ESD Clamping Voltage

Formula

Description

ESD protection devices like TVS diodes and varistors clamp transient voltages by conducting the surge current once their breakdown voltage is exceeded. The actual voltage seen by the protected circuit during an ESD event is higher than the breakdown voltage because of the device dynamic resistance. Lower dynamic resistance means better clamping and less voltage overshoot. The clamping voltage must remain below the absolute maximum rating of the protected IC to prevent damage.

Variables

• V_clamp — Peak voltage during ESD event (V)
• V_br — Breakdown or trigger voltage of the protection device (V)
• I_esd — Peak ESD current (A)
• R_dyn — Dynamic resistance of the protection device (Ω)

Practical Notes

IEC 61000-4-2 contact discharge at ±8 kV produces a peak current of 30 A with a rise time of less than 1 ns. TVS diodes typically have R_dyn of 0.3-2Ω, while varistors may have 1-10Ω. For USB, HDMI, and other high-speed interfaces, select ESD protection with low capacitance (< 1 pF) to avoid signal degradation.