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LT8331 データシート(PDF) 13 Page - Analog Devices |
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LT8331 データシート(HTML) 13 Page - Analog Devices |
13 / 30 page LT8331 13 Rev. C For more information www.analog.com APPLICATIONS INFORMATION Discontinuous conduction mode (DCM) provides higher conversion ratios at a given frequency at the cost of reduced efficiencies, higher switching currents, and lower available output power. Boost Converter: Maximum Output Current Capability and Inductor Selection For the boost topology, the maximum average inductor current is: IL(MAX)(AVE)= IO(MAX) • 1 1 − DMAX • 1 η where η (< 1.0) is the converter efficiency. Due to the current limit of its internal power switch, the LT8331 should be used in a boost converter whose maxi- mum output current (IO(MAX)) is: I O(MAX) ≤ VIN(MIN) VOUT • 0.5A − 0.5 • ΔISW ( ) • η Minimum possible inductor value and switching frequency should also be considered since they will increase inductor ripple current ∆ISW. The inductor ripple current ∆ISW has a direct effect on the choice of the inductor value and the converter’s maximum output current capability. Choosing smaller values of ∆ISW increases output current capability, but requires large inductances and reduces the current loop gain (the converter will approach voltage mode). Accepting larger values of ∆ISW provides fast transient response and allows the use of low inductances, but results in higher input current ripple and greater core losses, and reduces output current capability. It is recommended to choose a ∆ISW of approximately 0.2A to 0.3A. Given an operating input voltage range, and having cho- sen the operating frequency and ripple current in the inductor, the inductor value of the boost converter can be determined using the following equation: L = VIN(MIN) ΔISW • fOSC • DMAX The peak inductor current is the switch current limit (max- imum 0.7A), and the RMS inductor current is approxi- mately equal to IL(MAX)(AVE). Choose an inductor that can handle at least 0.7A without saturating, and ensure that the inductor has a low DCR (copper-wire resistance) to minimize I2R power losses. Note that in some applications, the current handling requirements of the inductor can be lower, such as in the SEPIC topology where each inductor only carries one- half of the total switch current. For better efficiency, use similar valued inductors with a larger volume. Many dif- ferent sizes and shapes are available from various manu- facturers (see Table 2). Choose a core material that has low losses at the programmed switching frequency, such as a ferrite core. The final value chosen for the inductor should not allow peak inductor currents to exceed 0.5A in steady state at maximum load. Due to tolerances, be sure to account for minimum possible inductance value, switching frequency and converter efficiency. Table 2. Inductor Manufacturers Sumida (847) 956-0666 www.sumida.com TDK (847) 803-6100 www.tdk.com Murata (714) 852-2001 www.murata.com Coilcraft (847) 639-6400 www.coilcraft.com Wurth (605) 886-4385 www.we-online.com BOOST CONVERTER: INPUT CAPACITOR SELECTION Bypass the input of the LT8331 circuit with a ceramic capacitor of X7R or X5R type placed as close as possible to the VIN and GND pins. Y5V types have poor performance over temperature and applied voltage, and should not be used. A 4.7µF to 10µF ceramic capacitor is adequate to bypass the LT8331 and will easily handle the ripple cur- rent. If the input power source has high impedance, or there is significant inductance due to long wires or cables, additional bulk capacitance may be necessary. This can be provided with a low performance electrolytic capacitor. A precaution regarding the ceramic input capacitor con- cerns the maximum input voltage rating of the LT8331. A ceramic input capacitor combined with trace or cable |
同様の部品番号 - LT8331 |
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同様の説明 - LT8331 |
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