Datasheet

TPS2379

www.ti.com

SLVSB98 –MARCH 2012

The type 2 hardware classification protocol of IEEE 802.3at specifies that a type 2 PSE drops its output voltage

into the detection range during the classification sequence. The PD is required to have an incorrect detection

signature in this condition, which is referred to as a mark event (see Figure 20). After the first mark event, the

TPS2379 will present a signature less than 12 kΩ until it has experienced a V

(VDD-VSS)

voltage below the mark

reset threshold (V

MSR

). This is explained more fully under Hardware Classification.

Hardware Classification

Hardware classification allows a PSE to determine a PD’s power requirements before powering, and helps with

power management once power is applied. Type 2 hardware classification permits high power PSEs and PDs to

determine whether the connected device can support high-power operation. A type 2 PD presents class 4 in

hardware to indicate that it is a high-power device. A type 1 PSE will treat a class 4 device like a class 0 device,

allotting 13 W if it chooses to power the PD. A PD that receives a 2-event class understands that it is powered

from a high-power PSE and it may draw up to 25.5 W immediately after the 80 ms startup period completes. A

type 2 PD that does not receive a 2-event hardware classification may choose to not start, or must start in a 13

W condition and request more power through the DLL after startup. The standard requires a type 2 PD to

indicate that it is underpowered if this occurs. Startup of a high-power PD under 13 W implicitly requires some

form of powering down sections of the application circuits.

The maximum power entries in Table 1 determine the class the PD must advertise. The PSE may disconnect a

PD if it draws more than its stated Class power, which may be the hardware class or a lower DLL-derived power

level. The standard permits the PD to draw limited current peaks that increase the instantaneous power above

the Table 1 limit, however the average power requirement always applies.

The TPS2379 implements two-event classification. Selecting an RCLS of 63.4 Ω provides a valid type 2

signature. TPS2379 may be used as a compatible type 1 device simply by programming class 0–3 per Table 1.

DLL communication is implemented by the Ethernet communication system in the PD and is not implemented by

the TPS2379.

The TPS2379 disables classification above V

CU_OFF

to avoid excessive power dissipation. CLS voltage is turned

off during PD thermal limiting or when DEN is active. The CLS output is inherently current limited, but should not

be shorted to V

for long periods of time.

Figure 21 shows how classification works for the TPS2379. Transition from state-to-state occurs when

comparator thresholds are crossed (see Figure 18 and Figure 19). These comparators have hysteresis, which

adds inherent memory to the machine. Operation begins at idle (unpowered by PSE) and proceeds with

increasing voltage from left to right. A 2-event classification follows the (heavy lined) path towards the bottom,

ending up with a latched type 2 decode along the lower branch that is highlighted. This state results in a low T2P

during normal operation. Once the valid path to type 2 PSE detection is broken, the input voltage must transition

below the mark reset threshold to start anew.

Product Folder Link(s) :TPS2379