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SP6136ER1 Datasheet(PDF) 11 Page - Exar Corporation |
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SP6136ER1 Datasheet(HTML) 11 Page - Exar Corporation |
11 / 18 page Oct 3-06 Rev L SP636 Synchronous Buck Controller © 2006 Sipex Corporation than internal resistors, then the Vin start threshold is given by: Vin(start) = 2.5 • (R4+R5)/R5................ (8) For example, if it is required to have a Vin start threshold of 7V, then let R5 = 5KW and using equation (9) we get R4 = 9.09KW. Inductor Selection There are many factors to consider in select- ing the inductor including cost, efficiency, size and EMI. In a typical SP636 circuit, the inductor is chosen primarily for value, saturation current and DC resistance. In- creasing the inductor value will decrease output voltage ripple, but degrade transient response. Low inductor values provide the smallest size, but cause large ripple cur- rents, poor efficiency and need more output capacitance to smooth out the larger ripple current. The inductor must also 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. 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: L = Vout • (Vin(max) - Vout) Vin(max) • Fs • Kr • Iout(max) 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: Ipp = Vout • (Vin(max) - Vout) Vin(max) • Fs • L Once the required inductor value is selected, the proper selection of core material is based on peak inductor current and efficiency re- quirements. The core must be large enough not to saturate at the peak inductor current IpEak = Iout(max) + Ipp /2 and provide low core loss at the high switch- ing 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 the better choice for all but the most cost sensi- tive 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.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 magnetic vendor. The copper loss in the inductor can be cal- culated using the following equation: PL(cu) = I 2 L (rms) • Rwinding where IL(rms) is the RMS inductor current that can be calculated as follows: APPLICATION INFORMATION |
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