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OPA564: 24V domain and 3.3V domain connection scheme

Prasanna Gore
Prodigy 10 points
Part Number: OPA564

Tool/software:

In my design +IN input of OPA is connected to 3.3V through a 6.8K pull up. And 24V supply will be provided when the communication is needed. 

Is there any problem in this scheme?

  • 0
    TI__Genius 14986 points

    Hey Prasanna, 

    Prasanna Gore said:
    In my design +IN input of OPA is connected to 3.3V through a 6.8K pull up

    It is fine to use a pull up on the inputs. 

    Prasanna Gore said:
    And 24V supply will be provided when the communication is neede

    Does this mean the pull-up will be continuously applied even if the device is powered off? This is non-ideal, but the 6.8kOhm resistor will help protect against damage. 

    Is this circuit function an integrator? What is the input communication signal?

    Thanks,

    Jacob

  • 0 in reply to Jacob Nogaj
    Prodigy 10 points

    Hi Jacob,

    Thanks for a quick response!

    The circuit input is a proprietary protocol like RS485 squarewave signal from microcontroller. That is fed into +IN pin of the opamp.

    One more modification I am going to provide is to supply all 3.3V domain with a resistor divider on 24V supply that way 3.3V would be available just before the 24V supply is up.

    Would that scheme be better this present circuit?

    Thanks,

    Prasanna

  • 0 in reply to Prasanna Gore
    TI__Genius 14986 points

    Hi Prasanna,

    Thanks for the details here!

    We can certainly reference the 3.3V off of the 24V rail to better sequence everything. This could help ensure we do not apply voltages before other components are ready. 

    If we are injecting a square wave, we would get a triangle wave out of this amp. Is that the goal here? 

    Thanks,

    Jacob

  • 0 in reply to Jacob Nogaj
    Prodigy 10 points

    Hi Jacob,

    Thanks for confirming!

    There is a circuit block further down that takes care of the OPA output. Currently I am getting the correct output as I need, my main concern was regarding the 24V and 3.3V domains.

    Regards,

    Prasanna

  • 0 in reply to Prasanna Gore
    Prodigy 10 points

    Jacob,

    One more question - can I leave the E/S (ENABLE) pin floating (unconnected), or do I need to connect it to 3.3V as shown in my schematics?

    -Prasanna

  • +1 in reply to Prasanna Gore
    TI__Genius 14986 points

    Prasanna,

    Good question. Yes, it can actually be left floating as the device has an internal weak pull up resistor to the Vdig pin. Some users like the ability to externally set this node to be extra safe. 

    You are correct regarding the supply sequencing for this device, it is especially important to ensure the supply sequencing is maintained. 

    Thanks,

    Jacob