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SN74LVC125A: Side affects when 5V is applied to output pin

Matthew Higgins
Prodigy 10 points
Part Number: SN74LVC125A
Other Parts Discussed in Thread: SN74LV125A, SN74LVC2G125,

Does anyone have experience using this chip where the output pin is connected to 5V, only when not driving? If the current can be limited using a resistor, is there any acceptable usage that will not damage the part? In testing it looks like the pin will draw ~20mA of current and be clamped to ~4V.  The driver is capable of supplying 20mA of current.

  • 0
    Guru 281000 points

    The output is diode-clamped to VCC. So you are not allowed to apply a voltage higher than VCC.

    The SN74LVC2G125 and SN74LV125A block reverse current in the power-off or high-impedance state.

    What is the actual problem you're trying to solve? An open-drain device might be more appropriate.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Our device uses this IC in an application which was initially specified at 3.3V, but has moved to 5V without our knowledge. After we've discovered the 5V use case and explained it is beyond the max operating level of the device; the end customer was interested in how the chip will/should/could fail. It's been used for 2+ years over the full temperature range with no noticeable degradation or loss of performance. We use a different IC in products designed to support 5V; this was more of a question of perhaps we are not seeing a failure due to the current being limited? Or they are just lucky.

  • 0 in reply to Matthew Higgins
    Guru 281000 points

    Is the output higher than VCC, or is this device run with VCC = 5 V?

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    The VCC is 3.3V. When the OE is on, the output of course is driven by this buffer and only 3.3V. When the OE is off (buffer not driving), the output signal will have a 50MHz 5V clock on it (driven by another IC) which is connected by 33 ohm resistor. 

  • 0 in reply to Matthew Higgins
    Prodigy 10 points

    In the original design, the external 50MHz clock used to be 3.3V. The customer has switched this to 5V (w/o our knowledge), which violates the spec of the part. In practice it seemed that the chip suffers no consequences though.

  • +1 in reply to Matthew Higgins
    Guru 281000 points

    There is a diode between the output and VCC. A current will flow into the 3.3 V supply, and if that current is not consumed, the voltage might rise. This is not really a problem for the SN74LVC125A, but other devices connected to the 3.3 V supply might get damaged.

    The SN74LV125A blocks reverse currents when the output is disabled (but has lower drive strength).