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Logic gate output current

  1. Apr 6, 2016 #1
    Hi, I have this logic gate
    http://www.ti.com/lit/ds/symlink/sn54hc32.pdf

    I am using 5V as my VCC. In the datasheet it says that the output current at 6V is 5.2mA. My question is, Is that the max current that it can output ?
     
  2. jcsd
  3. Apr 6, 2016 #2
    For sure not.
    http://www.nxp.com/documents/user_manual/HCT_USER_GUIDE.pdf [Broken] (page 34)
    You can drawn more current but Voh voltage will drop bellow Vih (High-level input voltage ), so the next gate will not be able to properly recognize the logic state at his input.
     
    Last edited by a moderator: May 7, 2017
  4. Apr 6, 2016 #3
    Ok, is there any way to know by how much it might drop ?, I am feeding it to the base of a transistor
     
    Last edited by a moderator: May 7, 2017
  5. Apr 7, 2016 #4
    How much base current do you need? Also if you drive a BJT base only then you do not have to worry about VIh any more because the BJT is not a logic circuit so you do not have to meet the logic level regiments. Also we can find in datasheet the info that Io_max = 20mA
    https://en.wikipedia.org/wiki/Logic_level#Logic_voltage_levels
     
  6. Apr 7, 2016 #5
    Assuming the output voltage from the ic is 5V which I used to calculate my base resistance. The current would be about 5.5mA
     
  7. Apr 7, 2016 #6
    No worries here the gate will survive this.
    As we can see from this characteristic
    Voh.png

    The voltage drop across the gate output will not be larger than 0.8V and typically it will be around 0.3V.
     
  8. Apr 7, 2016 #7
    Thanks, So I can assume the output voltage from the gate and hence the voltage to the base of my transistor will be about 4.7V ?

    Also, from the same datasheet, how much current does the ic draw when one of the pins is held high ?, I want to calculate a pull up resistor to ensure that the pin gets close to 5V as possible. I read around and they about 5k to 10k should be fine
     
  9. Apr 7, 2016 #8
    Yes.
    The current consumption will depend on the load resistance. Because CMOS circuit in static do not burned any power. Only the load will consume power from power supply.
    Do you really want to put a pull-up resistor at the gate output? Why ?
     
  10. Apr 9, 2016 #9
    I wanted to create a NOT gate using a transistor. But I don't need it now because I have an ic for it
     
  11. Apr 9, 2016 #10

    CWatters

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    Can I suggest you post a sketch of this bit of your finished circuit?
     
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