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AME5269 датащи(PDF) 10 Page - Analog Microelectronics |
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AME5269 датащи(HTML) 10 Page - Analog Microelectronics |
10 / 18 page AME 10 AME5269 Rev. B.01 2A, 28V, 340KHz Synchronous Rectified Step-Down Converter n Detailed Description (Contd.) Inductor The inductor is required to supply constant current to the load while being driven by the switched input voltage. A larger value inductor will have a larger physical size, higher series resistance, and lower saturation current. It will result in less ripple current that will in turn result in lower output ripple voltage. Make sure that the peak induc- tor current is below the maximum switch current limit. Determine inductance is to allow the peak-to peak ripple current to be approximately 30% of the maximum switch current limit. The inductance value can be calculated by: Where fs is the switching frequency, V IN is the input voltage, V OUT is the output voltage, and ∆IL is the peak-to- peak inductor ripple current. Choose an inductor that will not saturate under the maximum inductor peak current, calculated by: Where I LOAD is the load current. The choice of which style inductor to use mainly depends on the price vs. size re- quirements and any EMI constraints. Input Capacitor The input current to the step-down converter is discon- tinuous, therefore a capacitor is required to supply the AC current while maintaining the DC input voltage. Use low ESR capacitors for the best performance. Ceramic capaci- tors are preferred, but tantalum or low-ESR electrolytic capacitors will also be suggested. Choose X5R or X7R dielectrics when using ceramic capacitors. Since the input capacitor (C1) absorbs the input switching current, it requires an adequate ripple current rating. The RMS current in the input capacitor can be estimated by: At V IN = 2V OUT, where IC1 = ILOAD/2 is the worst-case condition occurs. For simplification, use an input capaci- tor with a RMS current rating greater than half of the maxi- mum load current. When using ceramic capacitors, make sure that they have enough capacitance to provide suffi- cient charge to prevent excessive voltage ripple at input. When using electrolytic or tantalum capacitors, a high quality, small ceramic capacitor, i.e. 0.1 µF, should be placed as close to the IC as possible. The input voltage ripple for low ESR capacitors can be estimated by: Where C1 is the input capacitance value. Output Capacitor The output capacitor (C2) is required to maintain the DC output voltage. Ceramic, tantalum, or low ESR electrolytic capacitors are recommended. Low ESR capacitors are preferred to keep the output voltage ripple low. The output voltage ripple can be estimated by: Where R ESR is the equivalent series resistance (ESR) value of the output capacitor and C2 is the output capaci- tance value. When using ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance which is the main cause for the output voltage ripple. For simpli- fication, the output voltage ripple can be estimated by: When using tantalum or electrolytic capacitors, the ESR dominates the impedance at the switching frequency. For simplification, the output ripple can be approximated to: The characteristics of the output capacitor also affect the stability of the regulation system. The AME5269 can be optimized for a wide range of capacitance and ESR values. − × × = IN OUT IN OUT LOAD C V V V V I I 1 1 − × ∆ × = IN OUT L s OUT V V I f V L 1 − × × × + = IN OUT s OUT LOAD LP V V L f V I I 1 2 − × × × = ∆ IN OUT IN OUT s LOAD IN V V V V f C I V 1 1 × × + × − × × = ∆ 2 8 1 1 C f R V V L f V V s ESR IN OUT s OUT OUT − × × × × = ∆ IN OUT s OUT OUT V V C L f V V 1 2 8 2 ESR IN OUT s OUT OUT R V V L f V V × − × × = ∆ 1 |
Аналогичный номер детали - AME5269 |
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Аналогичное описание - AME5269 |
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