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LM3352MTCX-3.0 Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LM3352MTCX-3.0 Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 11 page Filter Capacitor Selection a) CAPACITOR TECHNOLOGIES The three major technologies of capacitors that can be used as filter capacitors for LM3352 are: i) tantalum, ii) ceramic and iii) polymer electrolytic technologies. i) Tantalum Tantalum capacitors are widely used in switching regulators. Tantalum capacitors have the highest CV rating of any tech- nology; as a result, high values of capacitance can be ob- tained in relatively small package sizes. It is also possible to obtain high value tantalum capacitors in very low profile (<1.2 mm) packages. This makes the tantalums attractive for low-profile, small size applications. Tantalums also pos- sess very good temperature stability; i.e., the change in the capacitance value, and impedance over temperature is rela- tively small. However, the tantalum capacitors have relatively high ESR values which can lead to higher voltage ripple and their frequency stability (variation over frequency) is not very good, especially at high frequencies (>1 MHz). ii) Ceramic Ceramic capacitors have the lowest ESR of the three tech- nologies and their frequency stability is exceptionally good. These characteristics make the ceramics an attractive choice for low ripple, high frequency applications. However, the temperature stability of the ceramics is bad, except for the X7R and X5R dielectric types. High capacitance values (>1 µF) are achievable from companies such as Taiyo-yuden which are suitable for use with regulators. Ce- ramics are taller and larger than the tantalums of the same capacitance value. iii) Polymer Electrolytic Polymer electrolytic is a third suitable technology. Polymer capacitors provide some of the best features of both the ce- ramic and the tantalum technologies. They provide very low ESR values while still achieving high capacitance values. However, their ESR is still higher than the ceramics, and their capacitance value is lower than the tantalums of the same size. Polymers offer good frequency stability (compa- rable to ceramics) and good temperature stability (compa- rable to tantalums). The Aluminum Polymer Electrolytics of- fered by Cornell-Dubilier and Panasonic, and the POSCAPs 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 ca- pacitor 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 electrolyt- ics 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 15 µF polymer electrolytic, 22 µF ceramic, or 33 µF tantalum capacitor is recommended. This will ensure low input ripple at 200 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 ce- ramics 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 15 µF polymer electrolytics are physi- cally much larger than the 33 µF tantalums and 22 µF ceram- ics. www.national.com 8 |
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