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SC2618SKTRT Datasheet(PDF) 5 Page - Semtech Corporation |
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SC2618SKTRT Datasheet(HTML) 5 Page - Semtech Corporation |
5 / 9 page 5 2004 Semtech Corp. www.semtech.com POWER MANAGEMENT SC2618 Component Selection General design guideline of switching power supplies can be applied to the component selection for SC2618. Output Filter Components Output Filter Components Output Filter Components Output Filter Components Output Filter Components The purpose of soft start is to control inductor current during start up, this in turn controls the amount of ex- cess charge the output capacitor will receive and there- fore the output voltage overshoot that will occur at the end of soft start. GND L VIN Q2 2618-02 Co VO Q1 GND Fig. 2. Output filter components v C i L V O 0 0 t SS t P I L V P t V REF Fig. 3. Soft start, inductor current and overshoot Normally, controllers with an external soft start capaci- tor, have soft start set so long that overshoot is unde- tectable. Since the soft start for the SC2618 is internal, it is necessarily limited and the maximum values of L and Co should meet the constraints of: 50 2 S SS O X t C L ⋅ = ⋅ where tss is soft start time, 100us typically, and Xs is the percentage overshoot allowable. 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 if without adding extra cost and size. Higher inductance, lower ripple current, smaller core loss and higher efficiency. However, too high inductance slows down output transient response. It is recommended to choose the inductance that gives the inductor ripple cur- rent to be 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 from their Rdson, gate charge, and package. The SC2618 provides 0.5A gate drive cur- rent. To drive a 25nC gate charge MOSFET gives 25nC/ 0.5A=50ns 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 switching loss for the bottom MOSFET be- cause of its zero voltage switching. The conduction 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(s) Output Capacitor(s) Output Capacitor(s) Output Capacitor(s) Output Capacitor(s) 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 − ⋅ ⋅ ⋅ < |
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