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SP6123 Datasheet(PDF) 9 Page - Sipex Corporation |
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SP6123 Datasheet(HTML) 9 Page - Sipex Corporation |
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9 / 18 page  9 Date: 5/25/04 SP6123 Low Voltage, Synchronous Step Down PWM Controller © Copyright 2004 Sipex Corporation Inductor Selection There are many factors to consider in selecting the inductor including cost, efficiency, size and EMI. In a typical SP6123 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 induc- tor 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 oper- ating point determine the inductor value as fol- lows: (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 peak to peak inductor ripple current to the maximum output current The peak to peak inductor ripple current is: L F V V V V I S IN OUT IN OUT PP (max) (max) ) ( − = Once the required inductor value is selected, the proper selection of core material is based on peak inductor current and efficiency require- ments. The core material must be large enough not to saturate at the peak inductor current 2 (max) PP OUT PEAK I I I + = and provide low core loss at the high switching frequency. Low cost powdered iron cores have a gradual saturation characteristic but can intro- duce 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 induc- tance dropping sharply when the peak design current is exceeded. Nevertheless, they are pre- ferred at high switching frequencies because they present very low core loss and the design only needs to prevent saturation. The power dissipated in the inductor is equal to the sum of the core and copper losses. To mini- mize 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 cur- rent 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: WINDING RMS L Cu L R I P 2 ) ( ) ( = where IL(RMS) is the RMS inductor current that can be calculated as follows: IL(RMS) = IOUT(max) 1 + 1 ( IPP )2 3 IOUT(max) Output Capacitor Selection The required ESR (Equivalent Series Resis- tance) and capacitance drive the selection of the type and quantity of the output capacitors. The ESR must be small enough that both the resis- tive voltage deviation due to a step change in the load current and the output ripple voltage do not exceed the tolerance limits expected on the output voltage. During an output load transient, the output capacitor must supply all the addi- tional current demanded by the load until the SP6123 adjusts the inductor current to the new value. Therefore the capacitance must be large enough so that the output voltage is held up while the inductor current ramps up or down to APPLICATIONS INFORMATION |
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