltage char- acteristics of all the ceramics for a given value and size. V IN V OUT I L N-FET SW I OUT C IN C OUT I IN ESR P-FET I P-FET R2 ≤ 200kΩ APW7178 FB GND V OUT R1 ≤1MΩ |
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