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LTC3642IMS8E-5-PBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC3642IMS8E-5-PBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LTC3642 11 3642f APPLICATIONS INFORMATION Figure 3. Switching Frequency for VOUT = 3.3V Figure 2. Switching Frequency for VOUT = 5V Figure 4. Recommended Inductor Values for Maximum Efficiency where board area is not a limiting factor, inductors with larger cores can be used, which extends the recommended range of Figure 4 to larger values. Inductor Core Selection Once the value for 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 the more expensive ferrite cores. Actual core loss is independent of core size for a fixed inductor value but is very dependent of the inductance selected. As the inductance increases, core losses decrease. Un- fortunately, increased inductance requires more turns of wire and therefore copper losses will increase. Ferrite designs have very low core losses and are pre- ferred 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 ripple current and consequently output voltage ripple. Do not allow the core to saturate! Different core materials and shapes will change the size/current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy ma- terials are small and do not radiate energy but generally cost more than powdered iron core inductors with similar characteristics. The choice of which style inductor to use mainly depends on the price vs size requirements and any radiated field/EMI requirements. New designs for surface mount inductors are available from Coiltronics, Coilcraft, Toko, Sumida and Vishay. CIN and COUT Selection The input capacitor, CIN, is needed to filter the trapezoidal current at the source of the top high side MOSFET. To prevent large ripple voltage, a low ESR input capacitor sized for the maximum RMS current should be used. Approximate RMS current is given by: II V V V V RMS OUT MAX OUT IN IN OUT =− () •• 1 VIN INPUT VOLTAGE (V) 5 400 500 600 35 3642 F02 300 200 15 25 45 30 10 20 40 100 0 700 L = 47μH L = 68μH L = 100μH L = 150μH L = 220μH L = 470μH VOUT = 5V ISET OPEN VIN INPUT VOLTAGE (V) 5 0 50 150 200 250 500 350 15 25 30 3642 F03 100 400 450 300 10 20 35 40 45 L = 470μH L = 220μH L = 150μH L = 100μH L = 68μH L = 47μH VOUT = 3.3V ISET OPEN PEAK INDUCTOR CURRENT (mA) 100 1000 10000 10 100 3642 F04 |
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