LTC3417A-2 APPLICATIONS INFORMATION The inductor value will also have an effect on Burst Mode Input Capacitor (CIN) Selection operation. The transition from low current operation begins In continuous mode, the input current of the converter can when the peak inductor current falls below a level set by the be approximated by the sum of two square waves with burst clamp. Lower inductor values result in higher ripple duty cycles of approximately V current which causes this to occur at lower load currents. OUT1/VIN and VOUT2/VIN. To prevent large voltage transients, a low equivalent series This causes a dip in effi ciency in the upper range of low resistance (ESR) input capacitor sized for the maximum current operation. In Burst Mode operation, lower inductor RMS current must be used. Some capacitors have a values will cause the burst frequency to increase. de-rating spec for maximum RMS current. If the capaci- tor being used has this requirement, it is necessary to Inductor Core Selection calculate the maximum RMS current. The RMS current Different core materials and shapes will change the size/ calculation is different if the part is used in “in phase” or current relationship of an inductor. Toroid or shielded “out of phase”. pot cores in ferrite or permalloy materials are small and For “in phase”, there are two different equations: don’t radiate much energy, but generally cost more than powdered iron core inductors with similar electrical VOUT1 > VOUT2: characteristics. The choice of which style inductor to use 2 2 often depends more on the price vs size requirements IRMS = 2 •I1 •I2 •D2(1–D1)+I2 (D2 –D22)+I1 (D1–D12) of any radiated fi eld/EMI requirements than on what the LTC3417A-2 requires to operate. Table 1 shows some VOUT2 > VOUT1: typical surface mount inductors that work well in 2 2 LTC3417A-2 applications. IRMS = 2 •I1 •I2 •D1(1–D2)+I2 (D2 –D22)+I1 (D1–D12) where. V V D1= OUT1 and D2 = OUT2 VIN VIN Table 1MANUFACTURERPART NUMBERVALUE (μH)MAX DC CURRENT (A)DCRDIMENSIONS L × W × H (mm)L1 on OUT1 Toko A920CY-1R5M-D62CB 1.5 2.8 0.014 6 × 6 × 2.5 A918CY-1R5M-D62LCB 1.5 2.9 0.018 6 × 6 × 2 Coilcraft D01608C-152ML 1.5 2.6 0.06 6.6 × 4.5 × 2.9 Sumida CDRH4D22/HP 1R5 1.5 3.9 0.031 5 × 5 × 2.4 Midcom DUP-1813-1R4R 1.4 5.5 0.033 4.3 × 4.8 × 3.5 L2 on OUT2 Toko A915AY-2ROM-D53LC 2.0 3.9 0.027 5 × 5 × 3 Coilcraft D01608C-222ML 2.2 2.3 0.07 6.6 × 4.5 × 2.9 Sumida CDRH3D16/HP 2R2 2.2 1.75 0.047 4 × 4 × 1.8 2.2 1.6 0.035 3.2 × 3.2 × 2 Midcom DUP-1813-2R2R 2.2 3.9 0.047 4.3 × 4.8 × 3.5 3417a2fa 10
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