Open collector comparator

Thread Starter

mike _Jacobs

Joined Jun 9, 2021
7
Hey guys

What happens when you have an open collector comparator and your pull up resistor fails open.

I would of said the output is permanently low?
However, i am not seeing that in simulation. Is this an artifact of the sim or is this really what would happen? Essentially the output is floating and there is no load on it. It is connected to a mosfet gate with no pull down resistor.

What say you?
 

Papabravo

Joined Feb 24, 2006
17,316
Hey guys

What happens when you have an open collector comparator and your pull up resistor fails open.

I would of said the output is permanently low?
However, i am not seeing that in simulation. Is this an artifact of the sim or is this really what would happen? Essentially the output is floating and there is no load on it. It is connected to a mosfet gate with no pull down resistor.

What say you?
Floating and ground are both possibilities with a real part, depending on what the inputs are doing. If the inputs are in a condition where the output would be low, that output transistor will be on and it will sink current. On the other hand, when the inputs are such the the output would be high if the pullup were operational, that output transistor will be off and no current will flow to ground. In that case the output would float.

Now when it comes to simulation, I'm not too sure how a simulator does a floating output. A realistic simulation would do random voltages with no current flow and for the life of me I'm not certain how they would do that.

I think that for a logic gate with a tristate output they pick a deterministic intermediate voltage dependent on Vcc, but I am not certain about that.
 

BobTPH

Joined Jun 5, 2013
4,054
No, the output is floating, i.e. no connection (actually high impedance).

Edit: I see papa already explained it better than I did.

Bob
 

Papabravo

Joined Feb 24, 2006
17,316
I have a partial answer on high impedance tri-state output in LTspice for one example. It is approximately VCC/2. I made an unusual choice for VCC in this simulation on purpose. I'll see if I can dig up a comparator model and repeat the experiment.
 

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Thread Starter

mike _Jacobs

Joined Jun 9, 2021
7
interesting, so we think it would float.

So in the case of it being connected to a fet gate without a pulldown and seeing as the fet only needs minimal charge to turn on, could this floating support the FET gate enough to turn it on when its not suppose to be?
 

Papabravo

Joined Feb 24, 2006
17,316
It took me a few minutes to find the LM393 model. It turns out that the output voltage is a function of the actual pullup impedance. I can't easily simulate the infinite impedance of an open circuit, but you can see the trend as the pullup impedance gets larger and larger. IMHO, an LM393 output will head for zero in both the real part and the simulation if the pullup fails open. If the inputs allow it, the outputs will continue to sink current.
 

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Ian0

Joined Aug 7, 2020
3,785
It depends on which leakage is the greater.
If it's a high voltage MOSFET in a small package then the leakage across the surface of the pcb between drain and gate terminals might be the greater, and it will switch on.
If there's any AC on the drain, then the drain-gate capacitance might form a charge-pump with any diode in the circuit.
Needless to say - it's a situation well worth avoiding!
 
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