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SN74AHCT1G125: URGENT: Unwanted voltage on output pin above VCC+0.5V but low current => damage?

Siemen Smolders
Prodigy 20 points
Part Number: SN74AHCT1G125

Hello,

The datasheet for the SN74AHCT1G125 says that the absolute maximum rating for output is VCC+0.5V.
In a footnote, it says "The input and output voltage ratings may be exceeded if the input and output current ratings are observed."

We have a situation (in production) where the device is powered off (VCC = 0V), but the output pin of SN74AHCT1G125 sees a voltage from another device.
This voltage comes from a 5V with a series resistor of 4k7 ohm. It leads to a situation where there is max. 1mA flow into output pin of SN74AHCT1G125, and it sees ~2.0V.

My question: is this situation acceptable because of the low current 1mA going into the output pin of SN74AHCT1G125?
What are the critera I need to look at for this chip and its technology to verify whether damage will occur over some time?

  • 0
    TI__Guru* 96591 points
    Hi Siemen,
    Since the voltage applied to the output is larger than Vcc + 0.5V, then the output clamp diode will be active, causing current to sink into the device and cause a voltage to appear on the Vcc pin. Since the current is very low, there isn't danger to this device (this is specified as "Output clamp current, I_OK" in the Absolute Maximum Ratings table), however it can "back power" the entire rail that this device is connected to.

    Have you tried measuring the Vcc voltage when the 5V is on the output? If you are seeing 2V at the output, I would expect to see ~1.3V on the Vcc pin (about one diode drop difference). In many circumstances, this isn't a problem, however it can cause significant undesirable power consumption or worse could turn on some parts that are supposed to be off.
  • 0 in reply to Emrys Maier
    Prodigy 20 points

    Hi Emrys,

    Thanks for your prompt message.
    Unfortunately, I can't measure the voltage on VCC rail since I was handed this device and it is covered in epoxy as casing.
    In terms of back powering, it could indeed lead to quirky functional behaviour. But since it is supposed to be a 5V VCC rail, I assume that having 1.3V there won't damage anything.
    In that case, as long as we can confirm that functionally everything works, we are good to go.