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SC4605 Datasheet(PDF) 9 Page - Semtech Corporation |
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SC4605 Datasheet(HTML) 9 Page - Semtech Corporation |
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9 / 19 page  9 2004 Semtech Corp. www.semtech.com SC4605 POWER MANAGEMENT mode, the RMS value of the input capacitor can be cal- culated from: I 2 O I O 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 = VO/VI , duty ratio and DVI = 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. Power Mosfet Selection The SC4605 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: Applications Information (Cont.) 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: RG = gate drive resistor, QGD = the gate to drain charge of the top MOSFET, QGS2 = the gate to source charge of the top MOSFET, QGT = the total gate charge of the top MOSFET, QOSS = the output charge of the top MOSFET and Qrr = 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: QGB = the total gate charge of the bottom MOSFET and VF = the forward voltage drop of the bottom diode. For a low voltage and high output current application such as the 3.3V/1.5V@12A case, the conduction loss is of- ten dominant and selecting low RDS(ON) MOSFETs will noticeably improve the efficiency of the converter even though they give higher switching losses. The gate charge loss portion of the top/bottom MOSFET’s total power loss is derived from the SC4605. This gate charge loss is based on certain operating conditions (fs, VGATE, and I O). The thermal estimations have to be done for both MOSFETs to make sure that their junction temperatures do not exceed their thermal ratings according to their total power losses PTOTAL, ambient temperature Ta and their thermal resistance R θja as follows: ja TOTAL a (max) j R P T T θ + < |
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