Data Sheet

LT8440

analog.com

Rev. 0

11 of 22

OPERATION

Overview

Ethernet-APL (Advanced Physical Layer) is a standardized 2-Wire Ethernet physical layer based on 10BASE-T1L

(IEEE802.3cg). It can transmit both signal and power over long distances in hazardous or explosive environments.

APL enables direct and high-speed connection of field devices to Ethernet infrastructure for the process industry.

Figure 23 shows an exemplary APL topology. The field devices connect to field switches through the spur connections

up to the 200m distance, and field switches connect to the power switch or other field switches through the trunk

connection up to the 1000m distance.

Figure 23.Exemplary APL Topology

LT8440 power conditioner is designed for Class A field devices with intrinsic safety Ex ia (APL port class SLAA), which

should be used in Zone 0/Div 1 hazardous area. With three LT8440s in series, it protects the field device from two

countable faults, and optimizes the power delivery while maintaining compliance with APL and Intrinsic Safety.

Across the voltage range, it maintains ≥500mW power available to the load but also limits <650mW from the port at

fault.

Theory of Operation

The LT8440 should be positioned in the return path of the load current for a power port in an ethernet field device

such that the current enters the RTP pins and exits through the RTN pins. The part includes a resistor for measuring

the current drawn through the port and a pass transistor for regulating that current. The pass transistor will be driven

to full enhancement to minimize the voltage across its source and drain until the current rises to a limit that varies

with the line voltage sensed by LINEA and LINEB pins. The limitation of current and, ultimately, power by means of

the pass device facilitates certification for intrinsic safety because failing components would otherwise have to

dissipate the full power available from the power-sourcing equipment.

The collection of APL and Intrinsic Safety requirements limit the permitted current in the field device to between

55mA and 20mA, and the permitted power to between 500mW and 5.32W. Under all these conditions, including fault

scenarios, the field device must not exceed allowed self-heating temperatures. The optimal way to meet all the