10
LT3439
sn3439 3439fs
The primary inductance is then calculated:
LV
f
I
kHz
A
H
AA
PRI IN
OSC
PRI
=
∆
==µ
∆= = =
1
5
1
100
0 107
467
0 15 0 714 0 107
.
.•. .I 15% of I
PRI
Next, build a transformer with the calculated values of
turns ratio and primary inductance. Minimize resistance in
the windings. The turns ratio can be tweaked to get the
specified output voltage.
Capacitors
The DC transformer topology runs effectively at 100%
duty cycle (50% each side). This means that the input
supply current is approximately constant. Therefore, large
“hold-up 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. Low ESR, surface mount capaci-
tors such as ceramic, OS-CON of POSCAPs are recom-
mended. An additional LC filter can be added in addition to
the output capacitor to further reduce output noise.
Transformer winding capacitance between the isolated
primary and secondary have 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.
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.
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
f
OSC
= 100kHz, then f
CORNER
= 10kHz where:
f
LC
CORNER
1
2•π
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.
More Help
AN70: “A Monolithic Switching Regulator with 100µV
Output Noise” contains much information concerning
applications and noise measurement techniques.
AN19: “LT1070 Design Manual”
AN29: “Some Thoughts on DC-DC Converters” also have
general knowledge on switching regulators.
An LTC SwitcherCAD
TM
model is available to verify design
performance.
The LTC Applications department is always ready to lend
a helping hand.
APPLICATIO S I FOR ATIO
WUUU
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