I am looking to select an appropriate Zener diode from the ZMM range for MCU input pin protection.
They are specified at a Zener current I_ZT = 5 mA having a maximum impedance of Z_ZT = 90 ohm.
They also specify a maximum impedance (at working current threshold?) Z_ZK = 600 ohm @ I_ZK = 1 mA.
I am assuming absolute maximum input pin voltage of 5V.
My analysis is as follows:
The Zener diode requires at least 1mA to break down: 5V / 1mA = 5kOhm current limiting resistor. Below 1mA the Zener is not guaranteed to break down and my input pin could see the full voltage.
At 1mA the Zener impedance is 600 Ohm leading to a voltage drop of 0.6V.
5V permissible input voltage - 0.6V zener voltage drop: --> require zener diode with a maximum zener voltage of 4.4V.
From the ZMM range, the next best part would be the ZMM3V9 with a V_Z(max) = 4.1V @ I_ZT = 5mA.
My issue is, that steps 1-3 of my analysis were based on I_Z = 1mA but the Zener voltages are specified at 5mA.
Assuming we calculate an 'junction voltage' without the resistance voltage drop:
V_j = V_Z - 90ohm * 5mA
Does V_j vary with respect to the current I_Z?
Is it acceptable to calculate a Zener voltage at lower current as follows?
V_Z @ 1mA = V_j + 600 ohm * 1 mA
I started my analysis at the working threshold current, because a selection at 5mA gives insufficient over-voltage protection:
Assuming 0.25 W current limiting resistors:
V_input_max = 35V with 5kOhm (1mA analysis)
V_input_max = 15V with 1kOhm (equivalent 5mA analysis)
I am looking for protection from sustained voltages of 30V and an expected normal operating voltage of 15V.