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OPA1622: EN (Shutdown not working)

Leo A.
Intellectual 545 points
Part Number: OPA1622
Other Parts Discussed in Thread: OPA1632

Tool/software:

Hello,

I am using the OPA1622 in a design as unity gain buffer, as shown below.

The Enable pin is normally pulled up to 34V1 (34V) using a 22k resistor. Then I have an external switch that connects the EN pin to GND. When the switch is engaged the votlage on the enable pin is 0.077V (77mV).

The amplifier does go into a low power state however any input signal is also showing up on the output but with very high distortion.

So to me, it seems like the amplifier is not being fully turned off there is some bleed though. Is this normal behavior?

To me, it seems like the amplifier should be completely shut down and nothing should pass through it from input to output.

  • 0
    TI__Mastermind 23965 points

    Hi Leo, 

    The output should not be considered to be in high impedance. Applying signals can parasitically turn on the output stage. This is most likely why you are observing this behavior. 

    Best Regards, 

    Chris Featherstone

  • 0 in reply to Chris Featherstone
    Intellectual 545 points

    The datasheet says when applying a signal to the output, I am not applying any signals to the output while the device is in shutdown. Only to the + input.

  • +1 in reply to Leo A.
    TI__Mastermind 23965 points

    Hi Leo, 

    The input stage isn't shutoff in shutdown mode and I believe this is why you may be observing this behavior. The shutdown mode only affects the output transistors so I believe the feedforward path internally can still bias the output stage parasitically. Since it is a bipolar device it doesn't have a hard shutoff in the same way a cmos would. Is the signal that you are seeing very low level? I actually tested the output stage current in shutdown before and it isn't "high impedance" but also not super low. I would expect the signal that you are observing is not full scale. Is my assumption correct?

    I have brought this up to our team and they are aware of this for the future. Ideally the output stage would be completely shutdown. 

    Best Regards, 

    Chris Featherstone

  • 0 in reply to Chris Featherstone
    Intellectual 545 points

    The signal is about 2/3 of what it would be normally if the device was on. Looks like I will have to figure out some other way to completely shut down the output.

  • 0 in reply to Leo A.
    TI__Mastermind 23965 points

    Hi Leo, 

    Are you able to share your schematic? This may not be coming from the amplifier itself. You may be seeing the signal passing through the feedback components around the amplifier. It is possible that you could parasitically turn on the output stage via this path but you may just be seeing the signal conducting via the feedback components. 

    Best Regards, 

    Chris Featherstone

  • 0 in reply to Chris Featherstone
    Intellectual 545 points

    My schematic is in the first post. There is feedback components around the 1622 as it is being used as a unity gain buffer.

  • 0 in reply to Leo A.
    TI__Mastermind 23965 points

    Hey Leo, 

    Sorry I just realized the schematic you initially gave was there. I was thinking about this further this morning and thought of the feedback path. 

    Best Regards, 

    Chris Featherstone

  • 0 in reply to Chris Featherstone
    Intellectual 545 points

    Each positive input of the 1622 does have a 17V common mode on it from the OPA1632, could it have something to do with this?
    So basically it is always looking to put out 17V.