Datasheet
LTC4257-1
7
42571fb
summarized in Table 1. These various modes satisfy the
requirements defined in the IEEE 802.3af specification.
The input voltage is applied to the V
IN
pin and is with
reference to the GND pin. This input voltage is always
negative. To avoid confusion, voltages in this data sheet
are always referred to in terms of absolute magnitude.
Terms such as
maximum negative voltage
refer to the
largest negative voltage and a
rising negative voltage
refers to a voltage that is becoming more negative. Refer-
ences to electrical parameters in this applications section
use the nominal value. Refer to the Electrical Characteris-
tics section for the range of values a particular parameter
will have.
Table 1. LTC4257-1 Operational Mode
as a Function of Input Voltage
INPUT VOLTAGE
(V
IN
with RESPECT to GND) LTC4257-1 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 ≅ –36.0V
Falling input threshold ≅ –30.5V
Figure 2. PD Front End Using Diode Bridges On Main and Spare Inputs
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 LTC4257-1 (Figure
2). The LTC4257-1 takes this into account by compensat-
ing for these diode drops in the threshold points for each
range of operation. Since the voltage ranges specified in
the LTC4257-1 electrical specifications are with respect to
the IC pins, for both the signature and classification
ranges, the LTC4257-1 lower end extends two diode drops
below the IEEE 802.3af specification. A similar adjustment
is made for the UVLO voltages.
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
LTC4257-1 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.
APPLICATIO S I FOR ATIO
WUUU
RX
–
6
RX
+
3
TX
–
2
TX
+
RJ45
T1
POWERED DEVICE (PD)
INTERFACE
AS DEFINED
BY IEEE 802.3af
42571 F02
1
7
8
5
4
SPARE
–
SPARE
+
TO PHY
BR2
BR1
GND
8
4
D3
LTC4257-1
V
IN