Minimilist Level Shifting

Or single 1.8V series zener, if your 0V doesn't mind being ~0.7V or so.

You mean +0.7V, no?

Yeah, bad habit, I tend to use the tilde to mean approx.

Sorry. I'm having a poor vision morning and thought it was a -. Tilde I understand.

Thanks for your suggestions.

What's the zener drop?

What's the zener current?

Where can I buy a 1.8v zener?

tirsdag den 11. april 2023 kl. 16.27.42 UTC+2 skrev John Larkin:

I actually did that once. I had a board in production where I had forgotten about the voltage difference. I added a zener in series and let it play against the protection diode. It was a really ugly solution, but could be done without a board spin. Maybe that's why I don't like the series resistor idea. This is all too kludgy for me. Is 200 ohms enough? Is 600 ohms too many?

The logic parts are tiny and easy to buy. 5V tolerant regardless of Vcc. Yeah, that works great for me!

What will the zener voltage drop be in the proposed circuit?

tirsdag den 11. april 2023 kl. 17.01.21 UTC+2 skrev John Larkin:

does it matter? if a simple resistor is somehow not good enough there's no point in not using a 5V tolerant buffer

Depending on the current some LEDs are in that range. You get a zener and a light to boot.

tirsdag den 11. april 2023 kl. 17.38.41 UTC+2 skrev Ralph Mowery:

an LED is missing the diode in the other direction

If it makes anyone feel better, I did find the spec on the part involved. They allow 10 mA into the protection diodes for a pin and 120 mA total for a bank. But, since they have no app note or any further info in the data sheet, there is no specific exception given to the max Vin on the I/Os, beyond Vcc + 0.3V. There's also the power consumption issue. A resistor of any given value would have a power drain amount that varies 3 to 1 or more.

The two resistor approach has better potential. The ratio of resistors would be something like 3:1, drawing limited current when high, and a low impedance for speed. But even a pair of 0402 parts is as large as the dual inverter.

At this point, I'm good with the dual inverter with specified 5V tolerant I/Os. I'm not interested in exploring the analog properties of a large digital device.

with the series resistor the current taken from the 5V ends up in the 3.3V so if 3.3V is made with a linear regulator from 5V it is not wasted