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Original question:

[FAQ] Can the input voltage (Vi) to my logic device be higher than the supply voltage (Vcc)?

Over-Driving Inputs

John Fedak
Expert 6715 points
Other Parts Discussed in Thread: SN74AHC14Q-Q1

Hi Team,

I have a customer with a couple questions related to the post on driving the inputs above Vcc - the question is around driving the inputs above the abs max voltage, while the input current ratings are observed.

This is commonly stated in the abs max section as being OK, but isn't mentioned in the recommended operating conditions section. With this, here are the two questions:

    1. Are the outputs deterministic when the inputs are driven above the rail (assuming the current limit is respected)?
    2. Is there any degradation to lifetime when driving inputs above the rail (assuming the current limit is respected)?

This is specifically for the inputs on the SN74AHC14-Q1, but I believe would apply to all devices with this condition.

Regards,

~John

  • 0
    TI__Guru* 96591 points
    Hi John,
    It's important to note that the input voltage can be larger than the supply, however it _cannot_ be higher than the absolute maximum rating of the device.

    For example, applying 6.5V to the input of an SN74AHC14-Q1 when its supply is 3.3V is no problem, so long as the input current is limited to 20mA or less. The device will operate normally in this circumstance.

    Applying 120V to the input of the same part, even with limiting the current to 20mA, is very likely to destroy the device -- the gate oxide will instantaneously breakdown for the input FETs (before the clamp diodes can turn on and sink enough current to reduce the input voltage).
  • 0 in reply to Emrys Maier
    TI__Expert 6715 points

    Hi Emrys,

    Thank you for your feedback on this - I do have one follow-up on this:

    - The customer is OK with up to 7V on the input within regards to his questions (deterministic/long term reliability) and within IOK, correct? 

    The reason I ask is that the recommended operating conditions state up to 5.5V on the input voltage. Your post above indicates that 6.5V is no problem, but I just wanted to double check as I know there is a little gray area when we are between recommended operating conditions and abs max conditions.

    I do have a general question that I want to clear up on this and to dive into this specific case - there is no mention of VCC looking at the "Input voltage range" in abs max table for the SN74AHC14Q-Q1:

    Note 1, highlighted by VI, says as long as the input clamp current is respected the device can operate beyond the voltage conditions. The input clamp current is measured for VI < 0.

    Why is this note in there if -0.5 V can't be exceeded? Is this to highlight that operation is OK between 0 and -0.5V when IIK is within -20mA?

    Regards,

    ~John

  • 0 in reply to John Fedak
    TI__Guru* 96591 points
    Hi John,
    Yes, up to 7V will not damage the device.

    Theoretically you can exceed the abs max ratings and stay safe, but it's definitely not something I would recommend to our customers. If you can go to -0.7V, then can you go to -4kV?

    The fact is that you can put a -1V input on the device, and it will turn on that diode causing a huge amount of current (and likely burning the device). If you add a resistor to keep the current low, it will limit to ~-0.5V.

    The diodes there aren't very fast acting, so they won't be helpful for undershoot really.