LT4275
6
4275f
applicaTions inForMaTion
OVERVIEW
Power over Ethernet (PoE) continues to gain popularity as
products take advantage of DC power and high speed data
available from a single RJ45 connector. Powered device
(PD) equipment vendors are running into the 25.5W power
limit established by the IEEE 802.3 standard. The LT4275A
allows higher power while maintaining backwards com-
patibility with existing PSE systems. The LT4275 utilizes
a low R
DS(ON)
N-channel MOSFET to maximize efficiency
and delivered power. Heat is also reduced, easing thermal
design.
MODES OF OPERATION
The LT4275 has several modes of operation depending
on the input voltage sequence applied to the VPORT pin.
These modes include 25kΩ signature detection, classifica-
tion, mark, inrush and powered on.
DETECTION
During detection, the PSE looks for a 25kΩ signature
resistor which identifies the device as a PD. The PSE will
apply two voltages in the range of 2.8V to 10V and measure
the corresponding currents. Figure 1 shows the detection
voltages. The PSE calculates the signature resistance using
a ∆V/∆I measurement technique.
The LT4275 presents its precision, temperature-compen-
sated 24.4k resistor between the VPORT and GND pins,
allowing the PSE to recognize a PD is present and request-
ing power
to be applied. The LT4275 signature resistor is
smaller than 25k to compensate for the additional series
resistance introduced by the IEEE required bridge.
C
LASSIFICATION
The detection/classification process varies depending on
whether the PSE is Type 1, Type 2, or LTPoE
++
. A Type 2
PSE may use Type 1 classification signaling and later
renegotiate a higher power classification with the PD over
the data layer.
A Type 1 PSE, after a successful detection, may apply a
classification probe voltage of 15.5V to 20.5V and mea-
sure current.
A Type 2 PSE may declare the availability of high power
by performing 2-event (Physical Layer) classification
or by communicating over the (Data Link Layer) high
speed data line. A Type 2 PD must recognize both types
of communication. Since Layer 2 communications takes
place directly between the PSE and the PD application, the
LT4275A/LT4275B responsibility ends with supporting
2-event classification.
In 2-event classification, a Type 2 PSE probes for power
classification twice as shown in Figure 2. The LT4275A or
LT4275B recognizes this and pulls the T2P pin down to
signal the load that Type 2 power is available. If an LT4275A
senses an LTPoE
++
PSE it alternates between pulling T2P
down
and floating T2P at a rate of f
T2P
.
Figure 1. Type 1 Detect/Class Signaling Waveform
Figure 2. Type 2 Detect/Class Signaling Waveform
4275 F01
V
PORT
V
HSON
V
HSOFF
V
CLASSMIN
V
SIGMAX
V
SIGMIN
V
RESET
DETECT
CLASS
POWER ON
4275 F01
V
PORT
V
HSON
V
HSOFF
V
CLASSMIN
V
SIGMAX
V
SIGMIN
V
RESET
DETECT
1ST CLASS
1ST MARK 2ND MARK
2ND CLASS
POWER ON
