LTC3547 UUWUAPPLICATIO S I FOR ATIO A general LTC3547 application circuit is shown in Inductor Core Selection Figure 1. External component selection is driven by the Different core materials and shapes will change the load requirement, and begins with the selection of the size/current and price/current relationship of an induc- inductor L. Once the inductor is chosen, CIN and COUT tor. Toroid or shielded pot cores in ferrite or permalloy can be selected. materials are small and do not radiate much energy, but generally cost more than powdered iron core inductors Inductor Selection with similar electrical characteristics. The choice of which Although the inductor does not infl uence the operat- style inductor to use often depends more on the price vs ing frequency, the inductor value has a direct effect on size requirements, and any radiated fi eld/EMI requirements, ripple current. The inductor ripple current ΔIL decreases than on what the LTC3547 requires to operate. Table 1 with higher inductance and increases with higher VIN shows some typical surface mount inductors that work or VOUT: well in LTC3547 applications. ⎛ ⎞ ∆ V V I OUT OUT L = • 1− Table 1. Representative Surface Mount Inductors f L ⎝⎜ V ⎠⎟ • O IN (1) MANU-MAX DCFACTURERPART NUMBERVALUECURRENTDCRHEIGHT Accepting larger values of ΔIL allows the use of low Taiyo Yuden CB2016T2R2M 2.2µH 510mA 0.13Ω 1.6mm inductances, but results in higher output voltage ripple, CB2012T2R2M 2.2µH 530mA 0.33Ω 1.25mm greater core losses, and lower output current capability. CB2016T3R3M 3.3µH 410mA 0.27Ω 1.6mm A reasonable starting point for setting ripple current Panasonic ELT5KT4R7M 4.7µH 950mA 0.2Ω 1.2mm is 40% of the maximum output load current. So, for a Sumida CDRH2D18/LD 4.7µH 630mA 0.086Ω 2mm 300mA regulator, ΔIL = 120mA (40% of 300mA). Murata LQH32CN4R7M23 4.7µH 450mA 0.2Ω 2mm Taiyo Yuden NR30102R2M 2.2µH 1100mA 0.1 1mm The inductor value will also have an effect on Burst Mode Ω NR30104R7M 4.7µH 750mA 0.19Ω 1mm operation. The transition to low current operation begins FDK FDKMIPF2520D 4.7µH 1100mA 0.11Ω 1mm when the peak inductor current falls below a level set by FDKMIPF2520D 3.3µH 1200mA 0.1Ω 1mm the internal burst clamp. Lower inductor values result in FDKMIPF2520D 2.2µH 1300mA 0.08Ω 1mm higher ripple current which causes the transition to occur TDK VLF3010AT4R7- 4.7µH 700mA 0.24Ω 1mm MR70 at lower load currents. This causes a dip in effi ciency in VLF3010AT3R3- 3.3µH 870mA 0.17Ω 1mm the upper range of low current operation. Furthermore, MR87 lower inductance values will cause the bursts to occur VLF3010AT2R2- 2.2µH 1000mA 0.12Ω 1mm M1RD with increased frequency. VIN 2.5V TO 5.5V C1 RUN2 VIN RUN1 LTC3547 L2 L1 VOUT2 SW2 SW1 VOUT1 CF2 CF1 VFB2 VFB1 R4 GND R2 COUT2 R3 R1 COUT1 3547 F01 Figure 1. LTC3547 General Schematic 3547fa 8
