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SC4608EVB Datasheet(PDF) 11 Page - Semtech Corporation |
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SC4608EVB Datasheet(HTML) 11 Page - Semtech Corporation |
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11 / 19 page  11 2006 Semtech Corp. www.semtech.com SC4608 POWER MANAGEMENT Application Information (Cont.) mode, the RMS value of the input capacitor can be cal- culated from: IN 2 OUT IN OUT OMAX ) RMS ( CIN V ) V V ( V I I − ⋅ ⋅ = This current gives the capacitor’s power loss as follows: ) ESR ( CIN ) RMS ( CIN 2 CIN R I P ⋅ = This capacitor’s RMS loss can be a significant part of the total loss in the converter and reduce the overall con- verter efficiency. The input ripple voltage mainly depends on the input capacitor’s ESR and its capacitance for a given load, input voltage and output voltage. Assuming that the input current of the converter is constant, the required input capacitance for a given voltage ripple can be calculated by: ) R I V ( fs ) D 1 ( D I C ) ESR ( CIN OMAX I OMAX IN ⋅ − ∆ ⋅ − ⋅ ⋅ = Where: D = V O/VI , duty ratio and ∆V I = the given input voltage ripple. Because the input capacitor is exposed to the large surge current, attention is needed for the input capacitor. If tantalum capacitors are used at the input side of the converter, one needs to ensure that the RMS and surge ratings are not exceeded. For generic tantalum capaci- tors, it is wise to derate their voltage ratings at a ratio of 2 to protect these input capacitors. Boost Capacitor Selection The boost capacitor selection is based on its discharge ripple voltage, worst case conduction time and boost current. The worst case conduction time T w can be esti- mated as follows: max s D f 1 Tw ⋅ = Where: f s = the switching frequency and Dmax = maximum duty ratio. The required minimum capacitance for boost capacitor will be: W D B boost T V I C ⋅ = Where: I B = the boost current and V D= discharge ripple voltage. With f s = 300kH, VD=0.3V and IB = 50mA, the required capacitance for the boost capacitor is: nF 528 95 . 0 k 300 1 3 . 0 05 . 0 D f 1 V I C max s D B boost = ⋅ ⋅ = ⋅ ⋅ = Power MOSFET Selection The SC4608 can drive an N-MOSFET at the high side and an N-MOSFET synchronous rectifier at the low side. The use of the high side N-MOSFET will significantly re- duce its conduction loss for high current. For the top MOSFET, its total power loss includes its conduction loss, switching loss, gate charge loss, output capacitance loss and the loss related to the reverse recovery of the bot- tom diode, shown as follows: s I rr OSS s GATE GT 2 GS GD G GATE s I PEAK _ TOP ON _ TOP RMS _ TOP 2 TOTAL _ TOP f V ) Q Q ( f V Q ) Q Q ( R V f V I R I P ⋅ ⋅ + + ⋅ ⋅ + + ⋅ ⋅ ⋅ + ⋅ = Where: R G = gate drive resistor, Q GD = the gate to drain charge of the top MOSFET, Q GS2 = the gate to source charge of the top MOSFET, Q GT = the total gate charge of the top MOSFET, Q OSS = the output charge of the top MOSFET and Q rr = the reverse recovery charge of the bottom diode. For the top MOSFET, it experiences high current and high voltage overlap during each on/off transition. But for the bottom MOSFET, its switching voltage is the bottom diode’s forward drop during its on/off transition. So the switching loss for the bottom MOSFET is negligible. Its total power loss can be determined by: F AVG D s GATE GB ON _ BOT RMS _ BOT 2 TOTAL _ BOT V _ I f V Q R I P ⋅ + ⋅ ⋅ + ⋅ = Where: Q GB = the total gate charge of the bottom MOSFET and V F = the forward voltage drop of the bottom diode. |
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