I am trying to get a toggleable 2.0 to 2.6 ish voltage reference that can supply up to 45 mA.
I simulated the below circuit and found this to be acceptable. However upon building the circuit, I found my Vout voltage to be much higher, around 3.2V. The voltage drops slightly to 3.0V depending on the load (used 100 ohms to test).
So I think the Zener isn't operating correctly.
However, when measuring the voltage across the 50 ohm resistor (5.5V-3.2V)/50ohm = 46mA, I found that the Zener should be on! This Zener diode only needs like 5 mA to operate correctly. So what gives? Is the knee of this zener just really bad? What should I do to ensure the voltage never goes above 2.6V?
The PNP is toggled with a GPIO pin from an MCU.
Best Answer
A 2.4V (@5mA) Zener is going to have a heck of a lot more than 2.4V across it at close to 50mA. Just the way the physics works out for low voltage zeners.
3.3V or so (typical) looks about right.
Use a TL431 instead.
Just as a rule of thumb, think twice about using any zener < 5.1V.