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LT1370CT7 Datasheet(PDF) 9 Page - Linear Technology |
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LT1370CT7 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  9 LT1370 APPLICATIO S I FOR ATIO tested for low ESR, so they give the lowest ESR for a given volume. To further reduce ESR, multiple output capaci- tors can be used in parallel. The value in microfarads is not particularly critical, and values from 22 µF to greater than 500 µF 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 Many engineers have heard that solid tantalum capacitors are prone to failure if they undergo high surge currents. This is historically true and AVX type TPS capacitors are specially tested for surge capability, but surge ruggedness is not a critical issue with the output capacitor. Solid tantalum capacitors fail during very high turn-on surges, which do not occur at the output of regulators. High discharge surges, such as when the regulator output is dead-shorted, do not harm the capacitors. Single inductor boost regulators have large RMS ripple current in the output capacitor, which must be rated to handle the current. The formula to calculate this is: Output Capacitor Ripple Current (RMS) IRIPPLE (RMS) = IOUT = IOUT VOUT – VIN VIN DC 1 – DC DC = Switch duty cycle IPEAK = (IOUT) VIN = Minimum input voltage f = 500kHz 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 has 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 radiation will be a problem. 4. Start shopping for an inductor that meets the requirements 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 LTC Applications 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 needed for typical LT1370 applications is 0.025 Ω to 0.2 Ω. A typical output capacitor is an AVX type TPS, 22 µF at 25V (two each), 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 |
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