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參數(shù)資料
| 型號： | 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 |
| 文件頁數(shù)： | 13/23頁 |
| 文件大?。?/td> | 649K |
| 代理商： | 2377 |
www.ti.com
FAULT CONDITIONS IN TPS2375-1 vs. TPS2375
MAINTAIN POWER SIGNATURE
POWER GOOD
TPS2375-1
TPS2377-1
SLVS570–MARCH 2005
APPLICATION INFORMATION (continued)
because of the large thermal dissipation, which may be as high as 4.5 W. The MOSFET is turned off, and PG
goes low, until the die cools and the MOSFET is re-enabled. If V
(RTN-VSS)
is less than 10 V when re-enabled, the
current limit remains at 450 mA and PG goes open-drain. If the overload has caused V
(RTN-VSS)
to exceed 10 V
while the MOSFET was disabled, then the current limit is set to the inrush level and PG remains low.
Downstream converters that use PG control are turned off, permitting a normal start cycle. Converters that do not
use PG need to allow a restart by either drawing less current than the inrush current limit provides, or by
disabling long enough to allow the bulk capacitor to recharge. A converter that has bootstrap startup can be
designed to accomplish this goal.
A hard overload is one that forces the internal MOSFET into current limit with V
(RTN-VSS)
greater than 10 V for 150
μ
s. The MOSFET current limit switches to inrush level and PG goes low. Switching to the inrush current limit
under this condition reduces the stress to the TPS2375-1, other PD components, and the PSE. The peak power
dissipated depends on the inrush current programmed using the ILIM pin, which is the basis of choosing lower
current limits per the preceding recommendations. If the overload persists, the TPS2375-1 begins to thermally
cycle. The bulk capacitor recharges with a normal start cycle if the overload is removed, and the PD recovers.
The same comments about converter control with PG and restart apply.
Another possible condition is a rapid input voltage rise to the PD. The TPS2375-1 is forced into current limit
when the capacitor charges to the higher voltage, while also supplying the load. If the step voltage is small
enough, the capacitor recharges and operation is unaffected. If the load demand is close to the current limit, and
a large bulk capacitor is used, then thermal limit can be triggered. If V
is greater than 10 V, the current
limit drops to the inrush value and PG goes open-drain. Recovery occurs as previously discussed.
The TPS2375-1 uses the current limit, internal thermal shutdown, and foldback to inrush to protect itself, the
load, and the PSE during normal operation. The TPS2375-1 does not latch off under these fault conditions. By
contrast, the TPS2375 does latch off after eight overtemperature cycles, or after an overload or input transient
causes the voltage across the pass MOSFET to exceed 10 V for 150
μ
s. The TPS2375-1 automatically restarts
the PD if the load fault is removed or after a source transient, such as a voltage dropout. The TPS2375 provides
the best protection for itself, the load, and the PSE while the TPS2375-1 provides automatic recovery under
adverse conditions.
The TPS2375-1 satisfies system designs that require hotswap which automatically restarts after prolonged
overload or input voltage transients. In addition to differences in protection, there is a difference in how a PSE
handles the different devices. A PSE using dc MPS interprets a latched-off device as PD removal due to loss of
MPS, and recycles the input power. PSEs with ac MPS may interpret the latched-off PD impedance as
operational, and continue power. This operation is subject to wide variation due to the excessive range allowed in
the IEEE 802.3af standard. A continually cycling PD satisfies PSEs with both ac and dc MPS that the PD is
present, and the PSE does not have an indication that there is a problem with the PD.
Once a valid PD has been detected and powered, the PSE uses the maintain power signature (MPS) to
determine when to remove power from the PI. The PSE removes power from that output port if it detects loss of
MPS for 300 ms or more. A valid MPS requires the PD to draw at least 10 mA and also have an ac impedance
less than 26.25 k
in parallel with 0.05 μF. TI's reference designs meet the requirements necessary to maintain
power.
The TPS2375-1 includes a power-good circuit that can be used to signal the PD circuitry that the load capacitor
is fully charged. This pin is intended for use as an enable signal for downstream circuitry. If the converter tries to
start up while inrush is active, and draws a current equal to the inrush limit, a latchup condition occurs in which
the PD never successfully starts. Using the PG pin is the safest way to ensure that there are no undesired
interactions between the inrush limit, the converter startup characteristic, and the size of the bulk capacitor.
13
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