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LM2578AN Arkusz danych(PDF) 10 Page - National Semiconductor (TI) |
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LM2578AN Arkusz danych(HTML) 10 Page - National Semiconductor (TI) |
10 / 18 page Typical Applications (Continued) Figure 15 is a 15V to 5V buck regulator with an output cur- rent, I o, of 350 mA. The circuit becomes discontinuous at 20% of I o(max), has 10 mV of output voltage ripple, an effi- ciency of 75%, a load regulation of 30 mV (70 mA to 350 mA) and a line regulation of 10 mV (12 ≤ V in ≤ 18V). Component values are selected as follows: R1 = (V o − 1) x R2 where R2 = 10 kΩ R3 = V/I sw(max) R3 = 0.15 Ω where: V is the current limit sense voltage, 0.11V I sw(max) is the maximum allowable current thru the output transistor. L1 is the inductor and may be found from the inductance cal- culation chart ( Figure 16) as follows: Given V in = 15V V o = 5V I o(max) = 350 mA f OSC = 50 kHz Discontinuous at 20% of I o(max). Note that since the circuit will become discontinuous at 20% of I o(max), the load current must not be allowed to fall below 70 mA. Step 1: Calculate the maximum DC current through the in- ductor, I L(max). The necessary equations are indicated at the top of the chart and show that I L(max) = Io(max) for the buck configuration. Thus, I L(max) = 350 mA. Step 2: Calculate the inductor Volts-sec product, E-T op, ac- cording to the equations given from the chart. For the Buck: E-T op = (Vin −Vo)(Vo/Vin) (1000/fosc) =(15 − 5) (5/15) (1000/50) = 66V-µs. with the oscillator frequency, f osc, expressed in kHz. Step 3: Using the graph with axis labeled “Discontinuous At % I OUT” and “IL(max, DC)” find the point where the desired maximum inductor current, I L(max, DC) intercepts the desired discontinuity percentage. In this example, the point of interest is where the 0.35A line intersects with the 20% line. This is nearly the midpoint of the horizontal axis. Step 4: This last step is merely the translation of the point found in Step 3 to the graph directly below it. This is accom- plished by moving straight down the page to the point which intercepts the desired E-T op. For this example, E-Top is 66V-µs and the desired inductor value is 470 µH. Since this example was for 20% discontinuity, the bottom chart could have been used directly, as noted in step 3 of the chart instructions. DS008711-5 FIGURE 14. Basic Buck Regulator DS008711-6 Vin = 15V R3 = 0.15 Ω Vo = 5V C1 = 1820 pF Vripple = 10 mV C2 = 220 µF Io = 350 mA C3 = 20 pF fosc = 50 kHz L1 = 470 µH R1 = 40 k Ω D1 = 1N5818 R2 = 10 k Ω FIGURE 15. Buck or Step-Down Regulator www.national.com 10 |
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