The zener diode operation described in detail above now creates a set of characteristics which can be defined into parameters of interest. These typically include the zener voltage V_Z, zener impedance Z_Z, reverse leakage I_R, temperature coefficient S_Z, Power dissipation P_D, as well as capacitance C_D.

The zener voltage as discussed in the previous section is one of the most critical specifications when choosing a zener as this defines the effective breakdown voltage where the zener begins to conduct and regulate the protected line. For this reason, V_Z is specified at a given zener current or I_Z for which the V_Z is stable. More on this and stability at lower currents will be discussed later in description of TI zener diodes. It is a common misconception that zener diodes always clamp the protected line at V_Z. This is likely the case if the diode were an ideal zener diode, however every diode has a non-zero dynamic impedance, Z_Z, in the breakdown region which increases the regulated voltage level that can be calculated as using Equation 1.

Like V_Z, the zener impedance of Z_Z is specified at the same zener current. The zener impedance represents the ratio of the change in V_Z to to the change in I_Z within the breakdown region as shown in Figure 2-3. A low zener impendence can translate to more robust line regulation as this means large changes in line current that result in relatively small shifts in voltage. This is most critical in cases where the zener is used for DC voltage regulation.

Figure 2-3 Zener I-V Curve

As many zeners are used for powerline protection, specifying the leakage current, referred to as the reverse leakage I_R, is critical. This leakage current is verified at a voltage condition for which the zener is off (in a non-conducting state), called the reverse voltage or V_R. This leakage ranges from the nA to µA range depending on the V_Z of the selected zener.

Another critical parameter is the temperature coefficient, S_Z. This is referred to in terms of mV per degree, mV/C or mV/K. This is another parameter to understand the stability of your zener voltage but over the specified operating temperature range. Automotive applications where electronic components are introduced to wide range of temperatures within the car (near the engine block) or external weather conditions are examples of where the temperature coefficient can play a part if the zener is used for DC voltage regulation.

Many of the use cases discussed thus far involve DC voltage regulation and in these scenarios, the zener needs to conduct seemingly indefinitely. For this reason, understanding what the maximum power the diode can dissipate as the diode regulates is critical, and this is referred to the total power dissipation P_D. This is largely controlled by the package the zener is placed in and thus P_D varies as zeners are scaled from small packages such as DFN1006 to larger packages such as SMA-F, shown in Figure 2-4.

Figure 2-4 Comparison of a Small 0402 Package and Large SMA-F Package

The last specification to highlight is capacitance. In many power line applications the zener diode capacitance C_D is not critical and this is why zener diodes with capacitances up to hundreds of picoFarads are acceptable. However, since zeners can also be used for dataline protection, the selected device C_D is important to note and how that relates to the overall capacitive budget for the system and impact to signal integrity.