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LM2575T-5.0 Datasheet(PDF) 13 Page - National Semiconductor (TI) |
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LM2575T-5.0 Datasheet(HTML) 13 Page - National Semiconductor (TI) |
13 / 27 page LM2575 Series Buck Regulator Design Procedure PROCEDURE (Fixed Output Voltage Versions) EXAMPLE (Fixed Output Voltage Versions) Given: V OUT = Regulated Output Voltage (3.3V, 5V, 12V, or 15V) V IN(Max) = Maximum Input Voltage ILOAD(Max) = Maximum Load Current Given: V OUT =5VVIN(Max) = 20V ILOAD(Max) = 0.8A 1. Inductor Selection (L1) A. Select the correct Inductor value selection guide from Figures 3, 4, 5, 6 (Output voltages of 3.3V, 5V, 12V or 15V respectively). For other output voltages, see the design procedure for the adjustable version. B. From the inductor value selection guide, identify the inductance region intersected by V IN(Max) and I LOAD(Max), and note the inductor code for that region. C. 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 LM2575 switching frequency (52 kHz) and for a current rating of 1.15 x I LOAD. For additional inductor information, see the inductor section in the Application Hints section of this data sheet. 1. Inductor Selection (L1) A. Use the selection guide shown in Figure 4. B. From the selection guide, the inductance area intersected by the 20V line and 0.8A line is L330. C. Inductor value required is 330 µH. From the table in Figure 9, choose AIE 415-0926, Pulse Engineering PE-52627, or RL1952. 2. 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 and an acceptable output ripple voltage, (approximately 1% of the output voltage) a value between 100 µF and 470 µF is recommended. B. The capacitor’s voltage rating should be at least 1.5 times greater than the output voltage. For a 5V regulator, a rating of at least 8V is appropriate, and a 10V or 15V rating is recommended. Higher voltage electrolytic capacitors generally have lower ESR numbers, and for this reason it may be necessary to select a capacitor rated for a higher voltage than would normally be needed. 2. Output Capacitor Selection (C OUT)A. COUT = 100 µF to 470 µF standard aluminum electrolytic. B. Capacitor voltage rating = 20V. 3. 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 LM2575. 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. 3. Catch Diode Selection (D1) A. For this example, a 1A current rating is adequate. B. Use a 30V 1N5818 or SR103 Schottky diode, or any of the suggested fast-recovery diodes shown in Figure 8. 4. Input Capacitor (C IN) An aluminum or tantalum electrolytic bypass capacitor located close to the regulator is needed for stable operation. 4. Input Capacitor (C IN) A 47 µF, 25V aluminum electrolytic capacitor located near the input and ground pins provides sufficient bypassing. www.national.com 13 |
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