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LTC3873-5 Datasheet(PDF) 11 Page - Linear Technology |
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LTC3873-5 Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page LTC3873-5 11 38735fb Input Capacitors The input capacitor of a boost converter is less critical due to the fact that the input current waveform is triangular, and does not contain large square wave currents as found in the output capacitor. The input voltage source impedance determines the size of the capacitor that is typically 10μF to 100μF. A low ESR is recommended although not as critical as the output capacitor can be on the order of 0.3 Ω. The RMS input ripple current for a boost converter is: I V Lf D RMS CIN IN MIN MAX () () .• • • = 03 Please note that the input capacitor can see a very high surge current when a battery is suddenly connected to the input of the converter and solid tantalum capacitors can fail catastrophically under these conditions. In a flyback converter, the input flows in pulses placing severe demands on the input capacitors. Select an input capacitor with a ripple current rating greater than: I P V D D RMS IN IN MIN MAX MAX = () – 1 Duty Cycle Considerations The LTC3873-5 imposes a maximum duty cycle limit of 80% typical. For a flyback converter, the maximum duty cycle prevents the transformer core from saturation. In a boost converter application, however, it sets a limit on the maximum step-up ratio or maximum output voltage with the given input voltage of: V V V OUT MAX IN MIN D () () –. % – = 10 8 Current and voltage stress on the power switch and synchronous rectifiers, input and output capacitor RMS currents and transformer utilization (size vs power) are impacted by duty factor. Unfortunately duty factor can- not be adjusted to simultaneously optimize all of these requirements. In general, avoid extreme duty factors since this severely impacts the current stress on most of the components. A reasonable target for duty factor is 50% at nominal input voltage. Using this rule of thumb, the ideal transformer turns ratio is: N V V D D V V IDEAL OUT IN OUT IN == • – 1 Output Diode Selection To maximize efficiency, a fast switching diode with low forward drop and low reverse leakage is desired. The output diode in a boost converter conducts current during the switch off-time. The peak reverse voltage that the diode must withstand is equal to the regulator output voltage. The average forward current in normal operation is equal to the output current, and the peak current is equal to the peak inductor current. APPLICATIONS INFORMATION |
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