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SC4614 Datasheet(PDF) 7 Page - Semtech Corporation |
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SC4614 Datasheet(HTML) 7 Page - Semtech Corporation |
7 / 14 page 7 2005 Semtech Corp. www.semtech.com POWER MANAGEMENT SC4614 Applications Information (Cont.) termined by the output inductance and output capaci- tance. The values of output inductor and output bulk capacitors have to be properly selected so that the soft start peak current does not exceed the load trip point of the short circuit protection. Internal LDO for Gate Drive Internal LDO for Gate Drive Internal LDO for Gate Drive Internal LDO for Gate Drive Internal LDO for Gate Drive An internal LDO is designed in the SC4614 to lower the 12V supply voltage for gate drive. A 1uF external ce- ramic capacitor connected in between DRV pin to the ground is needed to support the LDO. The LDO output is connected to the low gate drive internally, and has to be connected to the high gate drive through an external bootstrap circuit. The LDO output voltage is set at 8.2V. The manufacture data and bench tested results show that, for low Rdson MOSFETs run at applied load current, the optimum gate drive voltage is around 8.2V, where the total power losses of power MOSFETs are minimized. COMPONENT SELECTION COMPONENT SELECTION COMPONENT SELECTION COMPONENT SELECTION COMPONENT SELECTION General design guideline of switching power supplies can be applied to the component selection for the SC4614. Induct Induct Induct Induct Inductor and MOSFET or and MOSFET or and MOSFET or and MOSFET or and MOSFETsssss The selection of inductor and MOSFETs should meet ther- mal requirements because they are power loss dominant components. Pick an inductor with as high inductance as possible without adding extra cost and size. The higher inductance, the lower ripple current, the smaller core loss and the higher efficiency will be. However, too high in- ductance slows down output transient response. It is rec- ommended to choose the inductance that creates an inductor ripple current of approximate 20% of maximum load current. So choose inductor value from: ) 1 ( 5 IN O O osc O V V V f I L - × × × = The MOSFETs are selected by their Rdson, gate charge, and package specifications. The SC4614 provides 1.5A gate drive current and gives 50nC/1.5A=33ns switching time for driving a 50nC gate charge MOSFET. The switch- ing time ts contributes to the top MOSFET switching loss: OSC S IN O S f t V I P × × × = There is no significant switching loss for the bottom MOSFET because of its zero voltage switching. The con- duction losses of the top and bottom MOSFETs are given by: D R I P dson O TOP C × × = 2 _ ) 1 ( 2 _ D R I P dson O BOT C - × × = If the requirement of total power losses for each MOSFET is given, the above equations can be used to calculate the values of Rdson and gate charge, then the devices can be determined accordingly. The solution should en- sure the MOSFET is within its maximum junction tem- perature at highest ambient temperature. Output Capacitor Output Capacitor Output Capacitor Output Capacitor Output Capacitor The output capacitors should be selected to meet both output ripple and transient response criteria. The output capacitor ESR causes output ripple VRIPPLE during the inductor ripple current flowing in. To meet output ripple criteria, the ESR value should be: ) 1 ( IN O O RIPPLE OSC ESR V V V V f L R - × × × < The output capacitor ESR also causes output voltage tran- sient VT during a transient load current IT flowing in. To meet output transient criteria, the ESR value should be: T T ESR I V R < To meet both criteria, the smaller one of above two ESRs is required. The output capacitor value also contributes to load tran- sient response. Based on a worst case where the induc- tor energy 100% dumps to the output capacitor during the load transient, the capacitance then can be calcu- lated by: 2 2 T T V I L C × > Input Capacitor Input Capacitor Input Capacitor Input Capacitor Input Capacitor The input capacitor should be chosen to handle the RMS ripple current of a synchronous buck converter. This value |
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