3-2 Key characteristics for protection against ESD events(1)

3-2(1) Low dynamic resistance (R_DYN)

In the event of an ESD strike, the ESD current flows to both the ESD protection diode and the device under protection (DUP). It is important to reduce the current flowing to the DUP (i.e., increase the current shunted through the ESD protection diodes).
Nowadays, the datasheets for ESD protection diodes show their dynamic resistance (R_DYN). R_DYN is the slope of the V_F–I_F curve in reverse conduction mode. In the event of an ESD strike, ESD protection diodes with lower dynamic resistance conduct more current at a given voltage.
From the connector side, the impedances of the ESD protection diode and the DUP can be regarded as being connected in parallel. If the ESD protection diode has low impedance (i.e., dynamic resistance), most of the surge current is shunted through the ESD protection diode, reducing the current that flows to the DUP and therefore the possibility of its destruction.

Figure 3.9 illustrates how to calculate dynamic resistance (R_DYN) of an ESD protection diode and where surge current flows in the event of an ESD strike. If the ESD protection diode has low impedance (i.e., dynamic resistance), most of the surge current is shunted to GND through the ESD protection diode, reducing the current that flows to the DUP. Therefore, the ESD protection diode helps prevent the destruction of the DUP due to an ESD strike. A transmission line pulse (TLP) test applies short pulses with a duration on the order of nanoseconds to study the current-voltage (I-V) characteristics of a diode based on the current-voltage relationship over time. In the following graph, TLP I and TLP V represent current and voltage, respectively.