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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(shè)備控制器，自動重試 |
| 文件頁數(shù)： | 14/23頁 |
| 文件大?。?/td> | 649K |
| 代理商： | 2377-1 |
www.ti.com
THERMAL PROTECTION
POWER SYSTEM DESIGN
AUXILIARY POWER SOURCE ORING
TPS2375-1
TPS2377-1
SLVS570–MARCH 2005
APPLICATION INFORMATION (continued)
The PG pin goes to an open-drain state approximately 150 μs after the inrush current falls 10% below the
regulated value. PG pulldown current is only ensured when the voltage difference between VDD and RTN
exceeds 4 V. This is not a limiting factor because the dc/dc converter should not be able to run from 4 V. The PG
output is pulled to RTN whenever the MOSFET is disabled, is in inrush current limiting, or the voltage rises
above 10 V.
Referencing PG to RTN simplifies the interface to the downstream dc/dc converter or other circuit because it is
referenced to RTN, not VSS. Care must be used in interfacing the PG pin to the downstream circuits. The pullup
to VDD shown in Figure 1 may not be appropriate for a particular dc/dc converter interface. The PG pin connects
to an internal open-drain, 100-V transistor capable of sinking 2 mA to a voltage below 0.4 V. The PG pin can be
left open if it is not used.
The controller may overheat after operation in inrush current-limit, or classification for an extended period of time,
or if the ambient temperature becomes excessive. The TPS2375-1 protects itself by disabling the RTN and
CLASS pins when the internal die temperature reaches about 140
°
C. It automatically restarts when the die
temperature has fallen approximately 20
°
C. This process is referred to as thermal cycling. Thermal protection is
active whenever the TPS2375 is not in detection.
Many PSEs do not recognize, and deactivate, a PD that fails with a short beyond the TPS2375-1, because the
MPS criteria are satisfied. The TPS2375-1 continues thermal cycles under this condition to protect itself and
other downstream components that repeatedly exceed the recommended junction temperature of 125
°
C. Short
periods of thermal cycling do not significantly impact the reliability or life expectancy, but prolonged periods may.
Other components in the power path can be overstressed if this condition exists for a prolonged time as well.
Additional protection for parts that cannot take the repetitive overstress is provided by the TPS2375 with latch-off.
The PSE is a power and current limited source, which imposes certain constraints on the PD power supply
design. DC/DC converters have both a constant input power characteristic that causes them to draw high
currents at low voltage, and they tend to go to a full input power mode during start-up that is often 25% or more
above their rated power. Improper design of the power system can cause the PD to not start up with all
combinations of Ethernet lines and PSE sources.
The following guidelines should be used:
1. Set the TPS2375-1 inrush to a moderate value as previously discussed.
2. Hold the dc/dc converter off during inrush as previously discussed.
3. The converter should have a softstart that keeps the peak input start-up current below 400 mA, and
preferably only a modest amount over the operating current, with a 44-V PSE source and a 20-
loop.
4. If step 3 cannot be met, the bulk input capacitor should not discharge more than 8 V during converter
start-up from a 400-mA limited, 44-V source with a 20-
line. Start-up must be completed in less than 50 mS
Step 4 requires a balance between the converter output capacitance, load, and input bulk capacitance. While
there are some cases which may not require all these measures, such as a 1-W PD with minimal converter
output capacitance, it is always a good practice to follow them.
Many PoE-capable devices are designed to operate from either a wall adapter or PoE power. A local power
solution adds cost and complexity, but allows a product to be used regardless of PoE availability. Attempting to
create solutions where the two power sources coexist in a specific controlled manner results in additional
complexity, and is not generally recommended. Figure 18 demonstrates three methods of diode ORing external
power into a PD. Option 1 inserts power on the output side of the PoE power conversion. Option 2 inserts power
on the TPS2375-1 output. Option 3 applies power to the TPS2375-1 input. Each of these options has
advantages and disadvantages. The wall adapter must meet a minimum 1500-Vac dielectric withstand test
voltage to the ac input power and to ground for options 2 and 3.
14
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