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LM2575-12 Datasheet(PDF) 11 Page - Motorola, Inc |
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LM2575-12 Datasheet(HTML) 11 Page - Motorola, Inc |
11 / 28 page LM2575 11 MOTOROLA ANALOG IC DEVICE DATA Procedure (Adjustable Output Version: LM2575–Adj) (continued) Procedure Example 4. Inductor Selection (L1) A. Use the following formula to calculate the inductor Volt x microsecond [V x µs] constant: B. Match the calculated E x T value with the corresponding number on the vertical axis of the Inductor Value Selection Guide shown in Figure 21. This E x T constant is a measure of the energy handling capability of an inductor and is dependent upon the type of core, the core area, the number of turns, and the duty cycle. C. Next step is to identify the inductance region intersected by the E x T value and the maximum load current value on the horizontal axis shown in Figure 21. D. From the inductor code, identify the inductor value. Then select an appropriate inductor from the Table 1 or Table 2. The inductor chosen must be rated for a switching frequency of 52 kHz and for a current rating of 1.15 x IIoad. The inductor current rating can also be determined by calculating the inductor peak current : where ton is the “on” time of the power switch and For additional information about the inductor, see the inductor section in the “External Components” section of this data sheet. ExT + V in –Vout Vout Von x 106 F[Hz] [V x ms] I p(max) + I Load(max) ) V in –Vout ton 2L ton + Vout V in x 1 fosc 4. Inductor Selection (L1) A. Calculate E x T [V x µs] constant: B. E x T = 51 [V x µs] C. ILoad(max) = 1.0 A Inductance Region = L220 D. Proper inductor value = 220 µH Choose the inductor from the Table 1 or Table 2. ExT + (12 – 8.0) x 8.0 12 x 1000 52 + 51 [V x ms] 5. Output Capacitor Selection (Cout) A. Since the LM2575 is a forward–mode switching regulator with voltage mode control, its open loop 2–pole–2–zero frequency characteristic has the dominant pole–pair determined by the output capacitor and inductor values. For stable operation, the capacitor must satisfy the following requirement: B. Capacitor values between 10 µF and 2000 µF will satisfy the loop requirements for stable operation. To achieve an acceptable output ripple voltage and transient response, the output capacitor may need to be several times larger than the above formula yields. C. Due to the fact that the higher voltage electrolytic capacitors generally have lower ESR (Equivalent Series Resistance) numbers, the output capacitor’s voltage rating should be at least 1.5 times greater than the output voltage. For a 5.0 V regulator, a rating of at least 8V is appropriate, and a 10 V or 16 V rating is recommended. C out w 7.785 V in(max) Vout xL [µH] [ µF] 5. Output Capacitor Selection (Cout) A. To achieve an acceptable ripple voltage, select Cout = 100 µF electrolytic capacitor. C out w 7.785 12 8.220 + 53 µF |
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