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LM3670 Datasheet(PDF) 13 Page - National Semiconductor (TI) |
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LM3670 Datasheet(HTML) 13 Page - National Semiconductor (TI) |
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13 / 17 page  Application Information (Continued) noise filtering of the input voltage spikes due to this rapidly changing current. Select an input filter capacitor with a surge current rating sufficient for the power-up surge from the input power source. The power-up surge current is approximately the capacitor’s value (µF) times the voltage rise rate (V/µs). The input current ripple can be calculated as: TABLE 1. Suggested Inductors and Their Suppliers Model Vendor Phone FAX IDC2512NB100M Vishay 408-727-2500 408-330-4098 DO1608C-103 Coilcraft 847-639-6400 847-639-1469 ELL6RH100M Panasonic 714-373-7366 714-373-7323 CDRH5D18-100 Sumida 847-956-0666 847-956-0702 OUTPUT CAPACITOR SELECTION The output filter capacitor smoothes out current flow from the inductor to the load, maintaining a steady output voltage during transient load changes and reduces output voltage ripple. These capacitors must be selected with sufficient capacitance and sufficiently low ESR to perform these func- tions. The output ripple current can be calculated as: Voltage peak-to-peak ripple due to capacitance = Voltage peak-to-peak ripple due to ESR = Voltage peak-to-peak ripple, root mean squared = Note that the output ripple is dependent on the current ripple and the equivalent series resistance of the output capacitor (R ESR). Because these two components are out of phase the rms value is used. The R ESR is frequency dependent (as well as temperature dependent); make sure the frequency of the R ESR given is the same order of magnitude as the switching frequency. TABLE 2. Suggested Capacitors and Their Suppliers Model Type Vendor Phone FAX 10 µF for C OUT VJ1812V106MXJAT Ceramic Vishay 408-727-2500 408-330-4098 LMK432BJ106MM Ceramic Taiyo-Yuden 847-925-0888 847-925-0899 JMK325BJ106MM Ceramic Taiyo-Yuden 847-925-0888 847-925-0899 4.7 µF for C IN VJ1812V475MXJAT Ceramic Vishay 408-727-2500 408-330-4098 EMK325BJ475MN Ceramic Taiyo-Yuden 847-925-0888 847-925-0899 C3216X5R0J475M Ceramic TDK 847-803-6100 847-803-6296 TABLE 3. Adjustable LM3670 Configurations for Various V OUT VOUT (V) R1 (K Ω)R2 (KΩ) C1 (pF) C2 (pF) L (µH) CIN (µF) COUT (µF) 0.7 80.6 200 200 150 4.7 4.7 10 0.8 120 200 130 none 4.7 4.7 10 0.9 160 200 100 none 4.7 4.7 10 1.0 200 200 82 none 4.7 4.7 10 www.national.com 13 |
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