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LM2674 Datasheet(PDF) 12 Page - National Semiconductor (TI) |
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LM2674 Datasheet(HTML) 12 Page - National Semiconductor (TI) |
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12 / 26 page  LM2674 Series Buck Regulator Design Procedure (Fixed Output) (Continued) PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version) A low ESR aluminum or tantalum bypass capacitor is needed between the input pin and ground to prevent large voltage transients from appearing at the input. This capacitor should be located close to the IC using short leads. In addition, the RMS current rating of the input capacitor should be selected to be at least 1⁄2 the DC load current. The capacitor manufacturer data sheet must be checked to assure that this current rating is not exceeded. The curves shown in Figure 14 show typical RMS current ratings for several different aluminum electrolytic capacitor values. A parallel connection of two or more capacitors may be required to increase the total minimum RMS current rating to suit the application requirements. For an aluminum electrolytic capacitor, the voltage rating should be at least 1.25 times the maximum input voltage. Caution must be exercised if solid tantalum capacitors are used. The tantalum capacitor voltage rating should be twice the maximum input voltage. The tables in Figure 15 show the recommended application voltage for AVX TPS and Sprague 594D tantalum capacitors. It is also recommended that they be surge current tested by the manufacturer. The TPS series available from AVX, and the 593D and 594D series from Sprague are all surge current tested. Another approach to minimize the surge current stresses on the input capacitor is to add a small inductor in series with the input supply line. Use caution when using ceramic capacitors for input bypassing, because it may cause severe ringing at the V IN pin. The important parameters for the input capacitor are the input voltage rating and the RMS current rating. With a maximum input voltage of 12V, an aluminum electrolytic capacitor with a voltage rating greater than 15V (1.25 x V IN) would be needed. The next higher capacitor voltage rating is 16V. The RMS current rating requirement for the input capacitor in a buck regulator is approximately 1⁄2 the DC load current. In this example, with a 500mA load, a capacitor with an RMS current rating of at least 250 mA is needed. The curves shown in Figure 14 can be used to select an appropriate input capacitor. From the curves, locate the 16V line and note which capacitor values have RMS current ratings greater than 250 mA. For a through hole design, a 100 µF/16V electrolytic capacitor (Panasonic HFQ series, Nichicon PL, Sanyo MV-GX series or equivalent) would be adequate. Other types or other manufacturers’ capacitors can be used provided the RMS ripple current ratings are adequate. Additionally, for a complete surface mount design, electrolytic capacitors such as the Sanyo CV-C or CV-BS and the Nichicon WF or UR and the NIC Components NACZ series could be considered. For surface mount designs, solid tantalum capacitors can be used, but caution must be exercised with regard to the capacitor surge current rating and voltage rating. In this example, checking Figure 15, and the Sprague 594D series datasheet, a Sprague 594D 15 µF, 25V capacitor is adequate. 5. Boost Capacitor (C B) 5. Boost Capacitor (C B) This capacitor develops the necessary voltage to turn the switch gate on fully. All applications should use a 0.01 µF, 50V ceramic capacitor. For this application, and all applications, use a 0.01 µF, 50V ceramic capacitor. www.national.com 12 |
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