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In order to improve system ESD design, designers need to understand the interaction between the IC and its system surroundings including external ESD protection. The diagram shown here is an example given an ESD strike on the connector or some interface on the system that routes to an I/O pin on the IC. The red path that is shown represents the energy flowing through the board and into the IC where it is supposed to be handled by the ESD structure in the IC. The goal is that the energy should be diverted safely away from the IC. The external ESD diode is supposed to act as a shunt element that is not supposed to interfere with normal operation. However, when an ESD or overvoltage event occurs, the ESD diode will turn-on and divert the transient energy to ground. In an ideal scenario, all of the energy is diverted to ground in order to keep the IC safe. This is represented by the green path through the external ESD diodes. However, in the real world, although most of the transient energy is diverted away from ground, there is some residual energy that will be exposed to the IC. As the ESD energy is entering the system, it will flow through the path to the ESD diode and also to the IC. After the ESD voltage passes the breakdown voltage of the diode and the IC, both will start to turn on. The two must work together to ensure that the majority of the ESD energy is diverted to ground through the external ESD diode. Diverting this potentially harmful energy though the IC requires proper handling from a circuit perspective, semiconductor process/design perspective, and thermal perspective where other main functions are not compromised. If both diodes are in the on-state, then the circuit acts an a voltage divider for the energy, where the majority of the current would flow through the least resistive path to ground. This is why the external ESD diode should always have a lower on-resistance or dynamic resistance than the inherent on-resistance of the IC.
