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LM2595 Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM2595 Datasheet(HTML) 10 Page - National Semiconductor (TI) |
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10 / 29 page  LM2595 Series Buck Regulator Design Procedure (Fixed Output) (Continued) PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version) 3. Catch Diode Selection (D1) A. The catch diode current rating must be at least 1.3 times greater than the maximum load current. Also, if the power supply design must withstand a continuous output short, the diode should have a current rating equal to the maximum current limit of the LM2595. The most stressful condition for this diode is an overload or shorted output condition. B. The reverse voltage rating of the diode should be at least 1.25 times the maximum input voltage. C. This diode must be fast (short reverse recovery time) and must be located close to the LM2595 using short leads and short printed circuit traces. Because of their fast switching speed and low forward voltage drop, Schottky diodes provide the best performance and efficiency, and should be the first choice, especially in low output voltage applications. Ultra-fast recovery, or High-Efficiency rectifiers also provide good results. Ultra-fast recovery diodes typically have re- verse recovery times of 50 ns or less. Rectifiers such as the 1N5400 series are much too slow and should not be used. 3. Catch Diode Selection (D1) A. Refer to the table shown in Figure 11 In this example, a 3A, 20V, 1N5820 Schottky diode will provide the best perfor- mance, and will not be overstressed even for a shorted out- put. 4. Input Capacitor (C IN) A low ESR aluminum or tantalum bypass capacitor is needed between the input pin and ground pin to prevent large volt- age transients from appearing at the input. This capacitor should be located close to the IC using short leads. In addi- tion, the RMS current rating of the input capacitor should be selected to be at least 1⁄2 the DC load current. The capacitor manufacturers data sheet must be checked to assure that this current rating is not exceeded. The curve shown in Fig- ure 13 shows typical RMS current ratings for several different aluminum electrolytic capacitor values. For an aluminum electrolytic, the capacitor voltage rating should be approximately 1.5 times the maximum input volt- age. Caution must be exercised if solid tantalum capacitors are used (see Application Information on input capacitor). The tantalum capacitor voltage rating should be 2 times the maximum input voltage and it is recommended that they be surge current tested by the manufacturer. Use caution when using ceramic capacitors for input bypass- ing, because it may cause severe ringing at the V IN pin. For additional information, see section on input capaci- tors in Application Information section. 4. Input Capacitor (C IN) The important parameters for the Input capacitor are the in- put voltage rating and the RMS current rating. With a nominal input voltage of 12V, an aluminum electrolytic capacitor with a voltage rating greater than 18V (1.5 x V IN) would be needed. The next higher capacitor voltage rating is 25V. 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 1A load, a capacitor with a RMS current rating of at least 500 mA is needed. The curves shown in Fig- ure 13 can be used to select an appropriate input capacitor. From the curves, locate the 25V line and note which capaci- tor values have RMS current ratings greater than 500 mA. Ei- ther a 180 µF or 220 µF, 25V capacitor could be used. For a through hole design, a 220 µF/25V electrolytic capaci- tor (Panasonic HFQ series or Nichicon PL series or equiva- lent) would be adequate. other types or other manufacturers capacitors can be used provided the RMS ripple current rat- ings are adequate. For surface mount designs, solid tantalum capacitors can be used, but caution must be exercised with regard to the ca- pacitor surge current rating (see Application Information on input capacitors in this data sheet). The TPS series available from AVX, and the 593D series from Sprague are both surge current tested. www.national.com 10 |
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