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CD74HC32: Can the IC get power by input pins?

Konrad Sledziewski
Prodigy 70 points
Part Number: CD74HC32
Other Parts Discussed in Thread: SN74LVC32A, CD4072B

Hi,

I have a problem with circuit below. When I plug "O" pins the gate is working without connected VCC. "O" pins comes from camera and they are pulled up to 3.3V inside camera. GND pins are connected together (gate and camera) but VCC pin from gate is not connected to 3.3V from camera. I don't really know why it's working? Is it because of the current flowing by gate input pins? Or inside pull-up resistor (1k) is connecting VCC with 3.3 in some way? If any additional informations are required, please ask me.

Best regards,

Konrad

  • 0
    TI__Guru* 96681 points
    Hi Konrad,
    Yes, the inputs to the CD74HC32 have positive clamp diodes which connect them to the positive supply.

    If you need an OR gate to operate at 3.3V and _not_ get powered by an input voltage when Vcc = 0V, then I would recommend using SN74LVC32A.
  • 0 in reply to Emrys Maier
    Prodigy 70 points

    Thank you for resolving my problem. I have a last question. Do you have 4 input OR gate IC?

     
    Best regards,
    Konrad

  • 0 in reply to Konrad Sledziewski
    TI__Guru* 96681 points
    Hi Konrad,
    I'm glad I could help.

    The CD4072B is a 4-input OR gate, however I would recommend instead going with a quad 2-input OR gate and wiring 3 of the gates together to form a 4-input OR. SN74LVC32A could easily accomplish this.

    Many-input logic gates aren't really made anymore because the multiple input structure requires a large bias voltage (ie 5V supplies). You can also get the same effect by tying open-drain outputs together from something like a hex open-drain inverter (it's technically a NOR gate though, you have to invert the output again with one of the channels to get an OR... so 5 inputs max).