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LM2576T-5.0 Folha de dados(PDF) 14 Page - National Semiconductor (TI) |
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LM2576T-5.0 Folha de dados(HTML) 14 Page - National Semiconductor (TI) |
14 / 24 page LM2576 Series Buck Regulator Design Procedure (Continued) PROCEDURE (Adjustable Output Voltage Versions) EXAMPLE (Adjustable Output Voltage Versions) 2. Inductor Selection (L1) A. Calculate the inductor Volt • microsecond constant, E • T(V • µs), from the following formula: 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, and note the inductor code for that region. E. Identify the inductor value from the inductor code, and select an appropriate inductor from the table shown in Figure 9. Part numbers are listed for three inductor manufacturers. The inductor chosen must be rated for operation at the LM2576 switching fre- quency (52 kHz) and for a current rating of 1.15 x I LOAD. For additional inductor information, see the inductor sec- tion in the application hints section of this data sheet. 2. Inductor Selection (L1) A. Calculate E • T(V • µs) B. E • T = 115 V • µs C. I LOAD(Max) = 3A D. Inductance Region = H150 E. Inductor Value = 150 µH Choose from AIE part #415-0936 Pulse Engineering part #PE-531115, or Renco part #RL2445. 3. Output Capacitor Selection (C OUT)A. The value of the output capacitor together with the inductor defines the dominate pole-pair of the switching regulator loop. For stable operation, the capacitor must satisfy the following requirement: The above formula yields capacitor values between 10 µF and 2200 µF that will satisfy the loop requirements for stable operation. But to achieve an acceptable output ripple voltage, (approximately 1% of the output voltage) and transient response, the output capacitor may need to be several times larger than the above formula yields. B. The capacitor’s voltage rating should be at last 1.5 times greater than the output voltage. For a 10V regulator, a rating of at least 15V or more is recommended. Higher voltage electrolytic capacitors generally have lower ESR numbers, and for this reason it may be necessary to select a capacitor rate for a higher voltage than would normally be needed. 3. Output Capacitor Selection (C OUT) However, for acceptable output ripple voltage select C OUT ≥ 680 µF C OUT = 680 µF electrolytic capacitor 4. Catch Diode Selection (D1) A. The catch-diode current rating must be at least 1.2 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 LM2576. The most stressful condition for this diode is an overload or shorted output. See diode selection guide in Figure 8. B. The reverse voltage rating of the diode should be at least 1.25 times the maximum input voltage. 4. Catch Diode Selection (D1) A. For this example, a 3.3A current rating is adequate. B. Use a 30V 31DQ03 Schottky diode, or any of the suggested fast-recovery diodes in Figure 8. 5. Input Capacitor (C IN) An aluminum or tantalum electrolytic bypass capacitor located close to the regulator is needed for stable operation. 5. Input Capacitor (C IN) A 100 µF aluminum electrolytic capacitor located near the input and ground pins provides sufficient bypassing. 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 of switching regulators. Switchers Made Simple (Version 3.3) is available on a (31⁄2") diskette for IBM compatible comput- ers from a National Semiconductor sales office in your area. www.national.com 14 |
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