參數(shù)資料
型號(hào)： 2377
廠商： Texas Instruments, Inc.
英文描述： IEEE 802.3af PoE POWERED DEVICE CONTROLLERS WITH AUTO-RETRY
中文描述： IEEE 802.3af標(biāo)準(zhǔn)的PoE用電設(shè)備控制器，自動(dòng)重試
文件頁(yè)數(shù)： 10/23頁(yè)
文件大?。?/td> 649K
代理商： 2377
www.ti.com
APPLICATION INFORMATION
OVERVIEW
The IEEE 802.3af specification defines a process for safely powering a powered device (PD) over a cable, and
then removing power if a PD is disconnected. The process proceeds through three operational states: detection,
classification, and operation. The intent behind the process is to leave an unterminated cable unpowered while
the PSE periodically checks for a plugged-in device; this is referred to as detection. The low power levels used
during detection are unlikely to cause damage to devices not designed for PoE. If a valid PD signature is
present, then the PSE may optionally inquire how much power the PD requires; this is referred to as
classification. The PD may return a default full-power signature, or one of four other choices. Knowing the power
demand of each PD allows the PSE to intelligently allocate power between PDs, and also to protect itself against
overload. The PSE powers up a valid PD, and then monitors its output for overloads. The maintain power
signature (MPS) is presented by the powered PD to assure the PSE that it is there. The PSE monitors its output
for the MPS to see if the PD is removed, and turns the port off, if it loses the MPS. Loss of MPS returns the PSE
to the initial state of detection. Figure 16 shows the operational states as a function of PD input voltage range.
Normal Operation
57
42
36
30
20.5
14.5
10.1
2.7
D
L
D
U
C
L
C
U
M
V
L
P
M
V
M
V
Detect
Classify
Shut -
down
PI Voltage (V)
0
INTERNAL THRESHOLDS
In order to implement the PoE functionality as shown in Figure 16, the TPS2375-1 has a number of internal
comparators with hysteresis for stable switching between the various states. Figure 17 relates the parameters in
the Electrical Characteristics section to the PoE states. The mode labeled
idle
between classification and
detection implies that the DET, CLASS, PG, and RTN pins are all high impedance.
TPS2375-1
TPS2377-1
SLVS570–MARCH 2005
The PD input is typically an RJ-45 (8-pin) connector, referred to as the power interface (PI). PD input
requirements differ from PSE output requirements to account for voltage drops in the cable and margin. The
specification uses a cable resistance of 20
to derive the voltage limits at the PD from the PSE output
requirements. Although the standard specifies an output power of 15.4 W at the PSE output, there is only
12.95 W available at the input of the PD due to the worst-case power loss in the cable.
The PSE can apply voltage either between the RX and TX pairs, or between the two spare pairs as shown in
Figure 1. The applied voltage can be of either polarity. The PSE cannot apply voltage to both paths at the same
time. The PD uses input diode bridges to accept power from any of the possible PSE configurations. The voltage
drops associated with the input bridges cause a difference between the IEEE 802.3af limits at the PI and the
TPS2375-1 specifications.
The PSE is required to current limit between 350 mA and 400 mA during normal operation, and it must
disconnect the PD if it draws this current for more than 75 ms. The PSE may set lower output current limits
based on the PD advertised power requirements, as discussed in the Classification section.
The following discussion is intended as an aid in understanding the operation of the TPS2375-1, but not as a
substitute for the actual IEEE 802.3af standard. Standards change and should always be referenced when
making design decisions.
Figure 16. IEEE 802.3 PD Limits
10
相關(guān)PDF資料
PDF描述
2377-1 IEEE 802.3af PoE POWERED DEVICE CONTROLLERS WITH AUTO-RETRY
238.2 5 Volt and 3.3 Volt MicroMonitor
238.25 Fuse
238.3 5 Volt and 3.3 Volt MicroMonitor
238.4 Fuse
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