ltc4267-3 Linear Technology Corporation, ltc4267-3 Datasheet - Page 11

• Manufacturer Part Number: ltc4267-3
• Description: Power Over Ethernet Ieee 802.3af Pd Interface With Integrated 300khz Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Series Diodes
The IEEE 802.3af-defi ned operating modes for a PD refer-
ence the input voltage at the RJ45 connector on the PD.
The PD must be able to accept power of either polarity
at each of its inputs, so it is common to install diode
bridges (Figure 2). The LTC4267-3 takes this into account
by compensating for these diode drops in the threshold
points for each range of operation. A similar adjustment
is made for the UVLO voltages.
Detection
During detection, the PSE will apply a voltage in the
range of –2.8V to –10V on the cable and look for a 25k
signature resistor. This identifi es the device at the end of
the cable as a PD. With the terminal voltage in this range,
the LTC4267-3 connects an internal 25k resistor between
the V
compensated resistor presents the proper signature to
alert the PSE that a PD is present and desires power to be
applied. The internal low-leakage UVLO switch prevents
the switching regulator circuitry from affecting the detec-
tion signature.
The LTC4267-3 is designed to compensate for the voltage
and resistance effects of the IEEE required diode bridge.
PORTP
and V
POWERED DEVICE (PD)
PORTN
U
BY IEEE 802.3af
pins. This precision, temperature
AS DEFINED
INTERFACE
U
RJ45
1
2
3
6
4
5
7
8
W
TX
TX
RX
RX
+
–
+
–
SPARE
SPARE
Figure 2. LTC4267-3 PD Front End Using
Diode Bridges on Main and Spare Inputs
+
–
U
T1
TO PHY
BR2
The signature range extends below the IEEE range to ac-
commodate the voltage drop of the two diodes. The IEEE
specifi cation requires the PSE to use a ∆V/∆I measurement
technique to keep the DC offset of these diodes from af-
fecting the signature resistance measurement. However,
the diode resistance appears in series with the signature
resistor and must be included in the overall signature
resistance of the PD. The LTC4267-3 compensates for
the two series diodes in the signature path by offsetting
the resistance so that a PD built using the LTC4267-3 will
meet the IEEE specifi cation.
In some applications it is necessary to control whether
or not the PD is detected. In this case, the 25k signature
resistor can be enabled and disabled with the use of the
SIGDISA pin (Figure 3). Disabling the signature via the
SIGDISA pin will change the signature resistor to 9k
(typical) which is an invalid signature per the IEEE 802.3af
specifi cation. This invalid signature is present for PD input
voltages from –2.8V to –10V. If the input rises above –10V,
the signature resistor reverts to 25k to minimize power
dissipation in the LTC4267-3. To disable the signature,
tie SIGDISA to V
be driven high with respect to V
high, all functions of the PD interface are disabled.
PORTP
D3
BR1
. Alternately, the SIGDISA pin can
V
LTC4267-3
PORTN
V
PORTP
42673 F02
PORTN
LTC4267-3
. When SIGDISA is
11
42673f