Datasheet

TPS2379
www.ti.com SLVSB98 –MARCH 2012
The PD input, typically an RJ-45 eight-lead connector, is referred to as the power interface (PI). PD input
requirements differ from PSE output requirements to account for voltage drops and operating margin. The
standard allots the maximum loss to the cable regardless of the actual installation to simplify implementation.
IEEE 802.3-2008 was designed to run over infrastructure including ISO/IEC 11801 class C (CAT3 per TIA/EIA-
568) that may have had AWG 26 conductors. IEEE 802.3at type 2 cabling power loss allotments and voltage
drops have been adjusted for 12.5 Ω power loops per ISO/IEC11801 class D (CAT5 or higher per TIA/EIA-568,
typically AWG #24 conductors). Table 2 shows key operational limits broken out for the two revisions of the
standard.
Table 2. Comparison of Operational Limits
PSE STATIC STATIC PD INPUT VOLTAGE
POWER LOOP PSE OUTPUT PD INPUT
RESISTANCE POWER POWER
STANDARD OUTPUT VOLTAGE
Power ≤12.95W Power >12.95W
(max) (min) (max)
(min)
IEEE802.3at-2008
20Ω 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 TPS2379 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 TPS2379 has a number of internal comparators with hysteresis for stable switching between the various
states. Figure 19 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.
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