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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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LTC1474 Fiches technique(PDF) 9 Page - Linear Technology |
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LTC1474 Fiches technique(HTML) 9 Page - Linear Technology |
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9 / 20 page  9 LTC1474/LTC1475 APPLICATIONS INFORMATION CIN and COUT Selection At higher load currents, when the inductor current is continuous, the source current of the P-channel MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum capacitor current is given by: C VV V V IN OUT IN OUT IN Required I = I RMS MAX − () []12/ This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is com- monly used for design because even significant deviations do not offer much relief. Note that capacitor manufacturer’s ripple current ratings are often based on 2000 hours of life. This makes it advisable to further derate the capacitor, or to choose a capacitor rated at a higher temperature than required. Do not underspecify this component. An addi- tional 0.1 µF ceramic capacitor is also required on VIN for high frequency decoupling. The selection of COUT is driven by the required effective series resistance (ESR) to meet the output voltage ripple and line regulation requirements. The output voltage ripple during a burst cycle is dominated by the output capacitor ESR and can be estimated from the following relation: 25mV < ∆VOUT, RIPPLE = ∆IL • ESR where ∆IL≤ IPEAK and the lower limit of 25mV is due to the voltage comparator hysteresis. Line regulation can also vary with COUT ESR in applications with a large input voltage range and high peak currents. ESR is a direct function of the volume of the capacitor. Manufacturers such as Nichicon, AVX and Sprague should be considered for high performance capacitors. The OS-CON semiconductor dielectric capacitor available from SANYO has the lowest ESR for its size at a somewhat higher price. Typically, once the ESR requirement is satis- fied, the capacitance is adequate for filtering. For lower current applications with peak currents less than 50mA, 10 µF ceramic capacitors provide adequate filtering and are a good choice due to their small size and almost negligible ESR. AVX and Marcon are good sources for these capacitors. In surface mount applications multiple capacitors may have to be paralleled to meet the ESR or RMS current handling requirements of the application. Aluminum elec- trolytic and dry tantalum capacitors are both available in surface mount configurations. In the case of tantalum, it is critical that the capacitors are surge tested for use in switching power supplies. An excellent choice is the AVX TPS series of surface mount tantalums, available in case heights ranging from 2mm to 4mm. Other capacitor types include SANYO OS-CON, Nichicon PL series and Sprague 595D series. Consult the manufacturer for other specific recommendations. To avoid overheating, the output capacitor must be sized to handle the ripple current generated by the inductor. The worst-case ripple current in the output capacitor is given by: IRMS = IPEAK/2 Once the ESR requirement for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement. Efficiency Considerations The efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting efficiency and which change would produce the most improvement. Efficiency can be expressed as: Efficiency = 100% – (L1 + L2 + L3 + ...) where L1, L2, etc. are the individual losses as a percentage of input power. Although all dissipative elements in the circuit produce losses, three main sources usually account for most of the losses in LTC1474/LTC1475 circuits: VIN current, I2R losses and catch diode losses. 1. The VIN current is due to two components: the DC bias current and the internal P-channel switch gate charge current. The DC bias current is 9 µA at no load and increases proportionally with load up to a constant 100 µA during continuous mode. This bias current is so |
Numéro de pièce similaire - LTC1474 |
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Description similaire - LTC1474 |
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