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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(biāo)準的PoE用電設(shè)備控制器，自動重試 |
| 文件頁數(shù)： | 11/23頁 |
| 文件大小： | 649K |
| 代理商： | 2377-1 |
www.ti.com
O
Detection
Classification
PD Powered
Idle
1.4V
V
(CL_ON)
V
(CU_OFF)
V
(CU_H)
V
(UVLO_F)
V
(UVLO_R)
V
(VDD)
DETECTION
This feature of IEEE 802.3af eliminates powering and potentially damaging Ethernet devices not intended for
application of 48 V. When a voltage in the range of 2.7 V to 10.1 V is applied to the PI, an incremental resistance
of 25 k
signals the PSE that the PD is capable of accepting power. A PD that is capable of accepting power,
but is not ready, may present an incorrect signature intentionally. The incremental resistance is measured by
applying two different voltages to the PI and measuring the current it draws. These two test voltages must be
within the specified range and be at least 1 V apart. The incremental resistance equals the difference between
the voltages divided by the difference between the currents. The allowed range of resistance is 23.75 k
to
26.25 k
.
The TPS2375-1 is in detection mode whenever the supply voltage is below the lower classification threshold. The
TPS2375-1 draws a minimum of bias power in this condition, while PG and RTN are high impedance and the
circuits associated with ILIM and CLASS are disabled. The DET pin is pulled to ground during detection. Current
flowing through R
to VSS ( Figure 1) produces the detection signature. For most applications, a 24.9-k
, 1%,
resistor is recommended. R
(DET)
can be a small, low-power resistor because it only sees a stress of about 5 mW.
When the input voltage rises above the 11.3 V lower classification comparator threshold, the DET pin goes to an
open-drain condition to conserve power.
CLASSIFICATION
Once the PSE has detected a PD, it may optionally classify the PD. This process allows a PSE to determine the
PD power requirements in order to allot only as much power as necessary from its fixed input power source. This
allows the PSE to power the maximum number of PDs from a particular power budget. This step is optional
because some PSEs can afford to allot the full power to every powered port.
TPS2375-1
TPS2377-1
SLVS570–MARCH 2005
APPLICATION INFORMATION (continued)
Figure 17. Threshold Voltages
The input diode bridge incremental resistance can be hundreds of ohms at the low currents seen at 2.7 V on the
PI. The bridge resistance is in series with R
(DET)
and increases the total resistance seen by the PSE. This varys
with the type of diode selected by the designer, and it is not usually specified on the diode data sheet. The value
of R
(DET)
may be adjusted downwards to accommodate a particular diode type.
The classification process applies a voltage between 14.5 V and 20.5 V to the input of the PD, which in turn
draws a fixed current set by R
(CLASS)
. The PSE measures the PD current to determine which of the five available
classes ( Table 1) that the PD is signalling. The total current drawn from the PSE during classification is the sum
of bias currents and current through R
(CLASS)
. The TPS2375-1 disconnects R
(CLASS)
at voltages above the
classification range to avoid excessive power dissipation ( Figure 16 and Figure 17).
The value of R
should be chosen from the values listed in Table 1 based on the average power
requirements of the PD. The power rating of this resistor should be chosen so that it is not overstressed for the
required 75-ms classification period, during which 10 V is applied. The PD could be in classification for extended
periods during bench test conditions, or if an auxiliary power source with voltage within the classification range is
connected to the PD front end. Thermal protection may activate and turn classification off if it continues for more
than 75 ms, but the design must not rely on this function to protect the resistor.
11
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