ltc4268cdkd-1-trpbf Linear Technology Corporation, ltc4268cdkd-1-trpbf Datasheet - Page 17

• Manufacturer Part Number: ltc4268cdkd-1-trpbf
• Description: High Power Pd With Synchronous Noopto Flyback Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
In some designs that include an auxiliary power option,
such as an external wall adapter, it is necessary to control
whether or not the PD is detected by a PSE. With the
LTC4268-1, the 25k signature resistor can be enabled or
disabled with the SHDN pin (Figure 5). Taking the SHDN
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 LTC4268-1. To disable the signature,
tie SHDN to V
driven high with respect to V
all functions are disabled. For normal operation tie SHDN
to V
PORTN
.
PORTP
.
Alternately, the SHDN pin can be
BY IEEE 802.3af
DEVICE (PD)
AS DEFINED
INTERFACE
POWERED
Figure 4. PD Front End Using Diode Bridges on Main and Spare Inputs
PORTN
RJ45
. When SHDN is high,
1
2
3
6
4
5
7
8
PSE
TX
TX
RX
RX
TO
Figure 5. 25k Signature Resistor with Disable
+
–
+
–
SPARE
SPARE
+
–
SHDN
LTC4268-1
T1
V
PORTN
BR2
TO PHY
16k
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 was reserved by the IEEE 802.3af
committee for future use and has been reassigned as a
high power indicator by IEEE 802.3at. The new Class 5
SIGNATURE DISABLE
25k SIGNATURE
RESISTOR
0.1μF
100V
V
PORTP
BR1
42681 F05
D3
V
LTC4268-1
PORTN
V
PORTP
42681 F04
LTC4268-1
17
42681fa