Using LT SPICE to study back feed voltages from digital inputs when power supply not operating

BACK FEED ISSUES I WANT TO AVOID

Digital abstractions for logic gates and other chips mask the analog reality of those devices, where the components inside have complicated behaviors that can produce results in the real world that don't exist under the abstraction.

For example, a NAND gate with two inputs and an output can feed power into its own chip and all the others on a board if the input to the gate is powered but the board is otherwise not powered. I would observe the board producing outputs but I hadn't turned on the bench power supply yet. 

This occurs because the input circuits of the NAND gate has protective diodes on the input - one of which will pass the power from the input to the VCC pin in the unpowered gate, powered input case I mentioned. That input pin is then providing all the current demanded by the chips on the board. The chip can easily be damaged because of that. There is even the chance that the power demands of the input can be more than the driving device (an Arduino Due in my case) can source, damaging the Arduino.

I set up a MOSFET to interrupt the low side (ground) of the board if the bench power supply isn't delivering voltage to the board. With the ground of the board isolated from the ground of the Arduino Due or other source to the input pins, no current will flow. 

The MOSFET is a complex device in reality, as is the voltage regulator providing the 3.3V from the 12V of the bench supply. Therefore I wanted to examine the real world behavior of these to see whether there is any back feed voltage and current pulled from the 

RAN LTSPICE SIMULATION OF THE BACK-FEED PROTECTION

I set up a schematic with the MOSFET circuit I will use for isolating the low side (ground) of my PCB. A diode representing an input protective diode on a logic gate as connected to a 3V voltage source to represent the source of back-feed. I then modeled the current flowing through my PCB circuits (represented by a simple load resistor in this model) while I alternated the control signal between 0V and 12V. It worked exactly as I expected, ensuring virtually no current flowed from the powered input pins of my PCB until the +12V was present. The simulation showed nanoamps of current flow due to the non-ideal nature of the MOSFET, but that is negligible.

I then set up a schematic with the LDL1117 voltage regulator I intend to use on future boards to replace the outboard voltage regulator module, dropping the +12V (or slightly higher) down to 3.3V for the chips on the board. I modeled what happened with the input pins of the board driven by 3 or 3.5V while the regulator chip was unpowered. I saw a slight voltage and only microamps flowing out of the regulators power input. This is insufficient to energize my MOSFET to connect the board ground to the grounds of the rest of the setup.