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LTC4257
7
4257fb
Table 1. LTC4257 Operational Mode
as a Function of Input Voltage
INPUT VOLTAGE
(V
IN
 with RESPECT to GND)    LTC4257 MODE OF OPERATION
0V to 1.4V
Inactive
1.5V to 10V
25k Signature Resistor Detection
11V to 12.4V
Classification Load Current Ramps up from
0% to 100%
12.5V to UVLO*
Classification Load Current Active
UVLO* to 57V
Power Applied to PD Load
*UVLO includes hysteresis.
Rising input threshold E 39.2V
Falling input threshold E 30.5V
Series Diodes
The IEEE 802.3af defined operating modes for a PD
reference the input voltage at the RJ45 connector on the
PD. However, PD circuitry must include diode bridges
between the RJ45 connector and the LTC4257 (Figure 2).
The LTC4257 takes this into account by compensating for
these diode drops in the threshold points for each range of
operation. Since the voltage ranges specified in the LTC4257
electrical specifications are with respect to the IC pins for
both the signature and classification ranges, the LTC4257
lower end extends two diode drops below the IEEE 802.3af
specification. A similar adjustment is made for the UVLO
voltages.
Figure 2. PD Front End Using Diode Bridges on Main and Spare Inputs
RX

6
RX
+
3
TX

2
TX
+
RJ45
T1
POWERED DEVICE (PD)
INTERFACE
AS DEFINED
BY IEEE 802.3af
4257 F02
1
7
8
5
4
SPARE

SPARE
+
TO PHY
BR2
BR1
GND
8
4
D3
LTC4257
V
IN
Detection
During detection, the PSE will apply a voltage in the range
of 2.8V to 10V on the cable and look for a 25k signature
resistor. This identifies the device at the end of the cable
as a PD. With the terminal voltage in this range, the LTC4257
connects an internal 25k resistor between GND and the V
IN
pins. This precision, temperature compensated resistor
presents the proper characteristics to alert the Power
Sourcing Equipment (PSE) at the other end of the cable that
a PD is present and desires power to be applied.
The power applied to a PD is allowed to use either of two
polarities and the PD must be able to accept this power so
it is common to install a diode bridge on the input. The
LTC4257 is designed to compensate for the voltage and
resistance effects of these two series diodes. The signa-
ture range extends below the IEEE range to accommodate
the voltage drop of the diodes. The IEEE specification
requires the PSE to use a V/I measurement technique
to keep the DC offset of these diodes from affecting the
signature resistance measurement. However, the diode
resistance appears in series with the signature resistor
and must be included in the overall signature resistance
of the PD. The LTC4257 compensates for the two series
diodes in the signature path by offsetting the resistance
so that a PD built using the LTC4257 will meet the IEEE
requirements.
APPLICATIO S I FOR ATIO
U
U
U
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