3.3V - is it HIGH for 5V chip?

Discussion in 'General Electronics Chat' started by palkapalka, Nov 2, 2015.

  1. palkapalka

    Thread Starter New Member

    Aug 15, 2015
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    Hi,

    I have ADC that I want to power with 5V.
    I have microcontroller that I power with 3.3V (it can not take higher).
    ADC is connected to MCU with I2C - SCL and SDA.
    ADC requires pull-ups on SCL and SDA.
    If I pull-up with 5V - I am afraid this may burn MCU since this voltage goes to MCU SCL and SDA pins.
    So, the question is can I pull-up with 3.3V and power ADC with 5V?

    Please do not suggest to power ADC with 3.3V - this works but I want to put it on 5V source because I already have 5V in my design that I can turn on and off that may save energy consumption - this is the whole idea here. 3.3V I can not turn on and off since it is the only power for MCU.
     
  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Which ADC and which MCU?

    Without such info, there is no possible way for your question to be answered.

    Links to datasheets would be useful, also.
     
  3. palkapalka

    Thread Starter New Member

    Aug 15, 2015
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    ADS1115 and RFD22301
     
  4. joeyd999

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    Jun 6, 2011
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    Would have been nice for you to post datasheets:

    ADS1115
    RFD22301

    There is not enough info on the RFD datasheet to make a determination. Best advice: call the manufacturer and ask.
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Well the RFD parta is specified for 3.3V so you need to make sure that it gets no more than 3.6V on its inputs. If you run the ADS at 5V exactly, then you are out of the assured range as the input should be at least 3.5V to make sure it will register as high level.
    I would simply use some level shifters and don´t have to guess.
     
  6. joeyd999

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    IIC level shifters? Wanna propose a suggestion?
     
  7. BillB3857

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  8. joeyd999

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    Jun 6, 2011
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    Clever.

    FYI to OP: There seem to be speed limitations using such a device, if you choose to do so.

    You should still ask the MCU manufacturer if the IIC lines are 5V tolerant. They may be.
     
  9. kubeek

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    The setup with 2x10K resistors are a bit on the slow side, I think I once did a bit of simulating it with values around 1k and it performed a lot better, but can´t remember the exact numbers.
     
  10. palkapalka

    Thread Starter New Member

    Aug 15, 2015
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    Thanks for the suggestion with the level shifter. Will have to read more about them.
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    There are various bus-wide level converters you can buy off the shelf for putting between 3.3V and 5V devices. Usually it isn't difficult to get a 3.3V device to switch a transistor on and off, you could simply tie its collector resistor to the 5V rail - but the signal would then be inverted. You can then invert it back again with a second transistor, but it starts getting a bit untidy! Not too bad for 1 or 2 lines of serial transfer, but for parallel lines a bus converter is the way to go.
     
  12. joeyd999

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    Jun 6, 2011
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    IIC.
     
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