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LTC3407AIDD-2-PBF Datasheet(PDF) 8 Page - Linear Technology |
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LTC3407AIDD-2-PBF Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  LTC3407A-2 8 3407a2f Input Capacitor (CIN) Selection In continuous mode, the input current of the converter is a square wave with a duty cycle of approximately VOUT/ VIN. To prevent large voltage transients, a low equivalent series resistance (ESR) input capacitor sized for the maxi- mum RMS current must be used. The maximum RMS capacitor current is given by: II VV V V RMS MAX OUT IN OUT IN ≈ (– ) where the maximum average output current IMAX equals the peak current minus half the peak-to-peak ripple cur- rent, IMAX = ILIM – ΔIL/2. This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case is commonly used to design because even significant deviations do not offer much relief. Note that capacitor manufacturer’s ripple current ratings are often based on only 2000 hours life- time. This makes it advisable to further derate the capaci- tor, or choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet the size or height requirements of the design. An additional 0.1 μF to 1μF ceramic capacitor is also recom- mended on VIN for high frequency decoupling, when not using an all ceramic capacitor solution. Table 1. Representative Surface Mount Inductors MANU- MAX DC FACTURER PART NUMBER VALUE CURRENT DCR HEIGHT Taiyo Yuden CB2016T2R2M 2.2 μH 510mA 0.13 Ω 1.6mm CB2012T2R2M 2.2 μH 530mA 0.33 Ω 1.25mm CB2016T3R3M 3.3 μH 410mA 0.27 Ω 1.6mm Panasonic ELT5KT4R7M 4.7 μH 950mA 0.2 Ω 1.2mm Sumida CDRH2D18/LD 4.7 μH 630mA 0.086 Ω 2mm Murata LQH32CN4R7M23 4.7 μH 450mA 0.2 Ω 2mm Taiyo Yuden NR30102R2M 2.2 μH 1100mA 0.1 Ω 1mm NR30104R7M 4.7 μH 750mA 0.19 Ω 1mm FDK FDKMIPF2520D 4.7 μH 1100mA 0.11 Ω 1mm FDKMIPF2520D 3.3 μH 1200mA 0.1 Ω 1mm FDKMIPF2520D 2.2 μH 1300mA 0.08 Ω 1mm TDK VLF3010AT4R7- 4.7 μH 700mA 0.28 Ω 1mm MR70 VLF3010AT3R3- 3.3 μH 870mA 0.17 Ω 1mm MR87 VLF3010AT2R2- 2.2 μH 1000mA 0.12 Ω 1mm M1R0 APPLICATIO S I FOR ATIO Inductor Selection Although the inductor does not influence the operating frequency, the inductor value has a direct effect on ripple current. The inductor ripple current ΔIL decreases with higher inductance and increases with higher VIN or VOUT: Δ = ⎛ ⎝⎜ ⎞ ⎠⎟ I V fL V V L OUT O OUT IN • •– 1 Accepting larger values of ΔIL allows the use of low inductances, but results in higher output voltage ripple, greater core losses, and lower output current capability. A reasonable starting point for setting ripple current is ΔIL = 0.3 • ILIM, where ILIM is the peak switch current limit. The largest ripple current ΔIL occurs at the maximum input voltage. To guarantee that the ripple current stays below a specified maximum, the inductor value should be chosen according to the following equation: L V fI V V OUT OL OUT IN MAX = Δ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ • •– () 1 The inductor value will also have an effect on Burst Mode operation. The transition from low current operation be- gins when the peak inductor current falls below a level set by the burst clamp. Lower inductor values result in higher ripple current which causes this transition to occur at lower load currents. This causes a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency to increase. Inductor Core Selection Different core materials and shapes will change the size/ current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy mate- rials are small and don’t radiate much energy, but gener- ally cost more than powdered iron core inductors with similar electrical characterisitics. The choice of which style inductor to use often depends more on the price vs size requirements and any radiated field/EMI require- ments than on what the LTC3407A-2 requires to operate. Table 1 shows some typical surface mount inductors that work well in LTC3407A-2 applications. |
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