AS1124 datasheet(10/21 Pages) ETC2

10 / 21 page

AS1124

Akros Silicon, inc.

6399 San Ignacio Avenue, Suite 250, San Jose, CA 95119 USA

408.746.9000



www.AkrosSilicon.com

FUNCTIONAL DESCRIPTION

Overview of PoE

Power over Ethernet (PoE) offers an economical

alternative for powering end network appliances such as

IP telephones, wireless access points, security and web

cameras, and other powered devices (PDs). The PoE

standard IEEE® Std. 802.3af-2003 is intended to

standardize the delivery of power over the Ethernet cables

in order to accommodate remotely powered client devices.

IEEE®

Std.

802.3af-2003

defines

method

for

recognizing PDs on the network and supplying different

power levels according to power level classes with which

each PD is identified. By employing this method, designers

can create systems that minimize power usage, allowing

more devices to be supported on an Ethernet network.

The end of the link that provides power through the Ethernet

cables is referred to as the power sourcing equipment

(PSE). The powered device (PD) is the end of the link that

receives the power. The PoE method for recognizing a PD

and determining the correct power level to allocate uses

the following sequence:

 Reset

, wherein power is withdrawn from the PD if the

applied voltage falls below a specified level.

 Signature Detection

, during which the PD is recognized

by the PSE.

 Classification

, during which the PSE reads the power

requirement of the PD. The Classification level of a PD

identifies how much power the PD requires from the

Ethernet line. This permits optimum use of the total

power available from the PSE. (Classification is

considered optional by IEEE® standard 802.3af-2003.)

 ON

operation, during which the allocated level of power

is provided to the PD.

This sequence occurs as progressively rising voltage

levels from the PSE are detected

To design PoE systems according to the PoE standard,

designers have the following constraints:

AS1124 POE Design

To help designers meet these requirements, the AS1124

is a fully integrated PoE PD controller. The AS1124 meets

all system requirements for the IEEE® 802.3 standard for

Ethernet and all power management requirements for

IEEE®

standard 802.3af-2003. The device has been

designed

also

address

pre-standard

802.3at

applications. EMI compliance has been verified for CISPR

22, FCC, class B radiated emissions and EN55022

conducted emissions.

The AS1124 acts as an interface to the PSE, performing

all detection, classification, and inrush current limiting

control necessary for compliance with the PoE standards.

An internal MOSFET and control circuit limits the inrush

and steady-state current drawn from the Ethernet line. An

integrated diode bridge is implemented to protect against

polarity reversal, to provide alternative A and B detection

and to provide improved high voltage protection. The

AS1124 passes 2kV ESD tests, as well as 8kv Contact

Discharge and 16.5 kV air Discharge tested per IEC

61000-4.2, 4.4, and 4.5.

Table 6– PoE Requirements

Requirement

Value

Maximum power to the PD interface

24 W

Voltage from Type 1 PSE

44-57 V

Voltage from Type 2 PSE

50-57 V

Maximum operating current

720 mA

Line resistance

10 Ω

Voltage drop due to series line resistance

8.8V

Min voltage at PD interface

36V

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