Data Sheet

LT8440

analog.com

Rev. 0

13 of 22

APPLICATIONS INFORMATION

Very few components are required to configure the LT8440 for use in a load power port of an intrinsically safe field

device and limit the power drawn by the port according to the Advanced Physical Layer standard. The following

sections are intended to guide the selection of those components.

Input Frontend Design

Figure 24 shows a typical field device frontend design. The cable shield is directly connected to the field device's

chassis. A TVS diode D1 protects any surge or ESD event on a twisted pair. The trip voltage of D1 should be higher

than 17.5V to not interfere with normal operation, while it should be low enough to still protect the circuits following.

A common-mode choke L1 suppresses the common-mode noise.

After L1, a diode bridge is used to eliminate polarity sensitivity at the module input. An additional diode, D6, is used

to block the spark energy when faults happen in the diode bridge. The LINEA/LINEB pins of LT8440 can be connected

either before or after the diode bridge. Connecting them before the diode bridge improves the accuracy of voltage

sensing. However, in this configuration, a two-point failure scenario where LINEA/LINEB shorts to SHUNT and RTP,

respectively, can form a leakage path from the input port to the load capacitor through LINEA/LINEB resistors. If the

total capacitance on the load side is large, LINEA/LINEB pins should be connected after the diode bridge, as shown

in Figure 25. Note that if a fault happens after the diode bridge, according to IEC60079, D2~D6 should be able to

withstand 1.5x of the worst-case stress. The worst-case voltage/current is 17.5V/380mA. Therefore, the diodes should

be rated to at least 26.25V/570mA. Also be aware that the diode drops reduce the total available power to the load,

as well as the minimum input voltage of LT8440. Schottky diodes can be used instead, to have less voltage drop.

However, Schottky diodes usually have a significantly higher leakage current at hot temperatures, and these

leakages cannot be sensed by LT8440.

Figure 24.Typical Field Device Frontend Design