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LM2594M-ADJ Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM2594M-ADJ Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 28 page LM2594/LM2594HV Series Buck Regulator Design Procedure (Fixed Output) PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version) Given: V OUT = Regulated Output Voltage (3.3V, 5V or 12V) V IN(max) = Maximum DC Input Voltage I LOAD(max) = Maximum Load Current Given: V OUT = 5V V IN(max) = 12V I LOAD(max) = 0.4A 1. Inductor Selection (L1) A. Select the correct inductor value selection guide from Fig- ures 4, 5 or Figure 6. (Output voltages of 3.3V, 5V, or 12V re- spectively.) For all other voltages, see the design procedure for the adjustable version. B. From the inductor value selection guide, identify the induc- tance region intersected by the Maximum Input Voltage line and the Maximum Load Current line. Each region is identified by an inductance value and an inductor code (LXX). C. Select an appropriate inductor from the four manufactur- er’s part numbers listed in Figure 8. 1. Inductor Selection (L1) A. Use the inductor selection guide for the 5V version shown in Figure 5. B. From the inductor value selection guide shown in Figure 5, the inductance region intersected by the 12V horizontal line and the 0.4A vertical line is 100 µH, and the inductor code is L20. C. The inductance value required is 100 µH. From the table in Figure 8, go to the L20 line and choose an inductor part number from any of the four manufacturers shown. (In most instance, both through hole and surface mount inductors are available.) 2. Output Capacitor Selection (C OUT) A. In the majority of applications, low ESR (Equivalent Series Resistance) electrolytic capacitors between 82 µF and 220 µF and low ESR solid tantalum capacitors between 15 µF and 100 µF provide the best results. This capacitor should be located close to the IC using short capacitor leads and short copper traces. Do not use capacitors larger than 220 µF. For additional information, see section on output capaci- tors in application information section. B. To simplify the capacitor selection procedure, refer to the quick design component selection table shown in Figure 2. This table contains different input voltages, output voltages, and load currents, and lists various inductors and output ca- pacitors that will provide the best design solutions. C. The capacitor voltage rating for electrolytic capacitors should be at least 1.5 times greater than the output voltage, and often much higher voltage ratings are needed to satisfy the low ESR requirements for low output ripple voltage. D. For computer aided design software, see Switchers Made Simple version 4.1 or later. 2. Output Capacitor Selection (C OUT) A. See section on output capacitors in application infor- mation section. B. From the quick design component selection table shown in Figure 2, locate the 5V output voltage section. In the load current column, choose the load current line that is closest to the current needed in your application, for this example, use the 0.5A line. In the maximum input voltage column, select the line that covers the input voltage needed in your applica- tion, in this example, use the 15V line. Continuing on this line are recommended inductors and capacitors that will provide the best overall performance. The capacitor list contains both through hole electrolytic and surface mount tantalum capacitors from four different capaci- tor manufacturers. It is recommended that both the manufac- turers and the manufacturer’s series that are listed in the table be used. In this example aluminum electrolytic capacitors from several different manufacturers are available with the range of ESR numbers needed. 120 µF 25V Panasonic HFQ Series 120 µF 25V Nichicon PL Series C. For a 5V output, a capacitor voltage rating at least 7.5V or more is needed. But, in this example, even a low ESR, switching grade, 120 µF 10V aluminum electrolytic capacitor would exhibit approximately 400 m Ω of ESR (see the curve in Figure 14 for the ESR vs voltage rating). This amount of ESR would result in relatively high output ripple voltage. To reduce the ripple to 1% of the output voltage, or less, a ca- pacitor with a higher voltage rating (lower ESR) should be se- lected. A 16V or 25V capacitor will reduce the ripple voltage by approximately half. www.national.com 9 |
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