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LT3999 Datasheet(PDF) 10 Page  Linear Technology 

LT3999 Datasheet(HTML) 10 Page  Linear Technology 
10 / 16 page LT3999 10 3999fa For more information www.linear.com/LT3999 APPLICATIONS INFORMATION Turns Ratio The turns ratio of the transformer determines the output voltage. The following equation is used as a first pass to calculate the turns ratio: NS NP = VOUT +VF 2 VIN – VSW ( )DC where VF is the forward voltage of the output diode, VSW is the voltage drop across the internal switches (see the Typical Performance curves) and DC is the duty cycle. Sufficient margin should be added to the turns ratio to account for voltage drops due to transformer winding resistance. Magnetizing Current The magnetizing inductance of the transformer causes a ripple current that is independent of load current. This ripple current is calculated by: ∆I= VIN •DC fSW •LM where∆IandLMareprimaryripplecurrentandmagnetizing inductance referred to the primary side of the transformer, respectively. Increasing the transformer magnetizing in ductance,LM,reducestheripplecurrent.Theripplecurrent formula shows the effect of the switching frequency on the magnetizing inductance. Setting the LT3999 at high switching frequency reduces the ripple current for the same magnetizing inductance. Therefore, it is possible to reduce the transformer turns and still achieve low ripple current.Thishelpstoreducethepowerconverterfootprint as well. The transformer magnetizing inductance should be designed for the worstcase duty cycle and input line voltage combination. A good rule of thumb is to set the primary current ripple amplitude 10% to 30% of the average primary current, IP: IP = POUT VIN •eff where POUT is the output power of the converter and eff is the converter efficiency, typically around 85%. Winding Resistance Resistance in either the primary or secondary winding reduces overall efficiency and degrades load regulation. If efficiency or load regulation is unsatisfactory, verify that the voltage drops in the transformer windings are not excessive. Capacitors In applications with full duty cycle operation, the input supply current is approximately constant. Therefore, large input “holdup type” capacitors are not necessary. A low value (>4.7µF), low ESR ceramic will be adequate to filter high frequency noise at the input. The output capacitors supply energy to the output load only during switch transitions. Therefore, large capacitance values are not necessary on the output. Transformer winding capacitance between the isolated primary and secondary has parasitic currents that can cause noise on the grounds. Providing a high frequency, low impedance path between the primary and secondary gives the parasitic currents a local return path. A 2.2nF, 1kV ceramic capacitor is recommended. Optional LC Filter An optional LC filter, as shown on the Typical Application on the first page of this data sheet, should be included if ultralow noise and ripple are required. It is recommended that the corner frequency of the filter should be set a decade below the switching frequency so that the switch noise is attenuated by a factor of 100. For example, if the fOSC = 100kHz, then fCORNER = 10kHz where: fCORNER = 1 2•π LC Switching Diode Selection A fast recovery, surface mount diode such as a Schottky is recommended. The proximity of the diodes to the transformer outputs is important and should be as close as possible with short, wide traces connecting them. 
