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LTC3412 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3412 Datasheet(HTML) 10 Page - Linear Technology |
10 / 28 page 10 LT1766/LT1766-5 1766fa or not the inductor must withstand continuous fault conditions. If maximum load current is 0.5A, for instance, a 0.5A inductor may not survive a continuous 2A overload condi- tion. Dead shorts will actually be more gentle on the inductor because the LT1766 has frequency and current limit foldback. Peak switch and inductor current can be significantly higher than output current, especially with smaller induc- tors and lighter loads, so don’t omit this step. Powdered iron cores are forgiving because they saturate softly, whereas ferrite cores saturate abruptly. Other core APPLICATIO S I FOR ATIO where: ESR = equivalent series resistance of the output capacitor ESL = equivalent series inductance of the output capacitor dI/dt = slew rate of inductor ripple current = VIN/L Peak-to-peak ripple current (ILP-P) through the inductor and into the output capacitor is typically chosen to be between 20% and 40% of the maximum load current. It is approximated by: I VV V Vf L LP P OUT IN OUT IN - = ()( ) ()( )( ) – Example: with VIN = 40V, VOUT = 5V, L = 47µH, ESR = 0.1Ω and ESL = 10nH, output ripple voltage can be approxi- mated as follows: IA dI dt VA mV RIPPLE P-P P-P = () − () () ()( ) = == = ()( )+ ()( )() =+= − − − 540 5 40 47 10 200 10 0 465 40 47 10 10 0 85 0 465 0 1 10 10 10 0 85 0 0465 0 0085 55 63 6 6 96 •• . • •. .. • . .. To reduce output ripple voltage further requires an in- crease in the inductor value or a reduction in the capacitor ESR. The latter can effect loop stability since the ESR forms a useful zero in the overall loop response. Typically the inductor value is adjusted with the trade-off being a physically larger inductor with the possibility of increased component height and cost. Choosing a smaller inductor with lighter loads may result in discontinuous operation but the LT1766 is designed to work well in both continu- ous or discontinuous mode. Peak Inductor Current and Fault Current To ensure that the inductor will not saturate, the peak inductor current should be calculated knowing the maxi- mum load current. An appropriate inductor should then be chosen. In addition, a decision should be made whether Table 2 VENDOR/ VALUE IDC DCR HEIGHT PART NO. ( µH) (Amps) (Ohms) (mm) Coiltronics CTX15-1P 15 1.4 0.087 4.2 CTX15-1 15 1.1 0.08 4.2 CTX33-2P 33 1.3 0.126 6 CTX33-2 33 1.4 0.106 6 UP2-330 33 2.4 0.099 5.9 UP2-470 47 1.9 0.146 5.9 UP2-680 68 1.7 0.19 5.9 UP2-101 100 1.4 0.277 5.9 Sumida CDRH6D28-150M 15 1.4 0.076 3 CDRH6D38-150M 15 1.6 0.062 4 CDRH6D28-330M 33 0.97 0.122 3 CDRH104R-330M 33 2.1 0.069 3.8 CDRH125-330M 33 2.1 0.044 6 CDRH104R-470M 47 2.1 0.095 3.8 CDRH125-470M 47 1.8 0.058 6 CDRH6D38-680M 68 0.75 0.173 4 CDRH104R-680M 68 1.5 0.158 3.8 CDRH125-680M 68 1.5 0.093 6 CDRH104R-101M 100 1.35 0.225 3.8 CDRH125-101M 100 1.3 0.120 6 Coilcraft DT3316P-153 15 1.8 0.06 5 DT3316P-333 33 1.3 0.09 5 DT3316P-473 47 1 0.11 5 |
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