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Data Sheet

LTC9101-2A/LTC9102

APPLICATIONS INFORMATION

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Rev. 0 | 22 of 34

LLDP Classification

Introduced in 802.3at and extended by 802.3bt, the PoE specifica-

tion defines a Link Layer Discovery Protocol (LLDP) method of

classification. The LLDP method adds extra fields to the Ethernet

LLDP data protocol.

Although the LTC9101-2A/LTC9102 is compatible with this classifi-

cation method, it cannot perform LLDP classification directly as it

does not have access to the data path. LLDP classification allows

the host to perform LLDP communication with the PD and update

the PD’s power allocation. The LTC9101-2A/LTC9102 supports

changing the "Police" levels dynamically, enabling system-level

LLDP support.

802.3at 2-Event Classification

In 802.3at, 802.3af classification is named Type 1 classification.

The 802.3at standard introduces an extension of Type 1 classifica-

tion: Type 2 (2-event) classification. Type 2 PSEs are required to

perform classification.

A Type 2 PD requesting 25.5W presents class signature 4 during

all class events. If a Type 2 PSE with 25.5W of available power

measures class signature 4 during the first class event, it forces the

PD to VMARK (9V typical), pauses briefly, and issues a second class

event, as shown in Figure 40. The second class event informs the

PD that the PSE has allocated 25.5W.

Figure 40. Type 2 PSE, 2-Event Class Sequence

Note that the second classification event only runs if required by the

IEEE classification procedure. For example, a Class 0 to 3 PD is

only issued a single class event, as shown in Figure 39.

The concept of demotion is introduced in 802.3at. A Type 2 PD

may be connected to a PSE only capable of delivering 13W,

perhaps due to power management limitations. In this case, the

PSE performs a single classification event, as shown in Figure

39, and note that 25.5W is requested. Due to the limited power

availability, the PSE does not issue a second event and proceeds

directly to power on the PD. The presence of a single class event

informs the Type 2 PD that it is demoted to 13W. If demoted, the

PD is subject to power limitations and may operate in a reduced

power mode.

Invalid Multi-Event Classification

Combinations

The 802.3at specification defines a set of valid class signature

combinations. All PDs return the same classification signature on

the first two class events.

Any individual class signature that exceeds the class current limit

is flagged as an invalid classification result. Any sequence of class

signatures that does not represent a legal sequence is likewise

flagged as an invalid classification result.

POWER CONTROL

The primary function of the LTC9101-2A/LTC9102 is to control

power delivery to the PSE port.

The LTC9101-2A/LTC9102 delivers power by controlling the gate

drive voltage of an external power MOSFET while monitoring the

current (through an external sense resistor) and the output voltage

(across the OUT pin).

The LTC9101-2A/LTC9102 connects the VEE power supply to the

PSE port in a controlled manner, meeting the power demands of

the PD while minimizing power dissipation in the external MOSFET

and disturbances to the VEE backplane.

Inrush Control

When commanded to apply power to a port, the LTC9101-2A/

LTC9102 ramp up the GATE pin, raising the external MOSFET gate

voltage in a controlled manner.

During a typical inrush, the MOSFET gate voltage rises until the

external MOSFET is fully enhanced or the port reaches the inrush

current limit (IINRUSH). IINRUSH is set automatically by the PSE.

IINRUSH is 425mA (typical).

The GATE pin is servoed if port current exceeds IINRUSH, actively

limiting current to IINRUSH. When the GATE pin is not being servoed,

the final VGS is 12V (typical).

During inrush, each port runs a timer (tSTART). The port stays in

inrush until tSTART expires. When tSTART expires, the PSE inspects

port voltage and current. Inrush is successful if the MOSFET is fully

enhanced and the port is drawing current below IINRUSH.

If inrush is not successful, power is removed and the corresponding

tSTART fault is set. Otherwise, the port advances to power on and

the programmed current limiting thresholds are used as described

in the Current Limit section.

Port Current Policing

The current policing threshold (Police) is monitored on a per-port

basis. When the output current over a tCUT moving average ex-

ceeds the specified threshold, power is removed from the port

and the corresponding ICUT fault is set. The "Police" threshold and

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