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CD40106B: high level input voltage on unpowered chip

Part Number: CD40106B

Hi,

I've got a situation where I can have a 9V signal present on one of the input pins of this chip when it's unpowered. I was thinking of adding a series resistor on that input, or potentially even a series resistor plus a diode from the input pin to the CD40106 rail (the idea being to use a beefier diode that I know the limits of, rather than rely on the chips internal protection circuitry).

I saw in the datasheet though that inputs already incorporate some series resistance in addition to the protection diodes (see image below for screenshot), and so I'm wondering if even any external protection is necessary? I understand that the safest option would probably be to have it, but this is an already existing board that I'll be reworking so I'd rather keep modifications to a minimum.

  • Hey Ryan,

    Unfortunately, that resistor isn't going to protect the device in your situation. As long as you limit the current into the diode under 10 mA you should be fine, but yes above all else i'd recommend your original strategy of using an external diode with specifications. I know its inconvenient to get implemented, but keeping the device protected through its lifetime will help in the long run.

  • Please note that a series resistor still allows a small current to flow into VDD, which might still be enough to power the device up.

  • What about if I limit the current to a value well below it's max, something like <1mA? Is there still a risk of long term damage?

    Regarding the device powering up through the input, as long as that doesn't mean a risk to the device itself, there's no risk to anything around it. Everything that the outputs are connected to is powered all the time.

  • If the diode current is limited (by a series resistor or something else) to an allowed value (< 10 mA), then the device will not be damaged.