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SP7663 Datasheet(PDF) 10 Page - Sipex Corporation |
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SP7663 Datasheet(HTML) 10 Page - Sipex Corporation |
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10 / 19 page  0 Feb2-07 Rev O SP7663 Wide Input Voltage Range 6A, 600kHz Buck Regulator © 2007 Sipex Corporation APPLICATIONS INFORMATION and provide low core loss at the high switching frequency. Low cost powdered- iron cores have a gradual saturation char- acteristic but can introduce considerable AC core loss, especially when the induc- tor value is relatively low and the ripple current is high. Ferrite materials, although more expensive, have an abrupt satura- tion characteristic with the inductance dropping sharply when the peak design current is exceeded. Nevertheless, they are preferred at high switching frequen- cies because they present very low core loss while the designer is only required to prevent saturation. In general, ferrite or molypermalloy materials are a better choice for all but the most cost sensitive applications. Optimizing Efficiency The power dissipated in the inductor is equal to the sum of the core and copper losses. To minimize copper losses, the winding resistance needs to be minimized, but this usually comes at the expense of a larger inductor.Corelosseshaveamoresignificant contribution at low output current where the copper losses are at a minimum, and can typically be neglected at higher output cur- rents where the copper losses dominate. Core loss information is usually available from the magnetics vendor. Proper induc- tor selection can affect the resulting power supplyefficiencybymorethan15%! The copper loss in the inductor can be cal- culatedusingthefollowingequation: PL(Cu) = I 2 L(rMS) • rwiNDiNg where IL(rMS)istheRMSinductorcurrent thatcanbecalculatedasfollows: IL(rMS) = IOut(MAx) • √+ (. IPP )2 3 IOut(MAx) Inductor Selection There are many factors to consider in se- lecting the inductor including core material, inductance vs. frequency, current handling capability,efficiency,sizeandEMI.Inatypi- cal SP7663 circuit, the inductor is chosen primarilyforvalue,saturationcurrentandDC resistance. Increasing the inductor value will decrease output voltage ripple, but degrade transientresponse.Lowinductorvaluespro- vide the smallest size, but cause large ripple currents,poorefficiencyandrequiremore output capacitance to smooth out the larger ripple current. The inductor must be able to handle the peak current at the switching frequencywithoutsaturating,andthecopper resistance in the winding should be kept as low as possible to minimize resistive power loss.Agood compromise between size, loss and cost is to set the inductor ripple current to be within 20% to 40% of the maximum output current. The switching frequency and the inductor operating point determine the inductor value asfollows: L = . VOut • (ViN(MAx) - VOut) ViN(MAx) • ƒs • Kr • IOut(MAx) where: ƒs = switching frequency Kr = ratio of the AC inductor ripple current to the maximum output current Thepeak-to-peakinductorripplecurrentis: IPP = . VOut • (ViN(MAx) - VOut) ViN(MAx) • ƒs •L Once the required inductor value is selected, theproperselectionofcorematerialisbased onpeakinductorcurrentandefficiencyre- quirements. The core must be large enough not to saturate at the peak inductor current IPeAk = IOut(MAx) + IPP 2 |
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