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Lets take under consideration a 3 state buffer, such as this one:

enter image description here

Since the datasheet doesnt provide the information about the internal build of the buffer, I was wondering either it is safe to assume that in case the chip is not powered up (0V between VCC and GND), the output is in High Z state? Should one worry about any load at the outputs in unpowered state?

I imagine there is a mosfet P + N half bridge at the output.

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No, it is not at all safe to assume that the output pins are floating when the device is not powered.

I would assume quite the opposite: all pins are clamped to Vcc through ESD diodes. Since Vcc is 0V when not powered, that means the output pins are clamped to ground through these diodes, and you won't be able to apply a voltage greater than about 0.5V to them.

Having first made that blanket assumption, check the datasheet. Some devices may be designed with different kinds of ESD protection. However, the specific part you mention states pretty clearly in the datasheet, under "output clamping current" that the output pin may draw 20mA if the voltage is 0.5V below ground or above Vcc.

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  • \$\begingroup\$ Thank you for your answer, this makes sense. A follow-up question arises- is it safe to assume floating outputs in case the VCC is floating (not tied to GND)? But it becomes clear that it is better to kep the chip powered at all times (and be ok with the quiescent and standby current draw) \$\endgroup\$ Commented Dec 16, 2019 at 12:57
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    \$\begingroup\$ Remember that there is a fairly large capacitance associated with the Vcc connections of a chip, and the ESD diodes will have some leakage current, so you can't really "float" Vcc as you would a signal line. \$\endgroup\$ Commented Dec 16, 2019 at 13:01
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Some devices such as the SN74AUP1G126 are designed to operate with outputs applied even without any power supply.

enter image description here Datasheet

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  • \$\begingroup\$ This one is interesting, as it seems that the I_off current is higher than I_I (idle I am guessing). \$\endgroup\$ Commented Dec 16, 2019 at 18:14
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No, you cannot make that assumption. Devices whose outputs are defined to be high-Z when not powered are sometimes referred to as being "cold spareable", which means you can connect something to them when they aren't powered (hence "cold"). Such devices find use in systems with redundant hardware. If devices have that capability, it's almost always called out on the data sheet.

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From the datasheet limiting values it can be seen that output cannot be higher than VCC+0.5V unless the clamping current is limited to 20mA. So output has protection diodes and it will try to back-feed the chip VCC from output if output is set externally high.

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