1
dc1145bf
DEMO MANUAL DC1145B
DESCRIPTION
LTC4267
Power over Ethernet Powered
Device With 12V Isolated Supply
Demonstration circuit 1145B, featuring the LTC
®
4267,
provides a complete IEEE 802.3af Powered Device (PD)
interface and isolated 12V at 0.9A power supply solution
for use in Power over Ethernet (PoE) applications. An
optional connection is included for a 12V auxiliary supply
on the secondary side that disables PoE when present.
The LTC4267 integrates the 25k signature resistor, clas-
sification current source, thermal overload protection,
signature disable and power good signal along with an
undervoltage lockout optimized for use with the IEEE
required diode bridge. The precision dual level input cur-
L
, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
PERFORMANCE SUMMARY
OPERATING PRINCIPLES
rent limit allows the LTC4267 to charge load capacitors
and interface with legacy PoE systems.
The LTC4267 combines the above features with a current
mode switching controller designed for driving a 6V rated
N-channel MOSFET. It features programmable slope com-
pensation, soft-start, and constant frequency operation,
minimizing electrical noise even with light loads.
Design files for this circuit board are available at
http://www.linear.com/demo
(T
A
= 25°C)
PARAMETER
CONDITIONS
VALUE
Minimum Turn-on Voltage
Input from PSE, I
OUT
=100mA
39V
Maximum Turn-off Voltage
Input from PSE
33V
Operating PoE Voltage
Input from PSE, I
OUT
= 0 to Maximum Output Current
37V to 57V
Maximum Input Current
Input from PSE
350mA
Maximum Output Current
Input from PSE, V
OUT
= 12V
0.9A
Output Voltage
Input from PSE, I
OUT
= 0 to Maximum Output Current
12V ±2%
PD Power Class
R
CLASS
= R7 = 45.3 Ohm
Class 3, 12.95W
Auxiliary input voltage
Input from auxiliary supply, dominant over PSE
10V to 16V
Invalid Signature
PD input, auxiliary supply present
9kΩ
Demonstration circuit 1145B interfaces with a customer’s
PoE test setup per Figure 1. A PSE connects to DC1145B
through RJ45 connector J1. The front end of the demo
circuit includes the required Ethernet input interface
transformer and common mode termination integrated in
J1. The LTC4267 after J1 performs IEEE 802.3af interface
functions while data is passed to connector J3 to connect
to an optional PHY. The switching regulator function is
also carried out by the LTC4267 to output an isolated 12V.
Diode bridges D2 and D6 provide the required auto-polarity
on the data pairs and spare pairs while C3 provides the
PD’s 0.1μF capacitance during detection. This demo circuit
allows detection and power classification of the PD per the
IEEE 802.3af specification. During the detection process
of a PD, the LTC4267 displays the proper 25k signature
resistor. Classification is programmed by the selection
of a single external resistor R7 connected to the R
CLASS
pin. DC1145B is set for Class 3, full power PD of 12.95W
at the input.
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