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LM2595 Datasheet(PDF) 12 Page - National Semiconductor (TI) |
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LM2595 Datasheet(HTML) 12 Page - National Semiconductor (TI) |
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12 / 29 page  LM2595 Series Buck Regulator Design Procedure (Adjustable Output) (Continued) PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version) 2. Inductor Selection (L1) A. Calculate the inductor Volt • microsecond constant E • T (V • µs), from the following formula: where V SAT = internal switch saturation voltage = 1V and V D = diode forward voltage drop = 0.5V B. Use the E • T value from the previous formula and match it with the E • T number on the vertical axis of the Inductor Value Selection Guide shown in Figure 7. C. on the horizontal axis, select the maximum load current. D. Identify the inductance region intersected by the E • T value and the Maximum Load Current value. Each region is identified by an inductance value and an inductor code (LXX). E. Select an appropriate inductor from the four manufactur- er’s part numbers listed in Figure 8. 2. Inductor Selection (L1) A. Calculate the inductor Volt • microsecond constant (E • T), B. E • T = 34.8 (V • µs) C. I LOAD(max) = 1A D. From the inductor value selection guide shown in Figure 7, the inductance region intersected by the 35 (V • µs) horizon- tal line and the 1A vertical line is 100 µH, and the inductor code is L29. E. From the table in Figure 8, locate line L29, and select an inductor part number from the list of manufacturers part num- bers. 3. Output Capacitor Selection (C OUT) A. In the majority of applications, low ESR electrolytic or solid tantalum capacitors between 47 µF and 330 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 330 µF. For additional informa- tion, see section on output capacitors in application in- formation section. B. To simplify the capacitor selection procedure, refer to the quick design table shown in Figure 3. This table contains dif- ferent output voltages, and lists various output capacitors that will provide the best design solutions. C. The capacitor voltage rating should be at least 1.5 times greater than the output voltage, and often much higher volt- age ratings are needed to satisfy the low ESR requirements needed for low output ripple voltage. 3. Output Capacitor SeIection (C OUT) A. See section on C OUT in Application Information section. B. From the quick design table shown in Figure 3, locate the output voltage column. From that column, locate the output voltage closest to the output voltage in your application. In this example, select the 24V line. Under the output capacitor section, select a capacitor from the list of through hole elec- trolytic or surface mount tantalum types from four different capacitor manufacturers. It is recommended that both the manufacturers and the manufacturers series that are listed in the table be used. In this example, through hole aluminum electrolytic capaci- tors from several different manufacturers are available. 82 µF, 35V Panasonic HFQ Series 82 µF, 35V Nichicon PL Series C. For a 20V output, a capacitor rating of at least 30V or more is needed. In this example, either a 35V or 50V capaci- tor would work. A 35V rating was chosen, although a 50V rat- ing could also be used if a lower output ripple voltage is needed. Other manufacturers or other types of capacitors may also be used, provided the capacitor specifications (especially the 100 kHz ESR) closely match the types listed in the table. Re- fer to the capacitor manufacturers data sheet for this informa- tion. 4. Feedforward Capacitor (C FF) (See Figure 1) For output voltages greater than approximately 10V, an ad- ditional capacitor is required. The compensation capacitor is typically between 50 pF and 10 nF, and is wired in parallel with the output voltage setting resistor, R 2. It provides addi- tional stability for high output voltages, low input-output volt- ages, and/or very low ESR output capacitors, such as solid tantalum capacitors. This capacitor type can be ceramic, plastic, silver mica, etc. (Because of the unstable characteristics of ceramic capaci- tors made with Z5U material, they are not recommended.) 4. Feedforward Capacitor (C FF) The table shown in Figure 3 contains feed forward capacitor values for various output voltages. In this example,a1nF capacitor is needed. www.national.com 12 |
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