Effect of RF Signals on TVS Diode Trigger Voltage for ESD Protection

Discrete transient voltage suppressor devices are used in addition to the on-chip ESD protection to protect the ICs from ESD damage. In applications involving snapback TVS devices, the trigger voltage is selected to be higher than the desired signals on the net. The presence of RF on the net affects the TVS behavior, even if the RF levels are less than the snapback trigger voltage. Known effects are RF intermodulation and harmonic generation, which diminish SNR. This work describes the other effect on the TVS diodes - the observed reduction of the snapback trigger voltage, monotonically dependent on the amplitude of the RF signal. This translates into snapback triggering at lower stress levels than specified in the datasheet or expected by the design engineer. TLP testing was performed using a 100 ns pulse with RF signals from 100 MHz to 2 GHz present at the diode terminals. The results show that the higher frequencies have a weaker impact on the reduction of the snapback trigger voltage. At lower frequencies such as 100 MHz, the overlap of the first few ns of the TLP pulse, the peak voltage, and the phase of the RF signal impact the amount of reduction in the trigger voltage. It is also observed that certain TVS diodes recover from snapback even when the RF amplitude is higher than the holding voltage of the diode.