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LM1771 データシート(PDF) 10 Page - National Semiconductor (TI) |
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LM1771 データシート(HTML) 10 Page - National Semiconductor (TI) |
10 / 17 page Design Guide (Continued) peak-to-peak ripple current equal to 30% of the maximum load current. The inductor current ripple ( ∆I L) can be calcu- lated by: Therefore, L can be initially set to the following by applying the 30% rule: The other features of the inductor that can be selected besides inductance value are saturation current and core material. Because the LM1771 does not have a current limit, it is recommended to have a saturation current higher than the maximum output current to handle any ripple or momen- tary over-current events. The core material also influences the saturation characteristics as ferrite materials have a hard saturation curve and care should be taken such that they never saturate during normal use. A shielded inductor or low profile unshielded inductor is recommended to reduce EMI. This also helps prevent any spurious noise from picking up on the feedback node resulting in unexpected tripping of the feedback comparator. OUTPUT CAPACITOR One of the most important components to select with the LM1771 is the output capacitor. This is because its size and ESR have a direct effect on the stability of the loop. A constant on-time control scheme works by sensing the out- put voltage ripple and switching the FETs appropriately. The output voltage ripple on a buck converter can be approxi- mated by stating that the AC inductor ripple flows entirely into the output capacitor and is created by the ESR of the capacitor. This can be expressed in the following equation: ∆V OUT = ∆I L xRESR To ensure stability, two constraints need to be met. The first is that there is sufficient ESR to create enough voltage ripple at the feedback pin. The recommendation is to have at least 10mV of ripple seen at the feedback pin. This can be calcu- lated by multiplying the output voltage ripple by the gain seen through the feedback resistors. This gain, H, can be calculated below: If the output voltage is fairly high, causing significant attenu- ation through the feedback resistors, a feed-forward capaci- tor can be used. This is actually recommended for most circuits as it improves performance. See the feed-forward capacitor section for more details. The second criteria is to ensure that there is sufficient ripple at the output that is in-phase with the switch. The problem exists that there is actually ripple caused by the capacitor charging and discharging, not only the ESR ripple. Since these are effectively out of phase, problems can exist. To avoid this issue it is recommended that the ratio of the two ripples ( β) is always greater than 5. To calculate the mini- mum ESR value needed, the following equation can be used. In general the best capacitors to use are chemistries that have a known and consistent ESR across the entire operat- ing temperature range. Tantalum capacitors or similar chem- istries such as Niobium Oxide perform well along with certain families of Aluminum Electrolytics. Small value POSCAPs and SP CAPs also work as they have sufficient ESR. When used in conjunction with a low value inductor it is possible to have an extremely stable design. The only capacitors that require modification to the circuit are ceramic capacitors. Ceramic capacitors cause problems meeting both criteria because they have low ESR and low capacitance. There- fore, if they are to be used, an external ESR resistor (R SNS) should be added. This can be seen below in the following circuit. www.national.com 10 |
同様の部品番号 - LM1771 |
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同様の説明 - LM1771 |
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