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参数资料
| 型号： | DC859A |
| 厂商： | Linear Technology |
| 文件页数： | 3/32页 |
| 文件大小： | 0K |
| 描述： | EVAL BOARD FOR LTC4267 |
| 设计资源： | DC859A Design File DC859A Schematic |
| 标准包装： | 1 |
| 主要目的： | 电源管理，以太网供电（POE） |
| 嵌入式： | 否 |
| 已用 IC / 零件： | LTC4267 |
| 已供物品： | 板 |
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LTC4267
11
4267fc
Series Diodes
The IEEE 802.3af-dened 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 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 25kΩ
signature resistor. This identies the device at the end of
the cable as a PD. With the terminal voltage in this range,
the LTC4267 connects an internal 25kΩ resistor between
the VPORTP and VPORTN 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 detec-
tion signature.
The LTC4267 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
specication requires the PSE to use a ΔV/ΔI 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 compensates for the
two series diodes in the signature path by offsetting the
resistance so that a PD built using the LTC4267 will meet
the IEEE specication.
In some applications it is necessary to control whether or
not the PD is detected. In this case, the 25kΩ 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 9kΩ
(typical) which is an invalid signature per the IEEE 802.3af
specication. 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 25kΩ to minimize power
dissipation in the LTC4267. To disable the signature, tie
SIGDISA to VPORTP. Alternately, the SIGDISA pin can be
driven high with respect to VPORTN. When SIGDISA is high,
all functions of the PD interface are disabled.
APPLICATIO S I FOR ATIO
WU
UU
RX–
6
RX+
3
TX–
2
TX+
RJ45
T1
POWERED DEVICE (PD)
INTERFACE
AS DEFINED
BY IEEE 802.3af
4267 F02
1
7
8
5
4
SPARE–
SPARE+
TO PHY
BR2
BR1
VPORTP
8
4
D3
LTC4267
VPORTN
Figure 2. LTC4267 PD Front End Using
Diode Bridges on Main and Spare Inputs
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