LTC4269-1 Linear Technology Corporation, LTC4269-1 Datasheet - Page 25

• Manufacturer Part Number: LTC4269-1
• Description: IEEE 802.3 At PD And Synchronous No-Opto Flyback Controller
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
in sensed output voltage, compensating for the IR drops.
Assuming relatively fi xed power supply effi ciency, Eff,
power balance gives:
Average primary-side current is expressed in terms of
output current as follows:
So, the effective change in V
where:
K1 = dimensionless variable related to V
fi ciency, as previously explained
R
SENSE
P
V
R1
R2
where:
thus
OUT
OUT
I
K1=
Δ
COMP I
16
FB
IN
Δ Δ
Δ
LOAD
V
V
I
= external sense resistor
:
OUT
= K I
= Eff • P
• I
OUT
OUT
V
Q3
OUT
22
Figure 11. Load Compensation Diagram
V
IN
1•
OUT
= 1
= 1
•
R
K
OUT
Eff
CMP
= Eff • V
K
Q1 Q2
IN
•
•
R
R
R
SENSE
R
CMP
SENSE
A1
CMP
IN
V
FB
+
–
• I
•
IN
•
R N
OUT
R N
1
1
•
21
•
target is:
C
SF
R
CMP
SF
CMPF
50k
•
Δ
SENSE
I
OUT
20
V
IN
FLBK
V
, V
IN
+
OUT
MP
R
42691 F11
•
•
SENSE
and ef-
•
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
subsequently in the Applications Information section.
Transformer Design
Transformer design/specifi cation is the most critical part of
a successful application of the LTC4269-1. 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 generally increase cur-
rent stresses. A reasonable target for duty cycle is 50%
at nominal input voltage.
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.
K
R
N
N
1
CMP
IDEAL
IDEAL
•
R
R
SENSE
=
CMP
=
=
K
48
V
1
5
V
CMP
OUT
•
IN
R
•
•
R N
ESR R
SENSE
1 0 5
gives:
1
•
−
0 5
•
1
.
−
DC
+
.
SF
DC
•
=
(
DS ON
=
1
9 6
−
ESR R
(
1
.
DC
CMP
)
1
)
S(OUT)
+
−
•
R N
DC
resistor are discussed
DS ON
LTC4269-1
1
•
(
:
SF
)
25
42691f