8
LT3439
sn3439 3439fs
APPLICATIO S I FOR ATIO
WUUU
mean greater switching losses in the internal output
switches. However, efficiency is only modestly reduced
for a large improvement in EMI.
Care should be taken to ensure that the worst-case input
voltage and load current conditions do not cause an
excessive die temperature. The total power dissipation of
the IC is dominated by three loss terms, regulator losses,
saturation losses and switching losses. The following
formulas may be used to approximate these losses:
1. Regulator Dissipation:
PVmA
I
VIN IN
=+
12
60
where I is the average switch current.
2. Switch Saturation Dissipation:
P
VSAT
= (V
SAT
)(I)
3. Switch Switching Dissipation:
PVIf
I
R
V
R
SW IN OSC
SL
SL
=
+
()
+
+
()
10
2 3 10 10 8
1 7 10 65 8
6
4
3
–
–
–
•••
–. • • .
–. • • .
Total IC power dissipation (P
D
) is the sum of these three
terms. Die junction temperature can be computed as
follows:
T
J
= T
AMB
+ (P
D
)(θ
JA
)
where T
AMB
is the ambient temperature, T
J
is the junction
temperature and θ
JA
is the thermal resistance from junc-
tion to ambient.
The LT3439 comes in the 16-pin TSSOP with exposed
backside package that has a very low junction-to-ambient
thermal resistance (θ
JA
) of approximately 40°C/W.
Transformer Design
Table 1 lists recommended center tapped transformers for
a variety of input voltage, output voltage and power
combinations.
These transformers will yield slightly high output voltages
so that they can accommodate an LDO regulator on the
output.
If your application is not listed, the LTC Applications group
is available to assist in the choice and/or the design of the
transformer.
In the design/selection of the transformer the following
characteristics are critical and should be considered.
Turns Ratio
The turns ratio of the transformer determines the output
voltage. The following equation can be used as a first pass
to calculate the turns ratio:
N
N
VV
VV
S
P
OUT F
IN SW
=
+
–
where V
F
is the forward voltage of the output diode and
V
SW
is the voltage drop across the internal switches (see
Typical Performance curves).
Sufficient margin should be added to the turns ratio to
account for voltage drops due to transformer winding
resistances. Also, if using an LDO for regulating the output
voltage, don’t forget to take into account the voltage drop
that should be added to V
OUT
.
Magnetizing Current
The primary inductance of the transformer causes a ripple
current that is independent of load current. The ripple
current manifests itself in the output voltage through the
parasitic resistances of the supply. Increasing the trans-
former magnetizing inductance can reduce the ripple
Table 1
NOMINAL NOMINAL
INPUT OUTPUT OUTPUT COILTRONICS
VOLTAGE VOLTAGE POWER PART NUMBER
5V 12V 1.5W CTX02-13716-X1
5V 12V 3.0W CTX02-13665-X1
5V ±15V 1.5W CTX02-13713-X1
5V ±15V 3.0W CTX02-13664-X1
5V 12V 1.5W CTX02-13834-X3
5V 12V 10W CTX02-13949-X1
12V –12V 6W CTX02-16076
