Datasheet

V
UVLO_R
Detection
Classification
PD Powered
Idle
V
CL_ON
V
CU_OFF
V
DD
-V
SS
Mark
V
MSR
Functional State
Type 2
Type 1
V
UVLO_H
V
CU_H
V
CL_H
TPS23751
TPS23752
www.ti.com SLVSB97C –JULY 2012–REVISED JANUARY 2014
Table 2. Comparison of Operational Limits
Power Loop PSE Static Static PD Input Voltage
PSE Output PD Input
Resistance Output
Standard
Power (min) Power (max)
Power ≤ 12.95W Power > 12.95W
(max) Voltage (min)
IEEE802.3at-2008
20 Ω 15.4W 44V 12.95W 37V – 57V N/A
802.3at (Type 1)
802.3at (Type 2) 12.5Ω 30W 50V 25.5W 37V – 57V 42.5V – 57V
The PSE can apply voltage either between the RX and TX pairs (pins 1 - 2 and 3 - 6 for 10baseT or 100baseT),
or between the two spare pairs (4 - 5 and 7 - 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 TPS23751 and TPS23752
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 13W for 80ms after the PSE applies operating voltage (power-up),
6. Must not draw more than 13W if it has not received a type 2 hardware classification or received permission
through DLL,
7. Must meet various operating and transient templates, and
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 TPS23751 and TPS23752 have a number of internal comparators with hysteresis for stable switching
between the various states. Figure 21 relates the parameters in the Electrical Characteristics section 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.
NOTE: Variable names refer to Electrical Characteristic table parameters
Figure 21. Threshold Voltages
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