LTC4263IS#PBF Linear Technology, LTC4263IS#PBF Datasheet - Page 11

• Manufacturer Part Number: LTC4263IS#PBF
• Description: IC IEEE 803.2AF CNTRLR 14-SOIC
• Manufacturer: Linear Technology
• Type: Power Over Ethernet (PoE)r

Specifications of LTC4263IS#PBF

• Applications: Power Interface Switch for Power Over Ethernet (PoE) Devices
• Voltage - Supply: 48V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 14-SOIC (3.9mm Width), 14-SOL
• Current - Supply: 1mA
• Interface: IEEE 802.3af
• Controller Type: Power Sourcing Equipment Controller (PSE)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIONS INFORMATION
LTC4263 OPERATION
Signature Detection
The IEEE 802.3af specifi cation defi nes a specifi c pair-to-
pair signature resistance used to identify a device that
can accept power via its Ethernet connection. When the
port voltage is below 10V, an IEEE 802.3af compliant
PD will have an input resistance of approximately 25kΩ.
Figure 2 illustrates the relationship between the PD sig-
nature resistance and the required resistance ranges the
PSE must accept and reject. According to the IEEE 802.3af
specifi cation, the PSE must accept PDs with signatures
between 19kΩ and 26.5kΩ and may or may not accept
resistances in the two ranges of 15kΩ to 19kΩ and 26.5kΩ
to 33kΩ. The black box in Figure 2 represents the typical
150Ω pair-to-pair termination used in Ethernet devices
like a computer’s network interface card (NIC) that cannot
accept power.
The LTC4263 checks for the signature resistance by forcing
two test currents on the port in sequence and measuring
the resulting voltages. It then subtracts the two V-I points
to determine the resistive slope while removing voltage
offset caused by any series diodes or current offset caused
by leakage at the port (see Figure 3). The LTC4263 will
typically accept any PD resistance between 17kΩ and
29.7kΩ as a valid PD. Values outside this range (exclud-
ing open and short-circuits) are reported to the user by a
code fl ashed via the LED pin.
The LTC4263 uses a force-current detection method in
order to reduce noise sensitivity and provide a more robust
detection algorithm. The fi rst test point is taken by forcing
a test current into the port, waiting a short time to allow
RESISTANCE
Figure 2. IEEE 802.3af Signature Resistance Ranges
PSE
PD
0Ω
150Ω (NIC)
REJECT
10k
15k
19k
20k
ACCEPT
23.75k
26.5k
26.25k
30k
33k
REJECT
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the line to settle and measuring the resulting voltage. This
result is stored and the second current is applied to the
port, allowed to settle and the voltage measured.
The LTC4263 will not power the port if the PD has more
than 5μF in parallel with its signature resistor unless legacy
mode is enabled.
The LTC4263 autonomously tests for a valid PD connected
to the port. It repeatedly queries the port every 580ms, or
every 3.2s if midspan backoff mode is active (see below).
If detection is successful, it performs classifi cation and
power management and then powers up the port.
Midspan Backoff
IEEE 802.3af requires the midspan PSE to wait two seconds
after a failed detection before attempting to detect again
unless the port resistance is greater than 500kΩ. This
requirement is to prevent the condition of an endpoint PSE
and a midspan PSE, connected to the same PD at the same
time, from each corrupting the PD signature and preventing
power-on. After the fi rst corrupted detection cycle, the
midspan PSE waits while the endpoint PSE completes
detection and turns the port on. If the midspan mode of
the LTC4263 is enabled by connecting the MIDSPAN pin to
V
cycle unless the result is an open circuit.
DD5
, a 3.2 second delay occurs after every failed detect
255
180
OFFSET
0V-2V
Figure 3. PD 2-Point Detection
VALID PD
25kΩ SLOPE
VOLTAGE
DETECTION
SECOND
POINT
DETECTION
POINT
FIRST
LTC4263
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