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SP6134 Datasheet(PDF) 8 Page - Exar Corporation |
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SP6134 Datasheet(HTML) 8 Page - Exar Corporation |
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8 / 15 page  S SP P6 61 13 34 4 6 60 00 0k kH Hz z S Sy yn nc ch hrro on no ou us s P PW WM M S Stte ep p D Do ow wn n C Co on nttrro olllle err © 2008 Exar Corporation 8/15 Rev. 2.0.0 APPLICATIONS INFORMATION INDUCTOR SELECTION There are many factors to consider in selecting the inductor including cost, efficiency, size and EMI. In a typical SP6134 circuit, the inductor is chosen primarily for value, saturation current and DC resistance. Increasing the inductor value will decrease output voltage ripple, but degrade transient response. Low inductor values provide the smallest size, but cause large ripple currents, poor efficiency and more output capacitance to smooth out the larger ripple current. The inductor must also 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 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: () ( ) () ( ) max max max OUT r S IN OUT IN OUT I K F V V V V L − = () 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: ( ) () L F V V V V L S IN OUT IN OUT max max − = () 2 max PP OUT PEAK I I I + = () () WINDING RMS L Cu L R I P 2 = () ( ) Once the required inductor value is selected, the proper selection of core material is based on peak inductor current and efficiency requirements. The core must be large enough not to saturate at the peak inductor current and provide low core loss at the high switching frequency. Low cost powdered iron cores have a gradual saturation characteristic but can introduce considerable ac core loss, especially when the inductor value is relatively low and the ripple current is high. 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 better choice for all but the most cost sensitive applications. 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. Core losses have a more significant contribution at low output current where the copper losses are at a minimum, and can typically be neglected at higher output currents where the copper losses dominate. Core loss information is usually available from the magnetic vendor. The copper loss in the inductor can be calculated using the following equation: where IL(RMS) is the RMS inductor current that can be calculated as follows: () 2 max max 3 1 1 ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ + − OUT PP OUT RMS L I I I I OUTPUT CAPACITOR SELECTION The required ESR (Equivalent Series Resistance) and capacitance drive the selection of the type and quantity of the output capacitors. The ESR must be small enough that both the resistive voltage deviation due to a step change in the load current and the output ripple voltage do not |
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