# What are the typical electrical characteristics of ESD protection diodes? (dynamic resistance and clamp voltage)

The following describes the key electrical characteristics of ESD protection diodes: clamp voltage, dynamic resistance, and working peak reverse voltage.

Clamp voltage (VC)
The clamp voltage (VC) of an ESD protection diode is similar to the Zener voltage (VZ) of a Zener diode, but VC is specified at higher current than VZ since the ESD protection diode is used for ESD protection.
VC is the maximum voltage to which an ESD protection diode is clamped when exposed to the specified peak pulse current (a transmission line pulse (TLP) or an 8/20 µs waveform specified in IEC 61000-4-5). Generally, VC is measured at multiple peak pulse current points.
For a description of the TLP waveform, see the FAQ entry “What is a TLP test?”
What is a TLP test?

Dynamic resistance (RDYN)
The current-voltage curve of the ESD protection diode rises nonlinearly and becomes linear in the high-current region as shown in Figure 2. The dynamic resistance specified in a datasheet is the slope of the current-voltage curve in the linear, high-current region (i.e., on-state series resistance at high current).
Since ESD is a high-current pulse, ESD protection in the high-current region is important for ESD protection diodes. The lower the dynamic resistance, the lower the rise of the clamp voltage at high current.
Dynamic resistance is calculated with the following equation by applying an ESD-pulse-like transmission line pulse (TLP) waveform to an ESD protection diode and measuring the resulting transmission line voltage at two current points:
RDYN = ΔVTLP / ΔITLP = (VTLP2 – VTLP1) / (ITLP2 – ITLP1)

Working peak reverse voltage (VRWM)
While the clamp voltage and dynamic resistance of an ESD protection diode represent its protection performance, the working peak reverse voltage (VRWM) provides a guide as to the maximum voltage allowed on the signal line to be protected. Below VRWM, an ESD protection diode acts as a device with extremely high impedance. When VRWM is applied to an ESD protection diode at room temperature, only leakage current lower than the specified IR flows.