A single
unidirectional TVS diode structure is designed for a wanted signal between ~0V and a “Maximum working voltage” specified for the TVS diode. In case of a negative signal is applied between signal line and GND, the device will become conductive if the signal level exceeds about minus 0.5V. Such a device can i.e. be used to protect a unipolar digital data signal. The ESD Absorption capability of the diode is granted for a positive AND a negative ESD strike.
In order to protect bipolar signals, i.e. signals providing both positive and negative voltage values,
a bidirectional TVS device is needed. The V-I curve of such device is symmetrical with respect to the origin, and the ESD absorption capability is granted for a positive AND a negative ESD strike in the same way. A bidirectional TVS diode can be created by using two identical unidirectional TVS diodes connected in series or by integrating the bi-directional functionality in one die.
Tip of the Week: to learn more about bidirectional and unidirectional protection diodes and their I/V characteristics, please take a look on the Application Note AN327:
"Bi-directional Ultra Low Capacitance Transient Voltage Suppression Diodes for High Power RF Applica...
