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I need to interface two devices, one runs at 5V and the other at 3.3V. In order to safely interface them, I'm using a simple logic level shifter (LLS) of this kind:

A boring, garden-variety, BSS138-based logic level shifter

It's a pretty straightforward circuit design, I have used it before, and I understand it reasonably well.

However, in this application the low voltage supply (LV=3.3V) will be provided by the 3.3V device, which will be switched off for long periods of time. OTOH, the high voltage supply (HV=5V) will be provided by the 5V device, which will be switched on at all times .

Thus, LV will be left floating (or weakly pulled-down, at most) often, with HV at 5V at the same time. Is it safe to do that, given the LLS design above? If not, what design changes would you suggest?

Thank you!

Enric

Additional info:

1) The 5V device can't provide a 3.3V supply line.

2) I'd like to avoid as much as possible any solution based on adding active components like, say, a 3.3V regulator. :)

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It's a pretty straightforward circuit design, I have used it before, and I understand it reasonably well.

When you should already be able to work out what happens when LV is at 0V.

Since LV will be pulled down LV and Vin will be the same voltage. With a VGS of zero the FET will be off. If the FET is off the high voltage side is blocked from reaching the low voltage side until you hit the transistor breakdown voltage which is around 50V.

In other words it'll be fine.

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