PD69108/F

Copyright © 2013

Microsemi

Rev. 1.6

Analog Mixed Signal Group

1 Enterprise, Aliso Viejo, CA 92656, USA; Within the USA: (800) 713-4113, Outside the USA: (949) 221-7100 Fax: (949) 756-0308

14

TM

®

Theory of Operation

The PD69108 performs IEEE802.3af and IEEE802.3at

functionality as well as legacy (capacitor) and Cisco’s

PDs detection and additional protections such as short

circuit and dV/dT protection upon startup.

Line Detection

The Line Detection feature detects a valid AF or AT

load, as specified in the AF / AT standard. The resistor

value should go from 19

kΩ to 26.5 kΩ. Line detection

is based on four different voltage levels generated over

the PD (the load) as illustrated in Figure 2.

v

t

2 Events

Classification Phase

Detection

Phase

Start-Up

(Inrush)

Power

“ON”

Power

“OFF”

Figure 2: Typical PoE Voltage Time Diagram

Legacy (Cap) Detection

In cases where legacy is set, the PD69108's Detection

mechanism detects and powers up legacy PDs as well

as AF/AT compliant PDs.

This mechanism is designed to detect and power up

CISCO legacy PDs as well.

Classification

The classification process takes place immediately after

resistor detection has successfully completed. The goal

of the classification process is to detect PD class, as

specified in the IEEE802.3AF and AT standards.

In the AF mode, classification mechanism is based on a

single voltage level (single finger).

In the AT mode, classification mechanism is based on

two voltage levels (dual finger) as defined in

IEEE802.3at-2009.

Port Start Up

Upon a successful detection and classification process,

power is applied to the load via a controlled Start Up

mechanism.

During this period current is limited to 425 mA for a

typical duration of 65 mS, which allows the PD load to

charge and allow steady state power condition.

Over-Load Detection and Port Shut Down

After power up, the PD69108 automatically initializes its

internal protection mechanisms utilized to monitor and

disconnect power from the load in cases where extreme

conditions such as over-current or short ports terminals

scenario occur, as specified in the IEEE802.3AF/AT

standard.

Disconnect Detection

The PD69108 supports DC Disconnect Function as per

the IEEE802.3AF/AT standard.

This mechanism continuously monitors load current and

disconnects power in cases where load current is below

7.5 mA (typical) for more than 322 mS.

Over-Temperature Protection

The PD69108 has internal temperature sensors that

continuously monitor the main MOSFET junction

temperature and disconnect load power in cases where