Zener Diode

A Zener diode relies on the reverse breakdown voltage. $V_F$ is generally poorly controlled, while $V_R$ can be well-controlled.

Zener diodes operate right on the edge of breakdown to provide a reasonably precise voltage.

• Given a specific $I_Z$, a Zener diode will drop $V_Z$ across it
• Choose a precision resistor, $R$, to drop the rest of the voltage This has much better voltage regulation than a resistor voltage divider. Use this when we need good voltage regulation but not a complete voltage supply!

Analyze just like a regular diode, using $V_Z$ instead of $V_F$.

Example

Problem: Say that $V_Z=3\text{ V}$, and the datasheet says to use $I_Z=1\text{ mA}$. Choose $R$ to drop excess voltage. Assume $V_{DD}$ is a $5\text{ V}$ supply.

The Zener diode is going to lock in at 3 volts, so the voltage across our resistor is the leftover 2 volts. Knowing that $I_Z=1\text{ mA}$, we have:

$$R=\frac{2\text{ V}}{1\text{ mA}}=2\text{ k}\Omega$$

Extra: we can find the static (“all the time”) power of the diode:

$$V_Z\cdot I_Z=3\text{ mW}$$