During normal operation
ESD protection diodes are normally placed between signal lines and GND. Therefore, these diodes act as capacitors in a steady state. Since their capacitance and the resistance of the signal lines form lowpass filters (LPFs), ESD protection diodes cause an insertion loss (IL), degrading the quality of signals, depending on their speed (especially the quality of high-speed signals such as USB 3.0 and USB 3.1).
In the event of a voltage surge
When a surge or external noise enters a system via a connector, its impact on the subsequent device (e.g., an IC) greatly depends on the presence or absence of ESD protection diodes. Without ESD protection diodes, all of the surge current flows directly to a sensitive device, causing a malfunction or device destruction. If a circuit has ESD protection diodes, much of the surge current is shunted to GND through them. The dynamic resistance (Rdyn) of an ESD protection diode represents the ease of shunting surge current to GND. ESD protection diodes with lower dynamic current shunt more of the surge current to GND. Such diodes also help reduce the voltage across their dynamic resistance, i.e., the resistance between their terminals (known as clamp voltage). Therefore, ESD protection diodes with lower dynamic resistance allow less surge current to flow to the device under protection (DUP), providing more robust protection for the DUP.