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LM2595S-3.3 Arkusz danych(PDF) 11 Page - Texas Instruments |
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LM2595S-3.3 Arkusz danych(HTML) 11 Page - Texas Instruments |
11 / 41 page LM2595 www.ti.com SNVS122B – MAY 1999 – REVISED APRIL 2013 If open core inductors are used, special care must be taken as to the location and positioning of this type of inductor. Allowing the inductor flux to intersect sensitive feedback, lC groundpath and COUT wiring can cause problems. When using the adjustable version, special care must be taken as to the location of the feedback resistors and the associated wiring. Physically locate both resistors near the IC, and route the wiring away from the inductor, especially an open core type of inductor. (See Application Information section for more information.) Table 1. LM2595 Series Buck Regulator Design Procedure (Fixed Output) PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version) Given: Given: VOUT = Regulated Output Voltage (3.3V, 5V or 12V) VOUT = 5V VIN(max) = Maximum DC Input Voltage VIN(max) = 12V ILOAD(max) = Maximum Load Current ILOAD(max) = 1A 1. Inductor Selection (L1) 1. Inductor Selection (L1) A. Select the correct inductor value selection guide from Figure 22 , A. Use the inductor selection guide for the 5V version shown in Figure 23, or Figure 24. (Output voltages of 3.3V, 5V, or 12V Figure 23. respectively.) For all other voltages, see the Design Procedure for B. From the inductor value selection guide shown in Figure 23, the the adjustable version. inductance region intersected by the 12V horizontal line and the 1A B. From the inductor value selection guide, identify the inductance vertical line is 68 μH, and the inductor code is L30. region intersected by the Maximum Input Voltage line and the C. The inductance value required is 68 μH. From the table in Maximum Load Current line. Each region is identified by an Table 5, go to the L30 line and choose an inductor part number from inductance value and an inductor code (LXX). any of the four manufacturers shown. (In most instance, both C. Select an appropriate inductor from the four manufacturer's part through hole and surface mount inductors are available.) numbers listed in Table 5. 2. Output Capacitor Selection (COUT) 2. Output Capacitor Selection (COUT) A. In the majority of applications, low ESR (Equivalent Series A. See section on output capacitors in Application Information Resistance) electrolytic capacitors between 47 μF and 330 μF and section. low ESR solid tantalum capacitors between 56 μF and 270 μF B. From the quick design component selection table shown in provide the best results. This capacitor should be located close to Table 2, locate the 5V output voltage section. In the load current the IC using short capacitor leads and short copper traces. Do not column, choose the load current line that is closest to the current use capacitors larger than 330 μF. needed in your application, for this example, use the 1A line. In the For additional information, see section on output capacitors in maximum input voltage column, select the line that covers the input Application Information section. voltage needed in your application, in this example, use the 15V line. Continuing on this line are recommended inductors and capacitors B. To simplify the capacitor selection procedure, refer to the quick that will provide the best overall performance. design component selection table shown in Table 2. This table contains different input voltages, output voltages, and load currents, The capacitor list contains both through hole electrolytic and surface and lists various inductors and output capacitors that will provide the mount tantalum capacitors from four different capacitor best design solutions. manufacturers. It is recommended that both the manufacturers and the manufacturer's series that are listed in the table be used. C. The capacitor voltage rating for electrolytic capacitors should be at least 1.5 times greater than the output voltage, and often much In this example aluminum electrolytic capacitors from several higher voltage ratings are needed to satisfy the low ESR different manufacturers are available with the range of ESR numbers requirements for low output ripple voltage. needed. D. For computer aided design software, see Switchers Made Simple 220 μF 25V Panasonic HFQ Series version 4.2 or later. 220 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, 220 μF 10V aluminum electrolytic capacitor would exhibit approximately 225 m Ω of ESR (see the curve in Figure 27 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 capacitor with a higher voltage rating (lower ESR) should be selected. A 16V or 25V capacitor will reduce the ripple voltage by approximately half. Copyright © 1999–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM2595 |
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