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LM3354MMX-1.8 Folha de dados(PDF) 8 Page - National Semiconductor (TI) |
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LM3354MMX-1.8 Folha de dados(HTML) 8 Page - National Semiconductor (TI) |
8 / 12 page Applications Information Operating Principle The LM3354 is designed to provide a step-up/step-down voltage regulation in battery powered systems. It combines switched capacitor circuitry, reference, comparator, and shutdown logic in a single 10-pin MSOP package. The LM3354 can provide a regulated voltage between 1.8V and 5.0V from an input voltage between 2.5V and 5.5V. It can supply a load current up to 90 mA (refer to Electrical Char- acteristics). As shown in Figure 1, the LM3354 employs two feedback loops to provide regulation in the most efficient manner possible. The first loop is from V OUT through the comparator COMP, the AND gate G 1, the phase generator, and the switch array. The comparator’s output is high when V OUT is less than the reference V REF. Regulation is provided by gating the clock to the switch array. In this manner, charge is transferred to the output only when needed. The second loop controls the gain configuration of the switch array. This loop consists of the comparator, the digital control block, the phase generator, and the switch array. The digital control block computes the most efficient gain from a set of five gains based on inputs from the A/D and the comparator. The gain signal is sent to the phase generator which then sends the appropriate timing and configuration signals to the switch array. This dual loop provides regulation over a wide range of loads efficiently. Since efficiency is automatically optimized, the curves for V OUT vs. VIN and Efficiency vs. VIN in the Typical Perfor- mance Characteristics section exhibit small variations. The reason is that as input voltage or output load changes, the digital control loops are making decisions on how to optimize efficiency. As the switch array is reconfigured, small varia- tions in output voltage and efficiency result. In all cases where these small variations are observed, the part is oper- ating correctly; minimizing output voltage changes and opti- mizing efficiency. Charge Pump Capacitor Selection A 0.33 µF ceramic capacitor is suggested for C1 and C2. To ensure proper operation over temperature variations, an X7R dielectric material is recommended. Filter Capacitor Selection a) CAPACITOR TECHNOLOGIES The three major technologies of capacitors that can be used as filter capacitors for LM3354 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 ceramic and the tantalum technologies. They provide very low ESR values while still achieving high capacitance val- ues. 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 (com- DS200188-3 FIGURE 1. Block Diagram www.national.com 8 |
Nº de peça semelhante - LM3354MMX-1.8 |
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Descrição semelhante - LM3354MMX-1.8 |
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