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參數(shù)資料
| 型號： | 2377-1 |
| 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標準的PoE用電設備控制器，自動重試 |
| 文件頁數(shù)： | 15/23頁 |
| 文件大小： | 649K |
| 代理商： | 2377-1 |
www.ti.com
T
S
R(DET)
R(CLASS)
V
V
CLASS
DET
RTN
Main
DC/DC
Converter
Output
R(ILIM)
R
Option 1
ILIM
DC/DC
Converter
UCC3809
or
UCC3813
Optional
Regulator
Option 2
Option 3
A Full Wave Bridge
Gives Flexibility To
Use Supply With Either
Polarity
For Option 2,
The Capacitor Must Be
Right At The Output
To Control The
Transients.
Auxiliary
Power
Input
Use only
one option
See TI Document SLVR030 For A Typical
Application Circuit.
~
~
+
~
~
+
0
μ
F
22
μ
F
Inserting a Diode in This Location
With Option 2, Allows PoE To Start
With Aux Power Present.
TPS2375-1
TPS2377-1
SLVS570–MARCH 2005
APPLICATION INFORMATION (continued)
Figure 18. Auxiliary Power ORing
Option 1 consists of ORing power to the output of the PoE dc/dc converter. This option is preferred in cases
where PoE is added to an existing design that uses a low-voltage wall adapter. The relatively large PD
capacitance reduces the potential for harmful transients when the adapter is plugged in. The wall adapter output
may be grounded if the PD incorporates an isolated converter. This solution requires two separate regulators, but
low-voltage adapters are readily available. The PoE power can be given priority by setting its output voltage
above that from the auxiliary source.
Option 2 has the benefits that the adapter voltage may be lower than the TPS2375-1 UVLO, and that the bulk
capacitor shown can control voltage transients caused by plugging in an adapter. The capacitor size and location
are chosen to control the amount of ringing that can occur on this node, which can be affected by additional
filtering components specific to a dc/dc converter design. The optional diode blocks the adapter voltage from
reverse biasing the input, and allows a PoE source to apply power provided that the PSE output voltage is
greater than the adapter voltage. The penalty of the diode is an additional power loss when running from PSE
power. The PSE may not be able to detect and start powering without the diode. This means that the adapter
may continue to power the PD until removed. Auxiliary voltage sources can be selected to be above or below the
PoE operational voltage range. If automatic PoE precedence is desired when using the low-voltage auxiliary
source option, make sure that the TPS2375-1 inrush program limit is set higher than the maximum converter
input current at its lowest operating voltage. It is difficult to use PG with the low-voltage auxiliary source because
the converter must operate during a condition when the TPS2375-1 would normally disable it. Circuits may be
designed to force operation from one source or the other depending on the desired operation and the auxiliary
source voltage chosen. However, they are not recommended because they increase complexity and thus cost.
Option 3 inserts the power before the TPS2375-1. It is necessary for the adapter to meet the TPS2375-1 UVLO
turnon requirement and to limit the maximum voltage to 57 V. This option provides a valid power-good signal and
simplifies power priority issues. The disadvantage of this method is that it is the most likely to cause transient
voltage problems. Plugging a powered adapter in applies a step input voltage to a node that has little
capacitance to control the dv/dt and voltage ringing. If the wall mount supply applies power to the PD before the
PSE, it prevents the PSE from detecting the PD. If the PSE is already powering the PD when the auxiliary source
is plugged in, priority is given to the higher supply voltage.
15
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