LT3439 LINER [Linear Technology], LT3439 Datasheet - Page 9

• Manufacturer Part Number: LT3439
• Description: Slew Rate Controlled Ultralow Noise1A Isolated DC/DC Transformer Driver
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
current. This can be accomplished by adding more turns
onto a given core or selecting a new core with a higher
inductance per turn squared characteristic (A
The following equation can be used to set the transformer
primary inductance:
t
set it between 10% to 30% of I
as follows:
Eff can be estimated at 70%.
Winding Resistance
Resistance in either the primary or secondary winding will
reduce overall efficiency and degrade load regulation. If
efficiency or load regulation is unsatisfactory, verify that
the voltage drops in the transformer windings are not
excessive.
Leakage Inductance
When the output switches turn off, the transformer leak-
age inductance causes a voltage spike on the output
switch collector. The size of the voltage spike is propor-
tional to the magnitude of the leakage inductance and to
the square of the load current (energy stored in the leakage
inductance). The voltage spike should be limited so that it
does not exceed the voltage breakdown of the output
switches. This can be accomplished by reducing the
transformer’s leakage inductance or by reducing the maxi-
mum slew rate. The voltage slew control will limit the
voltage spike by dissipating the leakage energy in the
power switches.
Transformer Imbalance
A common concern for the push-pull topology is trans-
former imbalance. If the volt/second products of each half
ON
I is somewhat arbitrary but a general rule of thumb is to
L
I
PRI
can be calculated by 1/f
PRI
V
V
OUT
IN
V Eff
IN
t
ON
•
I
I
OUT
U
U
OSC
PRI
.
where I
W
PRI
is calculated
L
U
).
of the switching cycle do not match, the transformer’s flux
level walks up the BH curve and the transformer goes into
saturation. This is undesirable because the effective mag-
netizing inductance drops off and the magnetizing current
increases rapidly. Fortunately, there are parasitics in the
circuit that counteract the transformer saturation. When
the transformer begins to saturate the magnetizing cur-
rent increases in one half of the switching cycle and
therefore, the IR drops increase thereby reducing the volt/
second product of that half cycle. The transformer balance
is maintained. Also, the losses in the transformer and the
main switches have positive temperature coefficients elimi-
nating the potential for thermal runaway. The LT3439 can
compensate for small circuit imbalances, however care
should be taken to balance both sides of the circuit
including transformer design and PCB layout.
Transformer Design Example
The following is an example of the design of a DC trans-
former for a 5V to 5V at 500mA supply.
Calculate the primary switch current (I
The “Switch Voltage Drop vs Switch Current” Typical
Performance curve gives a typical value of the switch
voltage drop (V
example, I
Next, calculate the turns ratio:
Add 15% margin to account for winding resistance of the
transformer:
Supply specs: V
Assume: V
I
N
N
N
N
PRI
P
P
S
S
1 22 15
V
V
V
.
PRI
IN
OUT
OUT
V Eff
Efficiency 70%
–
IN
F
SW
V
•
= 0.5V (forward voltage of output diode)
0.7A, therefore V
SW
I
OUT
V
f
) for a given switch current (I
OSC
F
%
IN
= 5V, V
= 100kHz
5
5
1 41
V
V
5
.
V
5
– .
V
•
0 5
0 5
500
•
.
OUT
70
V
V
mA
%
SW
= 5V, I
1 22
.
PRI
0.5V.
0 714
.
):
OUT
LT3439
A
PRI
= 500mA,
sn3439 3439fs
). In this
9