Zener diodes are handy devices which can be inserted into a circuit in reverse to clamp voltages to desired levels. But like all circuit components, Zener diodes do nor exhibit ideal behavior in all circumstances. The zener voltage varies significantly based upon temperature and through current, for instance. Thus, if one wants to use a zener diode to protect a low voltage, sensitive chip from over voltage on VDD, there are some important considerations. Assuming we have a 3.3V VDD and we select a 3.3Vz Zener diode. The diode Vz will increase with through current, but will decrease with increasing temperature. It is easy and wise to eliminate the current variability by using overcurrent protection devices like PPTC fuses, sized such that current through the zener diode does not exceed the power dissipation limit of the component nor cause the Vz to rise above the voltage maximim of the chip to be protected, whichever of those occurs first as current rises.
A more difficult to handle concern is the tendency of the zener voltage to decrease with temperature. If the curcuit is designed to operate in a wide range of temperatures, then the actual zener voltage has to assessed for all of those temperatures, most crucially to insure that the reduced zener voltage does not fall below the minimum voltage required by the chip, and that there are no other negative effects from reduced VDD (such as how it affects adc reference or digital output voltage).
In extreme cases, where zener protection is needed, but zener voltage variance is undesired, it becomes necessary to use a feedback controlled mosfet with a voltage reference that is temperature stable within the desired operating range, rather than a normal zener. The source of the n-channel mosfet is connected to ground, and the drain is connected to the VDD to be protected. The output of an opamp is connected to the mosfet gate, and the mosfet drain is connected to the + input of the opamp. The temperature stable voltage reference is connected to the negative input if the opamp. Thus if VDD falls below Vref, then the gate voltage will be brought low, making the mosfet high impedance. And if VDD rises above VRef the gate voltage will increase until it sets the mosfet impedance at the exact level required to reduce VDD to match Vref.
Amusingly, a temperature stable voltage reference can itself be made with a temperature feedback controlled current source through a zener diode.
