LTC4265
8
4265fb
For more information www.linear.com/LTC4265
applicaTions inForMaTion
These modes satisfy the requirements defined in the IEEE
802.3af/at specification.
Table 1. LTC4265 Modes of Operation as a Function
of Input Voltage
GND (V) LTC4265 MODES OF OPERATION
0V to 1.4V Inactive (Reset After 1st Classification Event)
1.5V to 9.8V
(5.4V to 9.8V)
25k Signature Resistor Detection Before 1st
Classification Event (Mark, 11k Signature
Corrupt After 1st Classification Event)
12.5V to ON/UVLO* Classification Load Current Active
ON/UVLO* to 60V Inrush and Power Applied T
o PD Load
>71V Over
voltage Lockout, 4265 Operations are Disabled
*ON/UVLO includes hysteresis. Rising input threshold, 37.2V Max.
Falling input threshold, 30V Min.
INPUT DIODE BRIDGE
In the IEEE 802.3af/at standard, the modes of operation
reference the input voltage at the PD’s RJ45 connector.
Since the PD must handle power received in either polarity
from either the data or the spare pair, input diode bridges
BR1 and BR2 are connected between the RJ45 connector
and the LTC4265 (Figure 2).
RX
–
6
RX
+
3
TX
–
2
TX
+
RJ45
T1
POWERED
DEVICE (PD)
INPUT
4265 F02
1
7
8
5
4
SPARE
–
SPARE
+
TO PHY
BR2
0.1µF
100V
BR1
GND
D3
LTC4265
V
IN
Figure 2. PD Front End Using Diode Bridges on Main and Spare Inputs
The input diode bridge introduces a voltage drop that
affects the range for each mode of operation. The LTC4265
compensates for these voltage drops so that a PD built with
the LTC4265 meets the IEEE 802.3af/at-established voltage
ranges. Note that the Electrical Specifications reference
with respect to the LTC4265 package pins.
DETECTION
During detection, the PSE looks for a 25k signature resistor
which identifies the device as a PD. The PSE will apply
two voltages in the range of 2.8V to 10V and measures
the corresponding currents. Figure 1 shows the detection
voltages V1 and V2 and the corresponding PD current.
The PSE calculates the signature resistance using the ΔV/
ΔI measurement technique.
The LTC4265 presents its precision, temperature-compen
-
sated 25k resistor between the GND
and V
IN
pins, alerting
the PSE that a PD is present and requests power to be
applied. The LTC4265 signature resistor also compensates
for the additional series resistance introduced by the input
diode bridge. Thus a PD built with the LTC4265 conforms
to the IEEE 802.3af/at detection specifications.