LT3999
APPLICATIONS INFORMATION
Output Voltage Regulation
The junction temperature is computed as:
T = T
+ P • θ
D
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.
J
AMB
JA
where:
P = P
+ P
+ P and θ is the package
SW JA
D
VIN
VCESAT
thermal resistance.
Layout Consideration Check List
The following is a list of recommended layout consider-
ations:
• Locate the bypass capacitor on the V pin of the trans-
IN
former close to the transformer.
Power Consideration
• Create a solid GND plane, preferably on layer two of
the PCB.
The current derived from the V pin and the SWA and
IN
SWB switching currents are the sources of the LT3999
• Use short wide traces to connect to the transformer.
power dissipation. The power dissipation is the sum of:
• The transformer and PCB routing should be care-
fully designed to maximize the symmetry between two
switching half cycles.
1) The quiescent current and switch drive power
dissipation:
ISW •DC
30
• SoldertheLT3999exposedpadtothePCB.Addmultiple
vias to connect the exposed pad to the GND plane.
IN
VIN = V
P
+4mA
More Help
where I is the average switch current.
SW
AN70: “A Monolithic Switching Regulator with 100mV
Output Noise” contains much information concerning
applications and noise measurement techniques.
2) Theconductingpowerdissipationoftheswitchesduring
on state:
P
= V • I • 2DC
CESAT SW
VCESAT
where DC is the duty cycle and V
is the collector
CESAT
to emitter voltage drop during the switch saturation.
3) The dynamic power dissipation due to the switching
transitions:
P
SW
= V • I • f
• (t + t )
IN SW OSC r f
where t and t are the rise and fall times.
r
f
3999fa
11
For more information www.linear.com/LT3999
Linear [ Linear ]
