LTC4269IDKD-1#TRPBF Linear Technology, LTC4269IDKD-1#TRPBF Datasheet - Page 24

IC PD/OPTO FLYBACK CTRLR 32-DFN

LTC4269IDKD-1#TRPBF

Manufacturer Part Number
LTC4269IDKD-1#TRPBF
Description
IC PD/OPTO FLYBACK CTRLR 32-DFN
Manufacturer
Linear Technology
Type
Power Over Ethernet (PoE)r
Datasheet

Specifications of LTC4269IDKD-1#TRPBF

Applications
Power Interface Switch for Power Over Ethernet (PoE) Devices
Voltage - Supply
14 V ~ 16 V
Operating Temperature
-40°C ~ 85°C
Mounting Type
Surface Mount
Package / Case
32-DFN
Current - Supply
1.35mA
Interface
IEEE 802.3af
Controller Type
Powered Device Interface Controller (PD)
Lead Free Status / RoHS Status
Lead free / RoHS Compliant

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APPLICATIONS INFORMATION
LTC4269-1
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
24
SENSE
P
V
R1
R2
where:
thus:
OUT
OUT
I
K1=
ΔV
COMP I
16
FB
IN
ΔV
ΔI
LOAD
= external sense resistor
OUT
= Eff • P
• I
= K1•I
OUT
OUT
V
Q3
OUT
22
Figure 11. Load Compensation Diagram
IN
V
OUT
= K1•
= K1•
• Eff
R
CMP
= Eff • V
OUT
Q1 Q2
IN
R
R
R
SENSE
R
CMP
SENSE
A1
CMP
IN
V
FB
+
• I
• R1• N
IN
• R1• N
OUT
21
target is:
C
R
SF
CMP
CMPF
50k
SF
• ΔI
SENSE
20
OUT
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 trade-offs. 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.
K1•
R
N
N
CMP
IDEAL
IDEAL
R
R
SENSE
= K1•
CMP
=
=
48
V
5
CMP
V
OUT
IN
R
• R1• N
ESR + R
1− 0.5
SENSE
gives:
0.5
1− DC
DC
SF
• 1− DC
=
(
DS(ON)
=
9.6
ESR + R
1
CMP
S(OUT)
1− DC
)
• R1• N
resistor are discussed
DS(ON)
:
SF
42691fb

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