Comparators with wired-OR outputs

Thread Starter

Swathi Sivakumar

Joined Apr 23, 2019
5
What will happen if the output pins of two comparators(LMV393) are shorted?

Dodgydave

Joined Jun 22, 2012
10,050
Nothing, as they are open Collector outputs.

Alec_t

Joined Sep 17, 2013
12,210
Welcome to AAC!
If the two outputs share a common pull-up resistor they can behave like an OR gate.

MrChips

Joined Oct 2, 2009
24,232
Depends on whether you use positive logic or negative logic.
With positive logic, it is an AND gate.
With negative logic, it is an OR gate, as per DeMorgan's transformation.

DickCappels

Joined Aug 21, 2008
7,741
*provided that the ground pins of both comparators are at the same potential, as they would be if they were in the same package.

Thread Starter

Swathi Sivakumar

Joined Apr 23, 2019
5
Is that don't create stability issues ?
Note - I'm using 2 LMV393 in closed loop configuration for controlling a TRIAC.

DickCappels

Joined Aug 21, 2008
7,741
Maybe this is a good time to show us your circuit so we can try to tell if there could be some sort of problems. If two outputs are connected and one comparator wants its output to go low, they will both go low because in a conventional application there is no active pull-up circuit, only that one resistor.

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Thread Starter

Swathi Sivakumar

Joined Apr 23, 2019
5
Welcome to AAC!
If the two outputs share a common pull-up resistor they can behave like an OR gate.
can you please elaborate on this ? I think this is the issue with my circuit.

Alec_t

Joined Sep 17, 2013
12,210
can you please elaborate on this ?
If either comparator A output goes low OR comparator B output goes low then, since the outputs are connected, the output C of the combination is low. The resistor is the only thing which can pull the output high.
As AlbertHall pointed out, and MrChips clarified, in positive Boolean logic this is an AND function :- C=A.B.
In negative logic this can be re-written as not-C = not-A OR not-B.

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