Datasheet

LTC4257-1
9
42571fb
specification. The LTC4257-1 is compatible with the IEEE
802.3af standard and also works with the obsolete Mea-
sured Voltage method.
In the Measured Current method (Figure 4), the PSE
presents a fixed voltage between –15.5V and –20.5V to the
PD. With the input voltage in this range, the LTC4257-1
asserts a load current from the GND pin through the
R
CLASS
resistor. The magnitude of the load current is set
with the selection of the R
CLASS
resistor. The resistor value
associated with each class is shown in Table 2.
In the Measured Voltage method (Figure 5), the PSE drives
a current into the PD and monitors the voltage across the
PD terminals. The PSE current steps between classifica-
tion load current values in order to classify the PD under
test. For PSE probe currents below the PD load current, the
LTC4257-1 will keep the PD terminal voltage below the
classification voltage range. For PSE probe currents above
the PD load current, the LTC4257-1 will force the PD
terminal voltage above the classification voltage range.
During classification, a moderate amount of power is
dissipated in the LTC4257-1. The IEEE 802.3af specifica-
tion limits the classification time to 75ms. The LTC4257-1
is designed to handle the power dissipation for this time
period. If the PSE probing exceeds 75ms, the LTC4257-1
may overheat. In this situation, the thermal protection
circuit will engage and disable the classification current
source in order to protect the part. The LTC4257-1 stays
in classification mode until the input voltage rises above
the UVLO turn-on voltage.
APPLICATIO S I FOR ATIO
WUUU
Figure 5. IEEE 802.af Measured-Voltage Method
of Classification Probing
GND
R
CLASS
V
IN
82
4
LTC4257-1
CONSTANT
LOAD
CURRENT
INTERNAL
TO LTC4257-1
42571 F05
R
CLASS
CURRENT PATH
V
PDPSE
PSE
VOLTAGE
MONITOR
PSE
PROBING
CURRENT
SOURCE
IF PSE PROBING CURRENT < LTC4257-1 LOAD CURRENT, PD TERMINAL VOLTAGE IS < 15V
IF PSE PROBING CURRENT > LTC4257-1 LOAD CURRENT, PD TERMINAL VOLTAGE IS > 20V
GND
R
CLASS
V
IN
82
4
LTC4257-1
CONSTANT
LOAD
CURRENT
INTERNAL
TO LTC4257-1
42571 F04
R
CLASS
CURRENT PATH
V
PD
PSE
PSE CURRENT MONITOR
PSE
PROBING
VOLTAGE
SOURCE
–15.5V TO –20.5V
Figure 4. IEEE 802.3af Measured-Current Method
of Classification Probing