Electronics > Projects, Designs, and Technical Stuff
Is it safe to connect inputs to VCC directly
<< < (6/14) > >>
iMo:

--- Quote from: wraper on February 05, 2020, 12:38:50 am ---
--- Quote from: MasterT on February 05, 2020, 12:30:02 am ---Inputs are not designed to pass much current, so there is "probability"  latching-up, and it's easier to install a resistor than estimate 12-20 inputs/outputs interference to each other .

--- End quote ---
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 ---
Have you ever seen how a CMOS input looks like?
wraper:

--- Quote from: imo on February 05, 2020, 12:45:20 am ---
--- Quote from: wraper on February 05, 2020, 12:38:50 am ---
--- Quote from: MasterT on February 05, 2020, 12:30:02 am ---Inputs are not designed to pass much current, so there is "probability"  latching-up, and it's easier to install a resistor than estimate 12-20 inputs/outputs interference to each other .

--- End quote ---
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 ---
Have you ever seen how a CMOS input looks like?

--- End quote ---
Yes I did and it's a non answer.
MasterT:

--- Quote from: ataradov on February 05, 2020, 12:34:37 am --- As I said, I'm not interested in catastrophic failures. If there is over-voltage or power pin connection loss, then device is broken. It does not really matter what happens after that.

I don't buy this "good designer" stuff.

I see this matter for aerospace or military applications, but there you have to think about much more than just a simple pull-up.

--- End quote ---
Failures happen all the time. It's saving on resistors what makes them catastrophic.  Cellular towers and mobile phones all around us, no need to be in aerospace.   
wraper:

--- Quote from: MasterT on February 05, 2020, 01:08:56 am ---It's saving on resistors what makes them catastrophic.

--- End quote ---
It's like saying saving on fur coat could make your life catastrophic. But I doubt it will make your life safer if you live in tropical forest. What I mean is that you should use stuff for a reason and I don't see how dumbly sprinkling resistors over unused inputs is supposed to make anything better.

--- Quote ---Cellular towers and mobile phones all around us, no need to be in aerospace.   
--- End quote ---
And in that case you better tie things hard with low impedance paths instead of using resistors which cannot quench RF interference.
MasterT:

--- Quote from: wraper on February 05, 2020, 12:38:50 am ---
--- Quote from: MasterT on February 05, 2020, 12:30:02 am ---Inputs are not designed to pass much current, so there is "probability"  latching-up, and it's easier to install a resistor than estimate 12-20 inputs/outputs interference to each other .

--- End quote ---
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 ---
I 'd refer to TI - White Paper
SCAA124 – April 2015
Latch-Up
Marty Johnson, Roger Cline, Scott Ward, Joe Schichl

--- Quote ---During the design of signal ESD protection structures, there can be
intentional SCRs employed that encourage selected parasitic PNPN diodes to trigger under an ESD event.
While ESD is an unpowered event, this is not a concern and actually desirable. However, during normal
operation since an intentional SCR could exist between a signal pin to ground and the signal pin could be
tied or driven to a “hi” state, care must be taken by using design rules on the product to limit effects of
Signal Latch-Up, thereby, mitigating the effect of excursion that could trigger the signal ESD SCR and
effectively short the signal to ground. If a signal pin is tied or driven to a “lo” state, then the signal voltage
is below the ESD SCR holding voltage and signal Latch-Up will not occur.
--- End quote ---
In short, there is certain "design rules".
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod