Datasheet LTC3417A-1 (Analog Devices) - 10

LTC3417A-1
APPLICATIONS INFORMATION
A reasonable starting point for setting ripple current is typical surface mount inductors that work well in ΔIL = 0.35ILOAD(MAX), where ILOAD(MAX) is the maximum LTC3417A-1 applications. current output. The largest ripple, ΔIL, occurs at the maxi- mum input voltage. To guarantee that the ripple current
Input Capacitor (CIN) Selection
stays below a specifi ed maximum, the inductor value In continuous mode, the input current of the converter can should be chosen according to the following equation: be approximated by the sum of two square waves with V duty cycles of approximately VOUT1/VIN and VOUT2/VIN. To L = VOUT 1– OUT prevent large voltage transients, a low equivalent series fO • IL V IN(MAX) resistance (ESR) input capacitor sized for the maximum RMS current must be used. Some capacitors have a The inductor value will also have an effect on Burst Mode de-rating spec for maximum RMS current. If the capaci- operation. The transition from low current operation begins tor being used has this requirement, it is necessary to when the peak inductor current falls below a level set by the calculate the maximum RMS current. The RMS current burst clamp. Lower inductor values result in higher ripple calculation is different if the part is used in “in phase” or current which causes this to occur at lower load currents. “out of phase”. This causes a dip in effi ciency in the upper range of low current operation. In Burst Mode operation, lower inductor For “in phase”, there are two different equations: values will cause the burst frequency to increase. VOUT1 > VOUT2:
Inductor Core Selection
I 2(D2 –D22) 2(D1–D12) RMS = 2 •I1 •I2 •D2(1– D1)+I2 +I1 Different core materials and shapes will change the size/ current relationship of an inductor. Toroid or shielded VOUT2 > VOUT1: pot cores in ferrite or permalloy materials are small and 2 2 don’t radiate much energy, but generally cost more than I (D2 – D22) (D1– D12) RMS = 2 •I1 •I2 •D1(1– D2)+I2 +I1 powdered iron core inductors with similar electrical characteristics. The choice of which style inductor to use where: often depends more on the price vs size requirements VOUT1 VOUT2 of any radiated fi eld/EMI requirements than on what the D1= and D2 = V V LTC3417A-1 requires to operate. Table 1 shows some IN IN
Table 1 MANUFACTURER PART NUMBER VALUE (μH) MAX DC CURRENT (A) DCR DIMENSIONS L × W × H (mm) L1 on OT1
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 3417a1fa 10