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CB2012T2R2M データシート(PDF) 8 Page - Linear Technology |
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CB2012T2R2M データシート(HTML) 8 Page - Linear Technology |
8 / 16 page 8 LTC3410 3410fb The basic LTC3410 application circuit is shown in Figure 1. External component selection is driven by the load require- ment and begins with the selection of L followed by CIN and COUT. Inductor Selection For most applications, the value of the inductor will fall in the range of 2.2 µH to 4.7µH. Its value is chosen based on the desired ripple current. Large value inductors lower ripple current and small value inductors result in higher ripple currents. Higher VIN or VOUT also increases the ripple current as shown in equation 1. A reasonable starting point for setting ripple current is ∆IL = 120mA (40% of 300mA). ∆ = ()( ) − ⎛ ⎝⎜ ⎞ ⎠⎟ I fL V V V L OUT OUT IN 1 11 () The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation. Thus, a 360mA rated inductor should be enough for most applications (300mA + 60mA). For better efficiency, choose a low DC-resistance inductor. The inductor value also has an effect on Burst Mode operation. The transition to low current operation begins when the inductor current peaks fall to approximately 100mA. Lower inductor values (higher ∆IL) will cause this to occur at lower load currents, which can cause 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. APPLICATIO S I FOR ATIO Inductor Core Selection Different core materials and shapes will change the size/ current and price/current relationship of an inductor. Tor- oid or shielded pot cores in ferrite or permalloy materials are small and don’t radiate much energy, but generally cost more than powdered iron core inductors with similar electrical characteristics. The choice of which style induc- tor to use often depends more on the price vs size require- ments and any radiated field/EMI requirements than on what the LTC3410 requires to operate. Table 1 shows some typical surface mount inductors that work well in LTC3410 applications. Table 1. Representative Surface Mount Inductors MAX DC MANUFACTURER PART NUMBER VALUE CURRENT DCR HEIGHT Taiyo Yuden CB2016T2R2M 2.2 µH 510mA 0.13Ω 1.6mm CB2012T2R2M 2.2 µH 530mA 0.33Ω 1.25mm LBC2016T3R3M 3.3 µH 410mA 0.27Ω 1.6mm Panasonic ELT5KT4R7M 4.7 µH 950mA 0.2Ω 1.2mm Sumida CDRH2D18/LD 4.7 µH 630mA 0.086Ω 2mm Murata LQH32CN4R7M23 4.7 µH 450mA 0.2Ω 2mm Taiyo Yuden NR30102R2M 2.2 µH 1100mA 0.1Ω 1mm NR30104R7M 4.7 µH 750mA 0.19Ω 1mm FDK FDKMIPF2520D 4.7 µH 1100mA 0.11Ω 1mm FDKMIPF2520D 3.3 µH 1200mA 0.1Ω 1mm FDKMIPF2520D 2.2 µH 1300mA 0.08Ω 1mm CIN and COUT Selection In continuous mode, the source current of the top MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: C required I I VV V V IN RMS OMAX OUT IN OUT IN ≅ − () []12/ This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is com- monly used for design because even significant deviations do not offer much relief. Note that the capacitor manufacturer’s ripple current ratings are often based on VIN CIN 4.7 µF CER VIN 2.7V TO 5.5V LTC3410 RUN 4.7 µH 10pF 232k 464k 3410 F01 SW VFB GND COUT 4.7 µF CER VOUT 1.2V Figure 1. High Efficiency Step-Down Converter |
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