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LM2674 Datasheet(PDF) 19 Page - National Semiconductor (TI) |
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LM2674 Datasheet(HTML) 19 Page - National Semiconductor (TI) |
19 / 26 page ![]() LM2674 Series Buck Regulator Design Procedure (Adjustable Output) (Continued) PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version) B. Select an appropriate capacitor value and voltage rating, using the capacitor code, from the output capacitor selection table in Figure 17. There are two solid tantalum (surface mount) capacitor manufacturers and four electrolytic (through hole) capacitor manufacturers to choose from. It is recommended that both the manufacturers and the manufacturer’s series that are listed in the table be used. A table listing the manufacturers’ phone numbers is located in Figure 11. B. From the output capacitor selection table in Figure 17, choose a capacitor value (and voltage rating) that intersects the capacitor code(s) selected in section A, C20. The capacitance and voltage rating values corresponding to the capacitor code C20 are the: Surface Mount: 33 µF/25V Sprague 594D Series. 33 µF/25V AVX TPS Series. Through Hole: 33 µF/25V Sanyo OS-CON SC Series. 120 µF/35V Sanyo MV-GX Series. 120 µF/35V Nichicon PL Series. 120 µF/35V Panasonic HFQ Series. Other manufacturers or other types of capacitors may also be used, provided the capacitor specifications (especially the 100 kHz ESR) closely match the characteristics of the capacitors listed in the output capacitor table. Refer to the capacitor manufacturers’ data sheet for this information. 4. Catch Diode Selection (D1) A. In normal operation, the average current of the catch diode is the load current times the catch diode duty cycle, 1-D (D is the switch duty cycle, which is approximately V OUT/VIN). The largest value of the catch diode average current occurs at the maximum input voltage (minimum D). For normal operation, the catch diode current rating must be at least 1.3 times greater than its maximum average current. However, if the power supply design must withstand a continuous output short, the diode should have a current rating greater than the maximum current limit of the LM2674. The most stressful condition for this diode is a shorted output condition. 4. Catch Diode Selection (D1) A. Refer to the table shown in Figure 12. Schottky diodes provide the best performance, and in this example a 500mA, 40V Schottky diode would be a good choice. If the circuit must withstand a continuous shorted output, a higher current (at least 1.2A) Schottky diode is recommended. B. The reverse voltage rating of the diode should be at least 1.25 times the maximum input voltage. C. Because of their fast switching speed and low forward voltage drop, Schottky diodes provide the best performance and efficiency. The Schottky diode must be located close to the LM2674 using short leads and short printed circuit traces. www.national.com 19 |
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