LTC4267-1
12
42671fa
For more information www.linear.com/4267-1
applicaTions inForMaTion
Classification
Once the PSE has detected a PD, the PSE may option-
ally 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 devices. The IEEE 802.3af specification 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 as-
sociated 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.
During classification (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 LTC4267-1 asserts a load
current from the V
PORTP
pin through the R
CLASS
resistor.
The magnitude of the load current is set by the R
CLASS
resistor. The resistor values associated with each class
are shown in Table 2. Note that the switching regulator
will not interfere with the classification measurement since
the LTC4267-1 has not passed power to the regulator.
Table 2. Summary of IEEE 802.3af Power Classifications and
LTC4267-1 R
CLASS
Resistor Selection
Class Usage
Maximum
Power Levels
at Input of PD
(W)
Nominal
Classification
Load Current
(mA)
LTC4267-1
R
CLASS
Resistor
(Ω, 1%)
0 Default 0.44 to 13.0 <5 Open
1 Optional 0.44 to 3.84 10.5 124
2 Optional 3.84 to 6.49 18.5 68.1
3 Optional 6.49 to 13.0 28 45.3
4 Reserved Reserved* 40 30.9
*Class 4 is currently reserved and should not be used.
The IEEE 802.3af specification limits the classification
time to 75ms because a significant amount of power is
dissipated in the PD. The LTC4267-1 is designed to handle
the power dissipation for this time period. If the PSE prob-
ing exceeds 75ms, the LTC4267-1 may overheat. In this
situation, the thermal protection circuit will engage and
disable the classification current source in order to protect
the part. The LTC4267-1 stays in classification mode until
the input voltage rises above the UVLO turn-on voltage.
V
PORTN
Undervoltage Lockout
The IEEE specification dictates a maximum turn-on voltage
of 42V and a minimum turn-off voltage of 30V for the PD.
In addition, the PD must maintain large on-off hysteresis to
prevent resistive losses in the wiring between the PSE and
the PD from causing start-up oscillation. The LTC
4267-1
incorporates
an undervoltage lockout (UVLO) circuit that
monitors the line voltage at V
PORTN
to determine when
to apply power to the integrated switching regulator
(Figure 5). Before the power is applied to the switching
regulator, the P
OUT
pin is high impedance and sitting at
the ground potential since there is no charge on capacitor
C1. When the input voltage rises above the UVLO turn-on
threshold, the LTC4267-1 removes the detection and clas-
sification loads and turns on the internal power MOSFET.
C1 charges up under the LTC4267-1 current limit control
and the P
OUT
pin transitions from 0V to V
PORTN
. This
sequence is shown in Figure 1. The LTC4267-1 includes
a hysteretic UVLO circuit on V
PORTN
that keeps power
applied to the load until the input voltage falls below the
UVLO turn-off threshold. Once the input voltage drops
below –30V, the internal power MOSFET is turned off and
Figure 4. IEEE 802.3af Classification Probing
Figure 3. 25k Signature Resistor with Disable
V
PORTP
V
PORTN
LTC4267-1
42671 F03
25k SIGNATURE
RESISTOR
SIGNATURE DISABLE
SIGDISA
9k
16k
TO
PSE
V
PORTP
R
CLASS
V
PORTN
LTC4267-1
CONSTANT
LOAD
CURRENT
INTERNAL
TO LTC4267-1
42671 F04
R
CLASS
CURRENT PATH
V
PDPSE
PSE CURRENT MONITOR
PSE
PROBING
VOLTAGE
SOURCE
–15.5V TO –20.5V
