LTC4269CDKD-1#TRPBF Linear Technology, LTC4269CDKD-1#TRPBF Datasheet - Page 26

• Manufacturer Part Number: LTC4269CDKD-1#TRPBF
• Description: IC PD/OPTO FLYBACK CTRLR 32-DFN
• Manufacturer: Linear Technology
• Type: Power Over Ethernet (PoE)r

Specifications of LTC4269CDKD-1#TRPBF

• Applications: Power Interface Switch for Power Over Ethernet (PoE) Devices
• Voltage - Supply: 14 V ~ 16 V
• Operating Temperature: 0°C ~ 70°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

APPLICATIONS INFORMATION
LTC4269-1
A fi nal note—the susceptibility of the system to bistable
behavior is somewhat a function of the load current/
voltage characteristics. A load with resistive—i.e., I = V/R
behavior—is the most apt to be bistable. Capacitive loads
that exhibit I = V
Secondary Leakage Inductance
Leakage inductance on the secondary forms an inductive
divider on the transformer secondary, reducing the size
of the fl yback pulse. This increases the output voltage
target by a similar percentage. Note that unlike leakage
spike behavior, this phenomenon is independent of load.
Since the secondary leakage inductance is a constant
percentage of mutual inductance (within manufacturing
variations), the solution is to adjust the feedback resistive
divider ratio to compensate.
Winding Resistance Effects
Primary or secondary winding resistance acts to reduce
overall effi ciency (P
increases effective output impedance, degrading load regu-
lation. Load compensation can mitigate this to some extent
but a good design keeps parasitic resistances low.
Bifi lar Winding
A bifi lar, or similar winding, is a good way to minimize
troublesome leakage inductances. Bifi lar windings also
improve coupling coeffi cients, and thus improve cross
regulation in multiple winding transformers. However,
tight coupling usually increases primary-to-secondary
capacitance and limits the primary-to-secondary
breakdown voltage, so is not always practical.
Primary Inductance
The transformer primary inductance, L
based on the peak-to-peak ripple current ratio (X) in the
transformer relative to its maximum value. As a general
rule, keep X in the range of 20% to 40% (i.e., X = 0.2 to
0.4). Higher values of ripple will increase conduction losses,
while lower values will require larger cores.
26
2
/R behavior are less susceptible.
OUT
/P
IN
). Secondary winding resistance
P
,
is selected
Ripple current and percentage ripple is largest at minimum
duty cycle; in other words, at the highest input voltage.
L
where:
Using common high power PoE values, a 48V (41V < V
< 57V) to 5V/5.3A converter with 90% effi ciency, P
26.5W and P
= 200kHz:
Optimization might show that a more effi cient solution
is obtained at higher peak current but lower inductance
and the associated winding series resistance. A simple
spreadsheet program is useful for looking at trade-offs.
Transformer Core Selection
Once L
effi ciency converters use ferrite cores to minimize core
loss. Actual core loss is independent of core size for a fi xed
inductance, but decreases as inductance increases. Since
increased inductance is accomplished through more turns
of wire, copper losses increase. Thus, transformer design
balances core and copper losses. Remember that increased
winding resistance will degrade cross regulation and
increase the amount of load compensation required.
The main design goals for core selection are reducing
copper losses and preventing saturation. Ferrite core
material saturates hard, rapidly reducing inductance
when the peak design current is exceeded. This results
L
P
P
f
DC
X
DC
L
is calculated from the following equation.
OSC
=
MAX
P
(
MIN
MIN
=
V
P
f
is the oscillator frequency
IN(MAX)
OSC
is known, the type of transformer is selected. High
200kHz • 0.4 • 26.5W
is ripple current ratio at maximum input voltage
is the DC at maximum input voltage
=
(
• X
1+
57V • 0.412
IN
MAX
N • V
• DC
= 29.5W. Using X = 0.4 N = 1/8 and f
V
1
• P
MIN
IN(MAX)
OUT
IN
)
2
)
2
=
=
(
V
1+
= 260µH
IN(MAX)
f
OSC
8
1
1
•
• X
57
5
• DC
MAX
= 41.2%
MIN
• P
OUT
)
2
• Eff
42691fb
OUT
OSC
IN
=