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LM2771 Datenblatt(PDF) 8 Page - National Semiconductor (TI) |
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LM2771 Datenblatt(HTML) 8 Page - National Semiconductor (TI) |
8 / 10 page Application Information (Continued) Net capacitance of a ceramic capacitor decreases with in- creased DC bias. This degradation can result in lower ca- pacitance than expected on the input and/or output, resulting in higher ripple voltages and currents. Using capacitors at DC bias voltages significantly below the capacitor voltage rating will usually minimize DC bias effects. Consult capaci- tor manufacturers for information on capacitor DC bias char- acteristics. Capacitance characteristics can vary quite dramatically with different application conditions, capacitor types, and capaci- tor manufacturers. It is strongly recommended that the LM2771 circuit be thoroughly evaluated early in the design-in process with the mass-production capacitors of choice. This will help ensure that any such variability in capacitance does not negatively impact circuit performance. The table below lists some leading ceramic capacitor manu- facturers. Manufacturer Contact Information AVX www.avx.com Murata www.murata.com Taiyo-Yuden www.t-yuden.com TDK www.component.tdk.com Vishay-Vitramon www.vishay.com OUTPUT CAPACITOR AND OUTPUT VOLTAGE RIPPLE The output capacitor in the LM2771 circuit (C OUT) directly impacts the magnitude of output voltage ripple. Other promi- nent factors also affecting output voltage ripple include input voltage, output current and flying capacitance. Due to the complexity of the regulation topology, providing equations or models to approximate the magnitude of the ripple can not be easily accomplished. But one important generalization can be made: increasing (decreasing) the output capaci- tance will result in a proportional decrease (increase) in output voltage ripple. In typical high-current applications, a 4.7µF low-ESR ce- ramic output capacitor is recommended. Different output capacitance values can be used to reduce ripple, shrink the solution size, and/or cut the cost of the solution. But chang- ing the output capacitor may also require changing the flying capacitor and/or input capacitor to maintain good overall circuit performance. Performance of the LM2771 with differ- ent capacitor setups in discussed in the section Recom- mended Capacitor Configurations. High ESR in the output capacitor increases output voltage ripple. If a ceramic capacitor is used at the output, this is usually not a concern because the ESR of a ceramic capaci- tor is typically very low and has only a minimal impact on ripple magnitudes. If a different capacitor type with higher ESR is used (tantalum, for example), the ESR could result in high ripple. To eliminate this effect, the net output ESR can be significantly reduced by placing a low-ESR ceramic ca- pacitor in parallel with the primary output capacitor. The low ESR of the ceramic capacitor will be in parallel with the higher ESR, resulting in a low net ESR based on the prin- ciples of parallel resistance reduction. INPUT CAPACITOR AND INPUT VOLTAGE RIPPLE The input capacitor (C IN) is a reservoir of charge that aids a quick transfer of charge from the supply to the flying capaci- tor during the charge phase of operation. The input capacitor helps to keep the input voltage from drooping at the start of the charge phase when the flying capacitor is connected to the input. It also filters noise on the input pin, keeping this noise out of sensitive internal analog circuitry that is biased off the input line. Much like the relationship between the output capacitance and output voltage ripple, input capacitance has a dominant and first-order effect on input ripple magnitude. Increasing (decreasing) the input capacitance will result in a propor- tional decrease (increase) in input voltage ripple. Input volt- age, output current, and flying capacitance also will affect input ripple levels to some degree. In typical high-current applications, a 1µF low-ESR ceramic capacitor is recommended on the input. Different input ca- pacitance values can be used to reduce ripple, shrink the solution size, and/or cut the cost of the solution. But chang- ing the input capacitor may also require changing the flying capacitor and/or output capacitor to maintain good overall circuit performance. Performance of the LM2771 with differ- ent capacitor setups is discussed below in Recommended Capacitor Configurations. FLYING CAPACITOR The flying capacitor (C 1) transfers charge from the input to the output. Flying capacitance can impact both output cur- rent capability and ripple magnitudes. If flying capacitance is too small, the LM2771 may not be able to regulate the output voltage when load currents are high. On the other hand, if the flying capacitance is too large, the flying capacitor might overwhelm the input and output capacitors, resulting in in- creased input and output ripple. Polarized capacitors (tantalum, aluminum electrolytic, etc.) must not be used for the flying capacitor, as they could become reverse-biased during LM2771 operation. RECOMMENDED CAPACITOR CONFIGURATIONS The data in Table 1 can be used to assist in the selection of a capacitor configuration that best balances solution size and cost with the electrical requirements of the application. As previously discussed, input and output ripple voltages will vary with output current and input voltage. The numbers provided show expected ripple voltage when V IN = 3.6V and load currents are between 10mA and 200mA. The table offers first look at approximate ripple levels and provides a comparison for the different capacitor configurations pre- sented, but is not intended to be a guarantee of perfor- mance. www.national.com 8 |
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