LTC3405A UUWUAPPLICATIO S I FOR ATIO The basic LTC3405A application circuit is shown in Fig- Table 1. Representative Surface Mount Inductors ure 1. External component selection is driven by the load MAX DC requirement and begins with the selection of L followed by MANUFACTURER PART NUMBERVALUE CURRENT DCR HEIGHT C Taiyo Yuden LB2016T2R2M 2.2µH 315mA 0.13Ω 1.6mm IN and COUT. LB2012T2R2M 2.2µH 240mA 0.23Ω 1.25mm LB2016T3R3M 3.3µH 280mA 0.2Ω 1.6mm Inductor Selection Panasonic ELT5KT4R7M 4.7µH 950mA 0.2Ω 1.2mm For most applications, the value of the inductor will fall in Murata LQH3C4R7M34 4.7µH 450mA 0.2Ω 2mm the range of 2.2µH to 10µH. Its value is chosen based on Taiyo Yuden LB2016T4R7M 4.7µH 210mA 0.25Ω 1.6mm the desired ripple current. Large value inductors lower Panasonic ELT5KT6R8M 6.8µH 760mA 0.3Ω 1.2mm ripple current and small value inductors result in higher Panasonic ELT5KT100M 10µH 680mA 0.36Ω 1.2mm ripple currents. Higher VIN or VOUT also increases the ripple Sumida CMD4D116R8MC 6.8µH 620mA 0.23Ω 1.2mm current as shown in equation 1. A reasonable starting point for setting ripple current is ∆IL = 120mA (40% of 300mA). electrical characteristics. The choice of which style induc- tor to use often depends more on the price vs size require- 1 ⎛ VOUT ⎞ ∆I ments and any radiated field/EMI requirements than on L = VOUT ⎜1− (f)( ) ⎟ L ⎝ V (1) IN ⎠ what the LTC3405A requires to operate. Table 1 shows some typical surface mount inductors that work well in The DC current rating of the inductor should be at least LTC3405A applications. equal to the maximum load current plus half the ripple current to prevent core saturation. Thus, a 360mA rated CIN and COUT Selection inductor should be enough for most applications (300mA In continuous mode, the source current of the top MOSFET + 60mA). For better efficiency, choose a low DC-resistance is a square wave of duty cycle V inductor. OUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the The inductor value also has an effect on Burst Mode maximum RMS current must be used. The maximum operation. The transition to low current operation begins RMS capacitor current is given by: when the inductor current peaks fall to approximately 100mA. Lower inductor values (higher ∆I / L) will cause this V V [ ( − V OUT IN OUT )]1 2 to occur at lower load currents, which can cause a dip in C required I I IN RMS ≅ OMAX efficiency in the upper range of low current operation. In VIN Burst Mode operation, lower inductance values will cause This formula has a maximum at VIN = 2VOUT, where the burst frequency to increase. IRMS = IOUT/2. This simple worst-case condition is com- monly used for design because even significant deviations Inductor Core Selection do not offer much relief. Note that the capacitor Different core materials and shapes will change the size/ manufacturer’s ripple current ratings are often based on current and price/current relationship of an inductor. Tor- 2000 hours of life. This makes it advisable to further derate oid or shielded pot cores in ferrite or permalloy materials the capacitor, or choose a capacitor rated at a higher are small and don’t radiate much energy, but generally cost temperature than required. Always consult the manufac- more than powdered iron core inductors with similar turer if there is any question. 3405afa 8
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