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LT1374IS8-5SYNC Datasheet(PDF) 11 Page - Linear Technology |
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LT1374IS8-5SYNC Datasheet(HTML) 11 Page - Linear Technology |
11 / 32 page 11 LT1374 1374fb When choosing an inductor you might have to consider maximum load current, core and copper losses, allowable component height, output voltage ripple, EMI, fault cur- rent in the inductor, saturation, and of course, cost. The following procedure is suggested as a way of handling these somewhat complicated and conflicting requirements. 1. Choose a value in microhenries from the graphs of maximum load current and core loss. Choosing a small inductor may result in discontinuous mode operation at lighter loads, but the LT1374 is designed to work well in either mode. Keep in mind that lower core loss means higher cost, at least for closed core geometries like toroids. The core loss graphs show both absolute loss and percent loss for a 5W output, so actual percent losses must be calculated for each situation. Assume that the average inductor current is equal to load current and decide whether or not the inductor must withstand continuous fault conditions. If maxi- mum load current is 0.5A, for instance, a 0.5A inductor may not survive a continuous 4.5A overload condition. Dead shorts will actually be more gentle on the induc- tor because the LT1374 has foldback current limiting. 2. Calculate peak inductor current at full load current to ensure that the inductor will not saturate. Peak current can be significantly higher than output current, espe- cially with smaller inductors and lighter loads, so don’t omit this step. Powdered iron cores are forgiving because they saturate softly, whereas ferrite cores saturate abruptly. Other core materials fall somewhere in between. The following formula assumes continu- ous mode of operation, but it errs only slightly on the high side for discontinuous mode, so it can be used for all conditions. II VV V fL V PEAK OUT OUT IN OUT IN =+ − () ()( )( ) 2 VIN = Maximum input voltage f = Switching frequency, 500kHz 3. Decide if the design can tolerate an “open” core geom- etry like a rod or barrel, with high magnetic field radiation, or whether it needs a closed core like a toroid to prevent EMI problems. One would not want an open APPLICATIONS INFORMATION core next to a magnetic storage media, for instance! This is a tough decision because the rods or barrels are temptingly cheap and small and there are no helpful guidelines to calculate when the magnetic field radia- tion will be a problem. 4. Start shopping for an inductor (see representative surface mount units in Table 2) which meets the requirements of core shape, peak current (to avoid Table 2 SERIES CORE VENDOR/ VALUE DC CORE RESIS- MATER- HEIGHT PART NO. ( µH) (Amps) TYPE TANCE(Ω) IAL (mm) Coiltronics CTX2-1 2 4.1 Tor 0.011 KM µ 4.2 CTX5-4 5 4.4 Tor 0.019 KM µ 6.4 CTX8-4 8 3.5 Tor 0.020 KM µ 6.4 CTX2-1P 2 3.4 Tor 0.014 52 4.2 CTX2-3P 2 4.6 Tor 0.012 52 4.8 CTX5-4P 5 3.3 Tor 0.027 52 6.4 Sumida CDRH125 10 4.0 SC 0.025 Fer 6 CDRH125 12 3.5 SC 0.027 Fer 6 CDRH125 15 3.3 SC 0.030 Fer 6 CDRH125 18 3.0 SC 0.034 Fer 6 Coilcraft DT3316-222 2.2 5 SC 0.035 Fer 5.1 DT3316-332 3.3 5 SC 0.040 Fer 5.1 DT3316-472 4.7 3 SC 0.045 Fer 5.1 Pulse PE-53650 4 4.8 Tor 0.017 Fer 9.1 PE-53651 5 5.4 Tor 0.018 Fer 9.1 PE-53652 9 5.5 Tor 0.022 Fer 10 PE-53653 16 5.1 Tor 0.032 Fer 10 Dale IHSM-4825 2.7 5.1 Open 0.034 Fer 5.6 IHSM-4825 4.7 4.0 Open 0.047 Fer 5.6 IHSM-5832 10 4.3 Open 0.053 Fer 7.1 IHSM-5832 15 3.5 Open 0.078 Fer 7.1 IHSM-7832 22 3.8 Open 0.054 Fer 7.1 Tor = Toroid SC = Semi-closed geometry Fer = Ferrite core material 52 = Type 52 powdered iron core material KM µ = Kool Mµ |
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