Author Topic: What happens if the output of an op-amp is pulled to VCC?  (Read 3483 times)

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Offline aiq25Topic starter

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What happens if the output of an op-amp is pulled to VCC?
« on: August 29, 2017, 08:38:30 pm »
I'm not too familiar with analog circuits so I have a question. What happens if the output of an op-amp is pulled to VCC while there is no power on VCC pin?

My guess is there should not be any damage to the op-amp but I'm not sure.

Reason I'm asking is because I have a case where the power to my circuit will be cut off but the output is pulled high via a resistor (1k Ohm). I don't want the op-amp to be damaged. Maybe adding a to the output diode will help.
« Last Edit: August 29, 2017, 08:55:08 pm by aiq25 »
 

Offline suicidaleggroll

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Re: What happens if the output of an op-amp is pulled to VCC?
« Reply #1 on: August 29, 2017, 09:14:26 pm »
"Pulled" how?  And to which Vcc?  Its own Vcc or some other power rail in the system?  I think we need a schematic.
 

Offline danadak

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Re: What happens if the output of an op-amp is pulled to VCC?
« Reply #2 on: August 29, 2017, 09:19:28 pm »
If OpAmp powered off Vcc, and Vcc is opened, no damage.

If OpAmp powered off Vcc, and Vcc is shorted to ground, possible damage from inputs being driven
from a voltage source.

If OpAmp powered off separate supply, and Vcc is opened, no damage.

If OpAmp powered off separate supply, and Vcc is shorted to ground, possible damage. Could be thermal,
or over current. Depends on protection designed into OpAmp.

You could supply a schematic to vendor FAE for analysis. You would have to include power sequencing
in the description.


Regards, Dana.
« Last Edit: August 29, 2017, 09:21:26 pm by danadak »
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 

Online Zero999

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Re: What happens if the output of an op-amp is pulled to VCC?
« Reply #3 on: August 29, 2017, 10:12:44 pm »
I'm not too familiar with analog circuits so I have a question. What happens if the output of an op-amp is pulled to VCC while there is no power on VCC pin?

My guess is there should not be any damage to the op-amp but I'm not sure.

Reason I'm asking is because I have a case where the power to my circuit will be cut off but the output is pulled high via a resistor (1k Ohm). I don't want the op-amp to be damaged. Maybe adding a to the output diode will help.
Yes, there's a possibility of damage, due to the upper output transistor being subjected to reverse bias, but it depends on the op-amp topology and the power supply voltage. If in doubt, connect the output pin to the Vcc pin via a diode: cathode to Vcc & anode to output. This will bypass the output transistor, if the output is dragged above Vcc.
 

Offline aiq25Topic starter

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Re: What happens if the output of an op-amp is pulled to VCC?
« Reply #4 on: August 30, 2017, 03:10:26 pm »
Attached is a block diagram. Sorry about it not being a proper schematic, this is something I was able to draw very quickly.

The VCC of the op-amp is NOT the same as the VIN of the system. The VCC of the op-amp is reverse voltage protected by a diode, so VCC of the op-amp has about a 0.2-0.3 volt drop.

The amplifier is just being used as a buffer. Perhaps it would be easier to replace this with a transistor based design so I don't have to worry about damaging the IC? I listed the 3 test cases in the block diagram.
 

Online Zero999

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Re: What happens if the output of an op-amp is pulled to VCC?
« Reply #5 on: August 30, 2017, 05:31:32 pm »
As I said above, a diode between the output and the positive power terminal will protect against this.

 
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Offline aiq25Topic starter

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Re: What happens if the output of an op-amp is pulled to VCC?
« Reply #6 on: August 30, 2017, 05:39:06 pm »
As I said above, a diode between the output and the positive power terminal will protect against this.

Thanks!
 


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