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LM2595 Datasheet(PDF) 13 Page - National Semiconductor (TI) |
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LM2595 Datasheet(HTML) 13 Page - National Semiconductor (TI) |
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13 / 29 page  LM2595 Series Buck Regulator Design Procedure (Adjustable Output) (Continued) PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version) Procedure continued on next page. Example continued on next page. 5. 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 are also a good choice, but some types with an abrupt turn-off charac- teristic may cause instability or EMl problems. Ultra-fast re- covery diodes typically have reverse recovery times of 50 ns or less. Rectifiers such as the 1N4001 series are much too slow and should not be used. 5. Catch Diode Selection (D1) A. Refer to the table shown in Figure 11 Schottky diodes pro- vide the best performance, and in this example a 3A, 40V, 1N5822 Schottky diode would be a good choice. The 3A di- ode rating is more than adequate and will not be over- stressed even for a shorted output. 6. Input Capacitor (C IN) 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. 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 manufacturers data sheet must be checked to assure that this current rating is not exceeded. The curve shown in Figure 13 shows typical RMS current ratings for several different aluminum electro- lytic capacitor values. This capacitor should be located close to the IC using short leads and the voltage rating should be approximately 1.5 times the maximum input voltage. If solid tantalum input capacitors are used, it is recomended that they be surge current tested by the manufacturer. se caution when using a high dielectric constant ceramic ca- pacitor for input bypassing, because it may cause severe ringing at the V IN pin. For additional information, see section on input capaci- tors in application information section. 6. 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 28V, an aluminum electrolytic aluminum elec- trolytic capacitor with a voltage rating greater than 42V (1.5 x V IN) would be needed. Since the the next higher capacitor voltage rating is 50V, a 50V capacitor should be used. The capacitor voltage rating of (1.5 x V IN) is a conservative guide- line, and can be modified somewhat if desired. The RMS current rating requirement for the input capacitor of 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 Figure 13 can be used to select an ap- propriate input capacitor. From the curves, locate the 50V line and note which capacitor values have RMS current rat- ings greater than 500 mA. Either a 100 µF or 120 µF, 50V ca- pacitor could be used. For a through hole design, a 120 µF/50V 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 rting (see Application Information or in- put capacitors in this data sheet). The TPS series available from AVX, and the 593D series from Sprague are both surge current tested. To further simplify the buck regulator design procedure, Na- tional Semiconductor is making available computer design software to be used with the Simple Switcher line ot switch- ing regulators. Switchers Made Simple (version 4.2 or later) is available on a 31⁄2" diskette for IBM compatible computers. www.national.com 13 |
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