In what types of circuits are Zener diodes (voltage regulator diodes, constant-voltage diodes) used?

A Zener diode is a type of pn junction diode. As the reverse bias voltage across the pn junction increases, a Zener diode enters either Zener or avalanche breakdown. This breakdown voltage increases a sharp rise as shown in Fig. 1. Reverse voltage remains nearly constant even when the reverse current changes. Because of this characteristic, Zener diodes are also called voltage regulator diodes and constant-voltage diodes.

• Voltage regulator circuits:
  Voltage regulator circuits using Zener diodes are employed as reference voltage sources for ICs because they provide almost constant voltage despite a simple circuit configuration. Nowadays, however, small high-precision power supply ICs such as low-dropout (LDO) regulators are more commonly used because voltage regulator circuits using Zener diodes are subject to variations in the electrical characteristics, temperature dependence.
  
  LDO product page
  
  
• Clipping circuits:
  The performance and operating frequency of ICs and other components for electronic applications are ever increasing. In line with this trend, these components are fabricated with progressively finer process geometries. As a result, they are becoming increasingly susceptible to permanent damage due to ESD, ringing, and other external events exceeding their electrical ratings. To prevent such damage, it is necessary to protect the input terminals of semiconductor devices. Nowadays, Zener diodes are used for this purpose. When voltage higher than Zener voltage is applied, a Zener diode turns on, clipping the signal as shown in Fig. 2.
  A protection circuit can be configured by inserting a Zener diode between the signal input terminal and the protected element such as an IC. ESD protection diodes, also called TVS diodes, specifically designed for ESD and surge protection are now commercially available. Toshiba also provides TVS (ESD protection) diodes.
  
  TVS Diodes (ESD Protection Diodes) product page

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