Datasheet LTC3410 (Analog Devices) - 8

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