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SP7651 Datasheet(PDF) 7 Page - Exar Corporation |
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SP7651 Datasheet(HTML) 7 Page - Exar Corporation |
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7 / 15 page  7 Rev J: 3/4/07 SP765 Wide Input Voltage Range 3A, 900kHz, Buck Regulator © Copyright 2007 Sipex Corporation APPLICATIONS INFORMATION Inductor Selection There are many factors to consider in se- lecting the inductor, including: core material, inductance vs. frequency, current handling capability, efficiency, size and EMI. In a typi- cal SP765 circuit, the inductor is chosen primarily by operating frequency, saturation current and DC resistance. Increasing the inductor value will decrease output voltage ripple, but degrade transient response. Low inductorvaluesprovidethesmallestsize,but cause large ripple currents, poor efficiency and require more output capacitance to smooth out the larger ripple current. The inductor must be able to handle the peak current at the switching frequency without saturating, and the copper resistance in the winding should be kept as low as possible to minimize resistive power loss. A good com- promise 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 as follows: where: Fs = switching frequency Kr = ratio of the AC inductor ripple current to the maximum output current The peak-to-peak inductor ripple current is: Once the required inductor value is se- lected, the proper selection of core mate- rial is based on peak inductor current and efficiency requirements. The core must be large enough not to saturate at the peak inductor current... ...andprovidelowcorelossatthehighswitch- ingfrequency.Lowcostpowdered-ironcores are inappropriate for 900kHz operation. Gappedferriteinductorsarewidelyavailable for consideration. Select devices that have operating data shown up to MHz. Ferrite materials, on the other hand, are more expensive and have an abrupt saturation characteristic with the inductance dropping sharply when the peak design current is exceeded. Nevertheless, they are preferred at high switching frequencies because they present very low core loss and the design only needs to prevent saturation. In general, ferrite or molypermalloy materials will be used with the SP765. 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 resis- tance needs to be minimized, but this usu- ally comes at the expense of using 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 inductor selection can affect the resulting power sup- ply efficiency by more than 15-20%! The copper loss in the inductor can be cal- culated using the following equation: where IL(RMS) is the RMS inductor current that can be calculated as follows: Vout (Vin(max) - Vout) Vin(max) fs Kr Iout(max) L= Ipp= Vout(Vin(max) - Vout) Vin(max) fs L Ipeak= iout(max) + I pp 2 PL(cu)=i2L(rms) rwinding IL(rms)=iout(max) 1 + ( )2 3 Ipp Iout(max) |
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