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LTC3405AES6 Datasheet(PDF) 8 Page - Linear Technology |
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LTC3405AES6 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  8 LTC3405A 3405afa APPLICATIO S I FOR ATIO The basic LTC3405A application circuit is shown in Fig- ure 1. External component selection is driven by the load requirement 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 2.2µH to 10µH. 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 1 (1) 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. 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 Table 1. Representative Surface Mount Inductors MAX DC MANUFACTURER PART NUMBER VALUE CURRENT DCR HEIGHT Taiyo Yuden LB2016T2R2M 2.2µH 315mA 0.13Ω 1.6mm LB2012T2R2M 2.2µH 240mA 0.23Ω 1.25mm LB2016T3R3M 3.3µH 280mA 0.2Ω 1.6mm Panasonic ELT5KT4R7M 4.7µH 950mA 0.2Ω 1.2mm Murata LQH3C4R7M34 4.7µH 450mA 0.2Ω 2mm Taiyo Yuden LB2016T4R7M 4.7µH 210mA 0.25Ω 1.6mm Panasonic ELT5KT6R8M 6.8µH 760mA 0.3Ω 1.2mm Panasonic ELT5KT100M 10µH 680mA 0.36Ω 1.2mm Sumida CMD4D116R8MC 6.8µH 620mA 0.23Ω 1.2mm 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 LTC3405A requires to operate. Table 1 shows some typical surface mount inductors that work well in LTC3405A applications. 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: CI VV V V IN OMAX OUT IN OUT IN required IRMS ≅ − () [] 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 2000 hours of life. This makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Always consult the manufac- turer if there is any question. |
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