I was looking at
4050 datasheet and I found these texts:
The input-signal high level (VIH) can exceed the VDD supply voltage when these devices are used for logic level conversions.
"ABSOLUTE MAXIMUM RATINGS" section I see:
Input Voltage: -0.5 to Vdd+0.5V
So you can't have more than Vdd on input pin (according to ABSOLUTE MAXIMUM RATINGS).
And the "RECOMMENDED OPERATING CONDITIONS":
Vin = 0 to VDD
What?
I can see there is an input resistor, but they don't give it's value or current for Vin>Vdd.
And can someone explain to me the reason for the two diodes around the resistor to ground? I couldn't figure it out . Maybe if you have negative voltage first diode will burn and then maybe second diode will be current limited by the resistor? why would you put the first diode anyway.
I had a look at
74HC4050 equivalent IC. It's absolute maximum ratings clearly show that you can use it as logic converter without any input current limiting:
Vdd = -0.5 .. 7V
Vin = -0.5 .. 16V