Application note

ST3237E

10/17

6

Application note

This application note describes the procedure for determining the susceptibility and the test

method to verify ST ESD advanced protection on RS-232 or RS485 I/O device.

Static electricity is defined as an electrical charge caused by an imbalance of electrons on

the surface of a material. This imbalance of electrons produces an electric field that can be

measured and that can influence other objects at a distance. Electrostatic discharge is

defined as the transfer of charge between bodies at different electrical potentials.

Electrostatic discharge (ESD) can change the electrical characteristics of a semiconductor

device, degrading or destroying it. Any input or output port (I/O) allows access

communication with other pieces of equipment by external connectors. These connectors

are directly linked by the I/O pins of RS-232 or RS485 interface. ST provides the E-series by

advanced high ESD protection structure. The protection functionality is tested in two

different conditions:

The first model is used to simulate the HUMAN BODY MODEL (HBM) event. A similar

discharge can occur from a charged conductive object, such as a metallic tool or fixture. The

model used to characterize this event is known as the Machine Model. A Human Body

Model circuit and waveform is presented in Figures below.

Figure 7.

Human body model circuit

Figure 8.

Human body model current waveform

The second model is IEC 1000-4-2 and is used to simulate the reaction of the device on

equipment when subjected to electrostatic discharges, which may occur from personnel to

objects near vital instrumentation. Direct (Contact) and indirect (Air Gap) applications of

discharges to the equipment under test (EUT) are possible. Test characteristics are shown

in circuit, waveform and table below.