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LT3999 Datasheet(PDF) 11 Page - Linear Technology

Part No. LT3999
Description  Low Noise, 1A, 1MHz Push-Pull DC/DC Driver with Duty Cycle Control
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LT3999 Datasheet(HTML) 11 Page - Linear Technology

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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.


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