LTC4257IS8 Linear Technology, LTC4257IS8 Datasheet - Page 8

• Manufacturer Part Number: LTC4257IS8
• Description: IC CONTROLLER POE INTERFAC 8SOIC
• Manufacturer: Linear Technology
• Type: Power over Ethernet Switch (PoE)r

Specifications of LTC4257IS8

• Applications: IP Phones, Power over LAN, Network Routers and Switches
• Internal Switch(s): Yes
• Current Limit: 350mA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
LTC4257
Classification
Once the PSE has detected a PD, the PSE may optionally
classify the PD. Classification provides a method for more
efficient allocation of power by allowing the PSE to identify
lower-power PDs and allocate less power for these de-
vices. IEEE 802.3af defines five classes (Table 2) with
varying power levels. The designer selects the appropriate
classification based on the power consumption of the PD.
For each class, there is an associated load current that the
PD asserts onto the line during classification probing. The
PSE measures the PD load current to determine the proper
classification and PD power requirements.
Table 2. Summary of IEEE 802.3af Power Classifications and
LTC4257 R
CLASS
0
1
2
3
4
*Class 4 is currently reserved and should not be used.
Early revisions of the IEEE 802.3af draft specification
defined two methods that a PSE could use in order to
perform PD classification. These methods are known as
–15.5V TO –20.5V
8
Figure 3. IEEE 802.3af Measured-Current Method
of Classification Probing
PROBING
VOLTAGE
SOURCE
Reserved
Optional
Optional
Optional
USAGE
Default
PSE
CLASS
PSE CURRENT MONITOR
Resistor Selection
AT INPUT OF PD
POWER LEVELS
PSE
V
0.44 to 12.95
6.49 to 12.95
0.44 to 3.84
3.84 to 6.49
MAXIMUM
Reserved*
U
(W)
U
CURRENT PATH
R
CLASS
CLASSIFICATION
LOAD CURRENT
2
4
W
NOMINAL
R
V
LTC4257
CLASS
IN
PD
(mA)
10.5
18.5
< 5
28
40
GND
4257 F03
U
8
RESISTOR
LTC4257
(Ω, 1%)
R
CONSTANT
LOAD
CURRENT
INTERNAL
TO LTC4257
Open
68.1
45.3
30.9
CLASS
124
IF PSE PROBING CURRENT < LTC4257 LOAD CURRENT, PD TERMINAL VOLTAGE IS < 15V
IF PSE PROBING CURRENT > LTC4257 LOAD CURRENT, PD TERMINAL VOLTAGE IS > 20V
Measured Current and Measured Voltage. The IEEE has
since removed the Measured Voltage method from the
specification. The LTC4257 is compatible with the IEEE
802.3af standard and also works with the obsolete Mea-
sured Voltage method.
In the Measured Current method (Figure 3), the PSE
presents a fixed voltage between –15.5V and –20V to the
PD. With the input voltage in this range, the LTC4257
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 4), 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 will keep the PD terminal voltage below the
classification voltage range. For PSE probe currents above
the PD load current, the LTC4257 will force the PD terminal
voltage above the classification voltage range.
During classification, a moderate amount of power is
dissipated in the LTC4257. IEEE 802.3af limits the classi-
fication time to 75ms. The LTC4257 is designed to handle
the power dissipation for this time period. If the PSE
CURRENT
PROBING
SOURCE
CLASS
PSE
Figure 4. IEEE 802.af Measured-Voltage Method
of Classification Probing
resistor. The magnitude of the load current is set
PSE
V
PSE
VOLTAGE
MONITOR
CURRENT PATH
CLASS
R
CLASS
resistor. The resistor value
2
4
PD
R
V
LTC4257
CLASS
IN
GND
8
CONSTANT
LOAD
CURRENT
INTERNAL
TO LTC4257
4257fb
4257 F04