ltc4268cdkd-1-trpbf Linear Technology Corporation, ltc4268cdkd-1-trpbf Datasheet - Page 31

• Manufacturer Part Number: ltc4268cdkd-1-trpbf
• Description: High Power Pd With Synchronous Noopto Flyback Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
where:
Nominal output impedance cancellation is obtained by
equating this expression with R
Solving for R
The practical aspects of applying this equation to determine
an appropriate value for the R
Applications Information.
Transformer Design
Transformer design/specifi cation is the most critical part of
a successful application of the LTC4268-1. The following
sections provide basic information about designing the
transformer and potential tradeoffs. If you need help, the
LTC Applications group is available to assist in the choice
and/or design of the transformer.
Turns Ratios
The design of the transformer starts with determining
duty cycle (DC). DC impacts the current and voltage stress
on the power switches, input and output capacitor RMS
currents and transformer utilization (size vs power). The
ideal turns ratio is:
Avoid extreme duty cycles as they, in general, increase
current stresses. A reasonable target for duty cycle is
50% at nominal input voltage.
K1 = dimensionless variable related to V
R
K
R
N
SENSE
1
CMP
DEAL
•
R
R
V
SENSE
=
OUT
CMP
=
= external sense resistor
K
V
1
V
CMP
OUT
and effi ciency as explained above
•
IN
R
•
R N
ESR R
SENSE
gives:
•
1
1
•
−
DC
+
SF
DC
•
(
DS ON
=
1
−
(
ESR R
DC
CMP
)
1
)
S(OUT)
+
−
•
resistor are found in the
R N
DC
DS ON
1
•
(
:
SF
)
IN
,
For instance, if we wanted a 48V to 5V converter at 50%
DC then:
In general, better performance is obtained with a lower
turns ratio. A DC of 45.5% yields a 1:8 ratio. Note the
use of the external feedback resistive divider ratio to set
output voltage provides the user additional freedom in
selecting a suitable transformer turns ratio. Turns ratios
that are the simple ratios of small integers; e.g., 1:1, 2:1,
3:2 help facilitate transformer construction and improve
performance. When building a supply with multiple
outputs derived through a multiple winding transformer,
lower duty cycle can improve cross regulation by keeping
the synchronous rectifi er on longer, and thus, keep
secondary windings coupled longer. For a multiple output
transformer, the turns ratio between output windings is
critical and affects the accuracy of the voltages. The ratio
between two output voltages is set with the formula V
= V
two windings. Also keep the secondary MOSFET R
small to improve cross regulation. The feedback winding
usually provides both the feedback voltage and power for
the LTC4268-1. Set the turns ratio between the output and
feedback winding to provide a rectifi ed voltage that under
worst-case conditions is greater than the 11V maximum
V
CC
N
For our example: N
where
We
OUT1
turn-off voltage.
DEAL
N
V
F
SF
will choose
=
• N21 where N21 is the turns ratio between the
>
:
=
Diode Forward V
11
48
5
V
OUT
+
•
V
1 0 5
F
−
0 5
.
3
1
.
SF
=
>
9 6
1
o o ltage
.
11 0 7
+
5
.
LTC4268-1
=
2 34
.
1
31
DS(ON)
42681fa
OUT2