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LTC1475CS8-3.3 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1475CS8-3.3 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 20 page  8 LTC1474/LTC1475 APPLICATIONS INFORMATION If the LMIN calculated is not practical, a larger IPEAK should be used. Although the above equation provides the mini- mum, better performance (efficiency, line/load regulation, noise) is usually gained with higher values. At higher inductances, peak current and frequency decrease (im- proving efficiency) and inductor ripple current decreases (improving noise and line/load regulation). For a given inductor type, however, as inductance is increased, DC resistance (DCR) increases, increasing copper losses, and current rating decreases, both effects placing an upper limit on the inductance. The recommended range of inductances for small surface mount inductors as a func- tion of peak current is shown in Figure 3. The values in this range are a good compromise between the trade-offs discussed above. If space is not a premium, inductors with larger cores can be used, which extends the recom- mended range of Figure 3 to larger values. section, increased inductance requires more turns of wire and therefore copper losses will increase. Ferrite and Kool M µdesigns have very low core loss and are preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates “hard,” which means that inductance collapses abruptly when the peak design current is exceeded. This results in an abrupt increase in inductor current above IPEAK and consequent increase in voltage ripple. Do not allow the core to satu- rate! Coiltronics, Coilcraft, Dale and Sumida make high performance inductors in small surface mount packages with low loss ferrite and Kool M µ cores and work well in LTC1474/LTC1475 regulators. Catch Diode Selection The catch diode carries load current during the off-time. The average diode current is therefore dependent on the P-channel switch duty cycle. At high input voltages the diode conducts most of the time. As VIN approaches VOUT the diode conducts only a small fraction of the time. The most stressful condition for the diode is when the output is short-circuited. Under this condition, the diode must safely handle IPEAK at close to 100% duty cycle. To maximize both low and high current efficiency, a fast switching diode with low forward drop and low reverse leakage should be used. Low reverse leakage current is critical to maximize low current efficiency since the leak- age can potentially approach the magnitude of the LTC1474/ LTC1475 supply current. Low forward drop is critical for high current efficiency since loss is proportional to for- ward drop. These are conflicting parameters (see Table 1), but a good compromise is the MBR0530 0.5A Schottky diode specified in the application circuits. Table 1. Effect of Catch Diode on Performance FORWARD NO LOAD DIODE (D1) LEAKAGE DROP SUPPLY CURRENT EFFICIENCY* BAS85 200nA 0.6V 9.7 µA 77.9% MBR0530 1 µA 0.4V 10 µA 83.3% MBRS130 20 µA 0.3V 16 µA 84.6% *Figure 1 circuit with VIN = 15V, IOUT = 0.1A Kool M µ is a registered trademark of Magnetics, Inc. PEAK INDUCTOR CURRENT (mA) 10 50 500 100 1000 100 1000 1474/75 F03 Inductor Core Selection Once the value of L is known, the type of inductor must be selected. High efficiency converters generally cannot afford the core loss found in low cost powdered iron cores, forcing the use of more expensive ferrite, molypermalloy or Kool M µ® cores. Actual core loss is independent of core size for a fixed inductor value, but is very dependent on inductance selected. As inductance increases, core losses go down. Unfortunately, as discussed in the previous Figure 3. Recommended Inductor Values |
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