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LM2853MH-0.8 Folha de dados(PDF) 9 Page - National Semiconductor (TI) |
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LM2853MH-0.8 Folha de dados(HTML) 9 Page - National Semiconductor (TI) |
9 / 12 page Applications Information (Continued) CHOOSING AN INDUCTANCE VALUE The current ripple present in the output filter inductor is determined by the input voltage, output voltage, switching frequency and inductance according to the following equa- tion: where ∆I L is the peak to peak current ripple, D is the duty cycle V OUT/VIN,VIN is the input voltage applied to the output stage, V OUT is the output voltage of the switcher, f is the switching frequency and L O is the inductance of the output filter inductor. Knowing the current ripple is important for inductor selection since the peak current through the induc- tor is the load current plus one half the ripple current. Care must be taken to ensure the peak inductor current does not reach a level high enough to trip the current limit circuitry of the LM2853. As an example, consider a 5V to 1.2V conver- sion and a 550 kHz switching frequency. According to Table 1, a 4.7 µH inductor may be used. Calculating the expected peak-to-peak ripple, The maximum inductor current for a 3A load would therefore be 3A plus 177 mA, 3.177A. As shown in the ripple equation, the current ripple is inversely proportional to inductance. OUTPUT FILTER INDUCTORS Once the inductance value is chosen, the key parameter for selecting the output filter inductor is its saturation current (I SAT) specification. Typically ISAT is given by the manufac- turer as the current at which the inductance of the coil falls to a certain percentage of the nominal inductance. The I SAT of an inductor used in an application should be greater than the maximum expected inductor current to avoid saturation. Be- low is a table of inductors that are suitable in LM2853 applications. TABLE 2. Recommended Inductors Inductance Part Number Vendor 4.7 µF DO3308P-472ML Coilcraft 4.7 µF DO3316P-472ML Coilcraft 4.7 µF MSS1260-472ML Coilcraft 5.2 µF MSS1038-522NL Coilcraft 5.6 µF MSS1260-562ML Coilcraft 6.8 µF DO3316P-682ML Coilcraft 6.8 µF MSS1260-682ML Coilcraft OUTPUT FILTER CAPACITORS The recommended capacitors that may be used in the output filter with the LM2853 are limited in value and ESR range according to Table 1. Below are some examples of capacitors that can typically be used in an LM2853 application. TABLE 3. Recommended Capacitors Capacitance (µF) Part Number Chemistry Vendor 100 594D107X_010C2T Tantalum Vishay-Sprague 100 593D107X_010D2_E3 Tantalum Vishay-Sprague 100 TPSC107M006#0075 Tantalum AVX 100 NOSD107M006#0080 Niobium Oxide AVX 100 NOSC107M004#0070 Niobium Oxide AVX 120 594D127X_6R3C2T Tantalum Vishay-Sprague 150 594D157X_010C2T Tantalum Vishay-Sprague 150 595D157X_010D2T Tantalum Vishay-Sprague 150 591D157X_6R3C2_20H Tantalum Vishay-Sprague 150 TPSD157M006#0050 Tantalum AVX 150 TPSC157M004#0070 Tantalum AVX 150 NOSD157M006#0070 Niobium Oxide AVX 220 594D227X_6R3D2T Tantalum Vishay-Sprague 220 591D227X_6R3D2_20H Tantalum Vishay-Sprague 220 591D227X_010D2_20H Tantalum Vishay-Sprague 220 593D227X_6R3D2_E3 Tantalum Vishay-Sprague www.national.com 9 |
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