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SP7652ER Datasheet(PDF) 7 Page - Sipex Corporation |
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SP7652ER Datasheet(HTML) 7 Page - Sipex Corporation |
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7 / 16 page  7 Rev F: /0/06 SP7652 Wide Input Voltage Range 6A, 600kHz, Buck Regulator © Copyright 2006 Sipex Corporation APPLICATIONS INFORMATION Inductor Selection There are many factors to consider in selecting the inductor including core material, inductance vs. frequency, cur- rent handling capability, efficiency, size and EMI. In a typical SP7652 circuit, the inductor is chosen primarily by operat- ing frequency, saturation current and DC resistance. Increasing the inductor value will decrease output voltage ripple, but degrade transient response. Low induc- tor values provide the smallest size, 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 pos- sible to minimize resistive power loss. A good 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 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 selected, theproperselectionofcorematerialisbased on peak inductor current and efficiency re- quirements. The core must be large enough not to saturate at the peak inductor current and provide low core loss at the high switch- ingfrequency.Lowcostpowdered-ironcores are inappropriate for 900kHz operation. Gapped ferrite inductorsare widely available 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 are bet- ter 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 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: L= VOut(Vin(max) - VOut) Vin(max)FS•KR•iOut(max) Ipp= VOut (Vin(max) - VOut) Vin(max)•Fs•L IpEak = iOut(max) + Ipp 2 Pl(cu) = i2l(RmS) RWinDing Il(RmS) = iOut(max) 1+ 3 ( )2 Ipp IOut(max) |
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