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LTC3417AEDHC-1-TRPBF Datasheet(PDF) 10 Page - Linear Technology |
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LTC3417AEDHC-1-TRPBF Datasheet(HTML) 10 Page - Linear Technology |
10 / 20 page LTC3417A-1 10 3417a1fa A reasonable starting point for setting ripple current is ΔIL = 0.35ILOAD(MAX), where ILOAD(MAX) is the maximum current output. The largest ripple, ΔIL, occurs at the maxi- mum input voltage. To guarantee that the ripple current stays below a specified maximum, the inductor value should be chosen according to the following equation: L = VOUT fO • IL 1– VOUT VIN(MAX) The inductor value will also have an effect on Burst Mode operation. The transition from low current operation begins when the peak inductor current falls below a level set by the burst clamp. Lower inductor values result in higher ripple current which causes this to occur at lower load currents. This causes a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower inductor values will cause the burst frequency to increase. Inductor Core Selection Different core materials and shapes will change the size/ current relationship of an inductor. Toroid 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 electrical characteristics. The choice of which style inductor to use often depends more on the price vs size requirements of any radiated field/EMI requirements than on what the LTC3417A-1 requires to operate. Table 1 shows some typical surface mount inductors that work well in LTC3417A-1 applications. Input Capacitor (CIN) Selection In continuous mode, the input current of the converter can be approximated by the sum of two square waves with duty cycles of approximately VOUT1/VIN and VOUT2/VIN. To prevent large voltage transients, a low equivalent series resistance (ESR) input capacitor sized for the maximum RMS current must be used. Some capacitors have a de-rating spec for maximum RMS current. If the capaci- tor being used has this requirement, it is necessary to calculate the maximum RMS current. The RMS current calculation is different if the part is used in “in phase” or “out of phase”. For “in phase”, there are two different equations: VOUT1 > VOUT2: VOUT2 > VOUT1: IRMS = 2•I1 •I2 •D1(1–D2)+I2 2(D2 –D22)+I 1 2(D1–D12) where: D1= VOUT1 VIN and D2 = VOUT2 VIN Table 1 MANUFACTURER PART NUMBER VALUE (μH) MAX DC CURRENT (A) DCR DIMENSIONS L × W × H (mm) L1 on OT1 Toko A920CY-1R5M-D62CB A918CY-1R5M-D62LCB 1.5 1.5 2.8 2.9 0.014 0.018 6 × 6 × 2.5 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 2.2 1.75 1.6 0.047 0.035 4 × 4 × 1.8 3.2 × 3.2 × 2 Midcom DUP-1813-2R2R 2.2 3.9 0.047 4.3 × 4.8 × 3.5 APPLICATIONS INFORMATION IRMS = 2•I1 •I2 •D2(1–D1)+I2 2(D2 –D22)+I 1 2(D1–D12) |
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