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LT3999 Datasheet(PDF) 11 Page - Linear Technology |
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LT3999 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LT3999 11 3999fa For more information www.linear.com/LT3999 APPLICATIONS INFORMATION Output Voltage Regulation The output voltage of the DC transformer topology is unregulated. Variations in the input voltage will cause the output voltage to vary because the output voltage is a function of the input voltage and the transformer turn ratio. Also, variations in the output load will cause the output voltage to change because of circuit parasitics, such as the transformer DC resistance and power switch on resistance. If regulation is necessary, a post regulator such as a linear regulator can be added to the output of the supply. See the Typical Applications for examples of adding a linear regulator. Power Consideration The current derived from the VIN pin and the SWA and SWB switching currents are the sources of the LT3999 power dissipation. The power dissipation is the sum of: 1) The quiescent current and switch drive power dissipation: PVIN = VIN ISW •DC 30 +4mA where ISW is the average switch current. 2) Theconductingpowerdissipationoftheswitchesduring on state: PVCESAT = VCESAT • ISW • 2DC where DC is the duty cycle and VCESAT is the collector to emitter voltage drop during the switch saturation. 3) The dynamic power dissipation due to the switching transitions: PSW = VIN • ISW • fOSC • (tr + tf) where tr and tf are the rise and fall times. The junction temperature is computed as: TJ = TAMB + PD • θJA where: PD = PVIN + PVCESAT + PSW and θJA is the package thermal resistance. Layout Consideration Check List The following is a list of recommended layout consider- ations: • Locate the bypass capacitor on the VIN pin of the trans- former close to the transformer. • Create a solid GND plane, preferably on layer two of the PCB. • Use short wide traces to connect to the transformer. • The transformer and PCB routing should be care- fully designed to maximize the symmetry between two switching half cycles. • SoldertheLT3999exposedpadtothePCB.Addmultiple vias to connect the exposed pad to the GND plane. More Help AN70: “A Monolithic Switching Regulator with 100mV Output Noise” contains much information concerning applications and noise measurement techniques. |
Similar Part No. - LT3999_15 |
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Similar Description - LT3999_15 |
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