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LM3354MMX-3.3 Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM3354MMX-3.3 Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 12 page Filter Capacitor Selection (Continued) parable to ceramics) and good temperature stability (compa- rable to tantalums). The Aluminum Polymer Electrolytics offered by Cornell-Dubilier and Panasonic, and the POS- CAPs offered by Sanyo fall under this category. Table 1 compares the features of the three capacitor tech- nologies. TABLE 1. Comparison of Capacitor Technologies Ceramic Tantalum Polymer Electrolytic ESR Lowest High Low Relative Height Low for Small Values (<10 µF); Taller for Higher Values Lowest Low Relative Footprint Large Small Largest Temperature Stability X7R/X5R-Acceptable Good Good Frequency Stability Good Acceptable Good V OUT Ripple Magnitude @ <50 mA Low High Low V OUT Ripple Magnitude @ >100 mA Low Slightly Higher Low dv/dt of V OUT Ripple @ All Loads Lowest High Low b) CAPACITOR SELECTION i) Output Capacitor (C OUT) The output capacitor C OUT directly affects the magnitude of the output ripple voltage so C OUT should be carefully se- lected. The graphs titled V OUT Ripple vs. COUT in the Typical Performance Characteristics section show how the ripple voltage magnitude is affected by the C OUT value and the capacitor technology. These graphs are taken at the gain at which worst case ripple is observed. In general, the higher the value of C OUT, the lower the output ripple magnitude. At lighter loads, the low ESR ceramics offer a much lower V OUT ripple than the higher ESR tantalums of the same value. At higher loads, the ceramics offer a slightly lower V OUT ripple magnitude than the tantalums of the same value. However, the dv/dt of the V OUT ripple with the ceramics and polymer electrolytics is much lower than the tantalums under all load conditions. The tantalums are suggested for very low profile, small size applications. The ceramics and polymer electro- lytics are a good choice for low ripple, low noise applications where size is less of a concern. ii) Input Capacitor (C IN) The input capacitor C IN directly affects the magnitude of the input ripple voltage, and to a lesser degree the V OUT ripple. A higher value C IN will give a lower VIN ripple. To optimize low input and output ripple as well as size a 10 µF polymer electrolytic or ceramic, or 15 µF tantalum capacitor is rec- ommended. This will ensure low input ripple at 90 mA load current. If lower currents will be used or higher input ripple can be tolerated then a smaller capacitor may be used to reduce the overall size of the circuit. The lower ESR ceram- ics and polymer electrolytics achieve a lower V IN ripple than the higher ESR tantalums of the same value. Tantalums make a good choice for small size, very low profile applica- tions. The ceramics and polymer electrolytics are a good choice for low ripple, low noise applications where size is less of a concern. The 10 µF polymer electrolytics are physi- cally much larger than the 15 µF tantalums and 10 µF ceramics. iii) C FIL A 1 µF, X7R ceramic capacitor should be connected to pin C FIL. This capacitor provides the filtering needed for the internal supply rail of the LM3354. Of the different capacitor technologies, a sample of vendors that have been verified as suitable for use with the LM3354 are shown in Table 2. TABLE 2. Capacitor Vendor Information Manufacturer Tel Fax Website Ceramic Taiyo-yuden (408) 573-4150 (408) 573-4159 www.t-yuden.com AVX (803) 448-9411 (803) 448-1943 www.avxcorp.com Sprague/Vishay (207) 324-4140 (207) 324-7223 www.vishay.com Tantalum Nichicon (847) 843-7500 (847) 843-2798 www.nichicon.com Polymer Electrolytic Cornell-Dubilier (ESRD) (508) 996-8561 (508) 996-3830 www.cornell-dubilier.com Sanyo (POSCAP) (619) 661-6322 (619) 661-1055 www.sanyovideo.com www.national.com 9 |
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