Datasheet

LTC4267-1
11
42671fa
For more information www.linear.com/4267-1
applicaTions inForMaTion
Series Diodes
The IEEE 802.3af-defined 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 LTC4267-1 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 identifies the device at the end of the cable as
a PD. With the terminal voltage in this range, the LTC4267-1
connects an internal 25k resistor between the V
PORTP
and
V
PORTN
pins. This precision, temperature 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 detection signature.
The LTC4267-1 is designed to compensate for the voltage
and resistance effects of the IEEE required diode bridge
.
The signature range extends below the IEEE range to ac-
commodate the voltage drop of the two diodes. The IEEE
specification 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 LTC4267-1 compensates for
the two series diodes in the signature path by offsetting
the resistance so that a PD built using the LTC4267-1 will
meet the IEEE specification.
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
specification. 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 LTC4267-1. To disable the signature
,
tie
SIGDISA to V
PORTP
. Alternately, the SIGDISA pin can
be driven high with respect to V
PORTN
. When SIGDISA is
high, all functions of the PD interface are disabled.
RX
–
6
RX
+
3
TX
–
2
TX
+
RJ45
T1
POWERED DEVICE (PD)
INTERFACE
AS DEFINED
BY IEEE 802.3af
42671 F02
1
7
8
5
4
SPARE
–
SPARE
+
TO PHY
BR2
BR1
V
PORTP
8
4
D3
LTC4267-1
V
PORTN
Figure 2. LTC4267-1 PD Front End Using
Diode Bridges on Main and Spare Inputs