LTC4278CDKD#PBF Linear Technology, LTC4278CDKD#PBF Datasheet - Page 21

• Manufacturer Part Number: LTC4278CDKD#PBF
• Description: IC PD IEEE 802.3AT 25.5W 32-DFN
• Manufacturer: Linear Technology

Specifications of LTC4278CDKD#PBF

• Controller Type: Powered Device Interface Controller (PD)
• Interface: IEEE 802.3af
• Voltage - Supply: 4.5 V ~ 20 V
• Current - Supply: 6.4mA
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 32-DFN
• Input Voltage: 60V
• Supply Current: 1.35mA
• Digital Ic Case Style: DFN
• No. Of Pins: 32
• Duty Cycle (%): 88%
• Frequency: 100kHz
• Operating Temperature Range: °C To +70°C
• Msl: MSL 1 - Unlimited
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIONS INFORMATION
for PoE dominance. Furthermore, PD applications may
also opt for a seamless transition — that is, without power
disruption — between PoE and auxiliary power.
The most common auxiliary power option injects power at
V
this example, the auxiliary port injects 48V onto the line via
diode D1. The components surrounding the SHDN pin are
selected so that the LTC4278 does not disconnect power
to the output until the auxiliary supply exceeds 36V.
This confi guration is an auxiliary-dominant confi guration.
That is, the auxiliary power source supplies the power even
if PoE power is already present. This confi guration also
provides a seamless transition from PoE to auxiliary power
when auxiliary power is applied, however, the removal of
auxiliary power to PoE power is not seamless.
Contact Linear Technology applications support for detail
information on implementing a custom auxiliary power
supply.
IEEE 802.3at SYSTEM POWER-UP REQUIREMENT
Under the IEEE 802.3at standard, a PD must operate under
12.95W in accordance with IEEE 802.3at standard until it
recognizes a Type 2 PSE. Initializing PD operation in 12.95W
mode eliminates interoperability issue in case a Type 2
PD connects to a Type 1 PSE. Once the PD recognizes a
NEG
. Figure 9 presents an example of this application.In
RJ45
1
2
3
6
4
5
7
8
TX
TX
RX
RX
+
–
+
–
SPARE
SPARE
+
–
TRANSFORMER
ISOLATED
Figure 9. Auxiliary Power Dominant PD Interface Example
T1
WALL
+
–
TO PHY
BR1
BR2
+
–
+
–
Type 2 PSE, the IEEE 802.3at standard requires the PD to
wait 80ms in 12.95W operation before 25.5W operation
can commence.
MAINTAIN POWER SIGNATURE
In an IEEE 802.3af/at system, the PSE uses the maintain
power signature (MPS) to determine if a PD continues to
require power. The MPS requires the PD to periodically draw
at least 10mA and also have an AC impedance less than
26.25k in parallel with 0.05μF . If one of these conditions
is not met, the PSE may disconnect power to the PD.
SWITCHING REGULATOR OVERVIEW
The LTC4278 includes a current mode converter designed
specifi cally for use in an isolated fl yback topology employing
synchronous rectifi cation. The LTC4278 operation is similar
to traditional current mode switchers. The major difference
is that output voltage feedback is derived via sensing the
output voltage through the transformer. This precludes
the need of an opto-isolator in isolated designs, thus
greatly improving dynamic response and reliability. The
LTC4278 has a unique feedback amplifi er that samples a
transformer winding voltage during the fl yback period and
uses that voltage to control output voltage. The internal
blocks are similar to many current mode controllers.
The differences lie in the feedback amplifi er and load
TVS
100k
10k
10k
36V
0.1μF
100V
SHDN
V
PORTN
LTC4278
V
PORTP
V
GND
NEG
D1
4278 F09
C1
LTC4278
21
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