Datasheet

V
UVLO_R
Detection
Classification
PDPowered
Idle
V
CL_ON
V
CL_H
V
CU_OFF
V
CU_H
Note:VariablenamesrefertoElectricalCharacteristic
Tableparameters
V
DD
-V
SS
V
UVLO_H
Mark
V
MSR
Functional
State
TPS23754
TPS23754-1
TPS23756
SLVS885G –OCTOBER 2008–REVISED OCTOBER 2013 www.ti.com
Table 2. Comparison of Operational Limits
POWER LOOP PSE PSE STATIC PD INPUT STATIC PD INPUT VOLTAGE
RESISTANCE
OUTPUT POWER OUTPUT VOLTAGE POWER
STANDARD
POWER ≤ POWER >
(max)
(min) (min) (max)
13.0 W 13.0 W
IEEE 802.3-2008
20 Ω 15.4 W 44 V 13.0 W 37 – 57 V N/A
802.3at (Type 1)
802.3at (Type 2) 12.5 Ω 30 W 50 V 25.5 W 37 – 57 V 42.5 – 57 V
The PSE can apply voltage either between the RX and TX pairs (pins 1 to 2 and 3 to 6 for 10baseT or
100baseT), or between the two spare pairs (4 to 5 and 7 to 8). Power application to the same pin combinations
in 1000baseT systems is recognized in IEEE 802.3at. 1000baseT systems can handle data on all pairs,
eliminating the spare pair terminology. The PSE may only apply voltage to one set of pairs at a 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 create a difference between the standard limits at the PI and the TPS23754
specifications.
A compliant type 2 PD has power management requirements not present with a type 1 PD. These requirements
include the following:
1. Must interpret type 2 hardware classification
2. Must present hardware class 4
3. Must implement DLL negotiation
4. Must behave like a type 1 PD during inrush and startup
5. Must not draw more than 13 W for 80 ms after PSE applies operating voltage (power-up)
6. Must not draw more than 13 W if it has not received a type 2 hardware classification or received permission
through DLL
7. Must meet various operating and transient templates
8. Optionally monitor for the presence or absence of an adapter (assume high power)
As a result of these requirements, the PD must be able to dynamically control its loading and monitor T2P for
changes. In cases where the design needs to know specifically if an adapter is plugged in and operational, the
adapter should be individually monitored, typically with an optocoupler.
Threshold Voltages
The TPS23754 has a number of internal comparators with hysteresis for stable switching between the various
states. Figure 22 relates the parameters in the ELECTRICAL CHARACTERISTICS to the PoE states. The mode
labeled idle between classification and operation implies that the DEN, CLS, and RTN pins are all high
impedance. The state labeled Mark, which is drawn in dashed lines, is part of the new type 2 hardware class
state machine.
Figure 22. Threshold Voltages
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