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LT1373HVIN8 Datasheet(PDF) 8 Page - Linear Technology |
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LT1373HVIN8 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LT1373 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. Assume that the average inductor current (for a boost converter) is equal to load current times VOUT/VIN and decide whether or not the inductor must withstand continuous overload conditions. If average inductor current at maximum load current is 0.5A, for instance, a 0.5A inductor may not survive a continuous 1.5A overload condition. Also, be aware that boost convert- ers are not short-circuit protected, and that under output short conditions, inductor current is limited only by the available current of the input supply. 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. Powered iron cores are forgiving be- cause they saturate softly, whereas ferrite cores satu- rate abruptly. Other core materials fall in between somewhere. The following formula assumes continu- ous mode operation, but it errors only slightly on the high side for discontinuous mode, so it can be used for all conditions. IPEAK = IOUT • VIN = minimum input voltage f = 250kHz switching frequency + VOUT VIN VIN (VOUT – VIN) 2(f)(L)(VOUT) 3. Decide if the design can tolerate an “open” core geom- etry like a rod or barrel, which have high magnetic field radiation, or whether it needs a closed core like a toroid to prevent EMI problems. One would not want an open 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 which meets the require- ments of core shape, peak current (to avoid saturation), average current (to limit heating), and fault current, (if the inductor gets too hot, wire insulation will melt and cause turn-to-turn shorts). Keep in mind that all good things like high efficiency, low profile and high temperature operation will increase cost, sometimes dramatically. 5. After making an initial choice, consider the secondary things like output voltage ripple, second sourcing, etc. Use the experts in the Linear Technology application department if you feel uncertain about the final choice. They have experience with a wide range of inductor types and can tell you about the latest developments in low profile, surface mounting, etc. Output Capacitor The output capacitor is normally chosen by its effective series resistance (ESR), because this is what determines output ripple voltage. At 500kHz, any polarized capacitor is essentially resistive. To get low ESR takes volume, so physically smaller capacitors have high ESR. The ESR range for typical LT1373 applications is 0.05 Ω to 0.5Ω. A typical output capacitor is an AVX type TPS, 22 µF at 25V, with a guaranteed ESR less than 0.2 Ω. This is a “D” size surface mount solid tantalum capacitor. TPS capacitors are specially constructed and tested for low ESR, so they give the lowest ESR for a given volume. To further reduce ESR, multiple output capacitors can be used in parallel. The value in microfarads is not particularly critical and values from 22 µF to greater than 500µF work well, but you cannot cheat mother nature on ESR. If you find a tiny 22 µF solid tantalum capacitor, it will have high ESR and output ripple voltage will be terrible. Table 1 shows some typical solid tantalum surface mount capacitors. Table 1. Surface Mount Solid Tantalum Capacitor ESR and Ripple Current E CASE SIZE ESR (MAX Ω) RIPPLE CURRENT (A) AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1 AVX TAJ 0.7 to 0.9 0.4 D CASE SIZE AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1 AVX TAJ 0.9 to 2.0 0.36 to 0.24 C CASE SIZE AVX TPS 0.2 (Typ) 0.5 (Typ) AVX TAJ 1.8 to 3.0 0.22 to 0.17 B CASE SIZE AVX TAJ 2.5 to 10 0.16 to 0.08 APPLICATIO S I FOR ATIO |
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