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LM2575T-5.0 Datasheet(PDF) 15 Page - National Semiconductor (TI) |
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LM2575T-5.0 Datasheet(HTML) 15 Page - National Semiconductor (TI) |
15 / 27 page Inductor Value Selection Guides (For Continuous Mode Operation) (Continued) PROCEDURE (Adjustable Output Voltage Versions) EXAMPLE (Adjustable Output Voltage Versions) Given: V OUT = Regulated Output Voltage VIN(Max) = Maximum Input Voltage I LOAD(Max) = Maximum Load Current F = Switching Frequency (Fixed at 52 kHz) Given: V OUT = 10V VIN(Max) = 25V ILOAD(Max) = 1A F = 52 kHz 1. Programming Output Voltage (Selecting R1 and R2, as shown in Figure 2 ) Use the following formula to select the appropriate resistor values. R 1 can be between 1k and 5k. (For best temperature coef- ficient and stability with time, use 1% metal film resistors) 1.Programming Output Voltage (Selecting R1 and R2) R2 = 1k (8.13 − 1) = 7.13k, closest 1% value is 7.15k 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 hori- zontal 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 manu- facturers. 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. 2. Inductor Selection (L1) A. Calculate E • T(V • µs) B. E • T = 115 V • µs C. I LOAD(Max) = 1A D. Inductance Region = H470 E. Inductor Value = 470 µH Choose from AIE part #430-0634, Pulse Engineering part #PE-53118, or Renco part #RL-1961. 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 2000 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 re- sponse, the output capacitor may need to be several times larger than the above formula yields. B. The capacitor’s volt- age 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 gen- erally 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)A. However, for acceptable output ripple voltage select C OUT ≥ 220 µF C OUT = 220 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 LM2575. 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 3A current rating is adequate. B. Use a 40V MBR340 or 31DQ04 Schottky diode, or any of the suggested fast-recovery diodes in Figure 8. www.national.com 15 |
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