Datasheet

LTC4264
10
4264f
APPLICATIONS INFORMATION
RX
–
6
RX
+
3
TX
–
2
TX
+
RJ45
T1
POWERED
DEVICE (PD)
INTERFACE
AS DEFINED
BY IEEE 802.3af
4264 F04
1
7
8
5
4
SPARE
–
SPARE
+
TO PHY
BR2
0.1µF
100V
BR1
GND
D3
LTC4264
V
IN
Figure 4. PD Front End Using Diode Bridges on Main and Spare Inputs
pin high will reduce the signature resistor to 10k which is
an invalid signature per the IEEE 802.3af specifi cations.
This will prevent a PSE from detecting and powering the
PD. This invalid signature is present in the PSE probing
range of –2.8V to –10V. When the input rises above –10V,
the signature resistor reverts to 25k to minimize power
dissipation in the LTC4264. To disable the signature, tie
SHDN to GND. Alternately, the SHDN pin can be driven
high with respect to V
IN
. When SHDN is high, all functions
are disabled. For normal operation tie SHDN to V
IN
.
CLASSIFICATION
Once the PSE has detected a PD, the PSE may option-
ally classify the PD. Classifi cation provides a method for
more effi cient allocation of power by allowing the PSE
to identify lower-power PDs and assign the appropriate
power level to these devices. For each class, there is an
associated load current that the PD asserts onto the line
during classifi cation probing. The PSE measures the PD
load current in order to assign the proper PD classifi ca-
tion. Class 0 is included in the IEEE 802.3af specifi cation
to cover PDs that do not support classifi cation. Class 1-3
partition PDs into three distinct power ranges as shown in
Table 2. Class 4 is reserved by the IEEE 802.3af committee
for future use. The new Class 5 defi ned here is available
for system vendors to implement a unique classifi cation
for use in closed systems and is not defi ned or supported
by the IEEE 802.3af. With the extended classifi cation
range available in the LTC4264, it is possible for system
designers to defi ne multiple classes using load currents
between 40mA and 75mA.
GND
V
IN
SHDN
LTC4264
SIGNATURE DISABLE
4264 F05
25k SIGNATURE
RESISTOR
16k
TO
PSE
Figure 5. 25k Signature Resistor with Disable