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LM1770TMFX bảng dữ liệu(PDF) 10 Page - National Semiconductor (TI) |
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LM1770TMFX bảng dữ liệu(HTML) 10 Page - National Semiconductor (TI) |
10 / 14 page Design Guide (Continued) 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 LM1770 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 (RSNS) should be added. This can be seen below in the following circuit. 20166218 This circuit uses an additional resistor in series with the inductor to add ripple at the output. It is placed in this location and used in combination with the feed-forward capacitor (C FF) to provide ripple to the feedback pin, without adding ripple or a DC offset to the output. The benefit of using a ceramic capacitor is still obtained with this technique. Be- cause the addition of the resistor results in power loss, this circuit implementation is only recommended for low currents (2A and below). The power loss and rating of the resistor should be taken into account when selecting this compo- nent. This circuit implementation utilizing the feed-forward capaci- tor begins to experience limitations when the output voltage is small. Previously the circuit relied on the C FF for all the ripple at the feedback node by assuming that the resistor divider was negligible. As V OUT decreases this can not be assumed. The resistor divider contributes a larger amount of ripple which is problematic as it is also out of phase. There- fore the resistor location should be changed to be in series with the output capacitor. This can be viewed as adding an effective ESR to the output capacitor. www.national.com 10 |
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