Electronics > Projects, Designs, and Technical Stuff
Is it safe to connect inputs to VCC directly
Siwastaja:
--- Quote from: wraper on February 05, 2020, 12:38:50 am ---Again, please explain how MOSFET gates tied to Vdd/Vss are supposed to pass any current. Latch-up does not result in current passing through the gates.
--- End quote ---
It's the case of the inputs being connected to a different Vdd, with wrong power-up sequencing.
Some logic IC families specify in the datasheet that they are safe against inputting voltages on unpowered chip. That's an actual marketing argument.
Needless to say, this does not apply to most use cases, including the OP's.
wraper:
--- Quote from: Siwastaja on February 05, 2020, 12:02:07 pm ---It's the case of the inputs being connected to a different Vdd
--- End quote ---
It's a completely different case from what was asked, doing so is simply stupid and those who do this should be burned on stake.
Siwastaja:
Most of the "best practice" bullshit is based on mental copy-paste of something that usually is not related to the actual problem, often not even close, and given some engineering discipline, the disconnection is trivial to see.
It's a colossally bad idea to add random components because someone somewhere thinks that they are necessary in a completely different circuit, and to call that "good practice" is disgrace. Engineering just isn't that simple. Understanding the circuit is paramount.
tkamiya:
imo,
That's a bunch of obvious but good points. I think most folks (me too) do this without thinking. Pick the closest low impedance (ie. power) point. But taken literally, nothing stops folks from just daisy chaining empty input pins and just ground or pull up at one point. That would be problematic.
MasterT:
If I asked, is it safe to apply 5.5V? Everyone would say, yes, DS specifies acceptable voltage level. But reliability numbers would fall from 10^-7 to 10^-6 or like that. So, from reliability point of view, increasing voltage (or using safety resistors) should be considered based on Acceptable failure rates. And this strongly depends on a market, cost of device, and things that difficult to estimate - brand name ratings. Nobody would care if led-blinking toy breaks down, OTOH, if a scope failed because someone save on single resistor - would you buy another one from this maker?
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