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SC4603IMSTRT Datasheet(PDF) 10 Page - Semtech Corporation |
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SC4603IMSTRT Datasheet(HTML) 10 Page - Semtech Corporation |
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10 / 16 page  10 2004 Semtech Corp. www.semtech.com SC4603 POWER MANAGEMENT Applications Information - (Cont.) during a load transient, the output capacitor delivers all the additional current needed by the load. The ESR and ESL of the output capacitor, the loop parasitic inductance between the output capacitor and the load combined with inductor ripple current are all major contributors to the output voltage ripple. Surface mount speciality poly- mer aluminum electrolytic chip capacitors in UE series from Panasonic provide low ESR and reduce the total capacitance required for a fast transient response. POSCAP from Sanyo is a solid electrolytic chip capacitor which has a low ESR and good performance for high fre- quency with a low profile and high capacitance. Above mentioned capacitors are recommended to use in SC4603 applications. Input Capacitor Selection The input capacitor selection is based on its ripple cur- rent level, required capacitance and voltage rating. This capacitor must be able to provide the ripple current by the switching actions. For the continuous conduction mode, the RMS value of the input capacitor can be cal- culated from: 2 in 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 converter efficiency. The input ripple voltage mainly de- pends on the input capacitor’s ESR and its capacitance for a given load, input voltage and output voltage. As- suming that the input current of the converter is con- stant, 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. Power MOSFET Selection The SC4603 can drive a P-MOSFET at the high side and an N-MOSFET synchronous rectifier at the low side. The use of the high side P-MOSFET eliminates the need for an external charge pump and simplifies the high side gate driver circuit. For the top MOSFET, its total power loss includes its con- duction loss, switching loss, gate charge loss, output ca- pacitance loss and the loss related to the reverse recovery of the bottom 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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