LTC4257-1 Linear Technology, LTC4257-1 Datasheet - Page 9

• Manufacturer Part Number: LTC4257-1
• Description: Power over Ethernet Interface Controller with Dual Current Limit
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
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 –20V to the
PD. With the input voltage in this range, the LTC4257-1
asserts a load current from the GND pin through the
R
with the selection of the R
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
CLASS
resistor. The magnitude of the load current is set
U
CLASS
U
–15.5V TO –20V
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
resistor. The resistor value
CURRENT
PROBING
SOURCE
PROBING
VOLTAGE
SOURCE
W
PSE
Figure 4. IEEE 802.3af Measured-Current Method
of Classification Probing
PSE
Figure 5. IEEE 802.af Measured-Voltage Method
of Classification Probing
PSE CURRENT MONITOR
PSE
V
PSE
V
U
PSE
VOLTAGE
MONITOR
CURRENT PATH
CURRENT PATH
R
R
CLASS
CLASS
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.
2
4
2
4
PD
R
V
LTC4257-1
R
V
LTC4257-1
CLASS
IN
IN
CLASS
PD
GND
42571 F04
GND
8
8
CONSTANT
LOAD
CURRENT
INTERNAL
TO LTC4257-1
CONSTANT
LOAD
CURRENT
INTERNAL
TO LTC4257-1
42571 F05
LTC4257-1
42571fa
9