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SC2621 Datasheet(PDF) 7 Page - Semtech Corporation |
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SC2621 Datasheet(HTML) 7 Page - Semtech Corporation |
7 / 10 page 7 2003 Semtech Corp. www.semtech.com POWER MANAGEMENT SC2621 Applications Information (Cont.) 150 175 200 225 250 275 300 325 350 0 100 200 300 400 500 600 Rset (k-ohm) 150 175 200 225 250 275 300 325 350 0 100 200 300 400 500 600 Rset (k-ohm) Fig. 2. Pull down resistor for current limit setting 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 SC2621. 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 requirement 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 gives the in- ductor ripple current to be approximate 20% of maxi- mum load current. So choose inductor value from: ) 1 ( 5 IN O O osc O V V V f I L - × × × = The MOSFETs are selected from their Rdson, gate charge, and package. The SC2621 provides 1.5A gate drive cur- rent. To drive a 50nC gate charge MOSFET gives 50nC/ 1.5A=33ns switching time. The switching time ts contrib- utes 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 can be calculated using above equations, then the devices can be deter- mined accordingly. The solution should ensure the MOSFET is within its maximum junction temperature 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 × > |
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