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TINA/Spice/SN74AHC1G04: Excess current draw when negative voltage applied to input

Part Number: SN74AHC1G04
Other Parts Discussed in Thread: TINA-TI,

Tool/software: TINA-TI or Spice Models

I am currently using the AHC inverter gate in a circuit that has approximately -1.8V present on the input pin. The datasheet states that Vin can go below -0.5V as long as the input current isn't  > -20mA. The input  appears to be limited to less than 1mA.

Is there another caveat to the input voltage limit? Could we be forward biasing the input clamping diode?

The circuit was modeled in a spice simulator using the TI provided spice model, but no issues were seen in the simulation. Are the input clamping diodes present in the SN74AHC1G04 Spice model?

  • Hi Matt,

    FYI, One of our team will take a look at this.

    Herman
  • Hi Matt,

    To answer some of your questions:
    We don't model the input clamp in this part's spice model.

    Based on the datasheet section "6.1 Absolute Maximum Ratings":
    Vi, Input Voltage Range is recommended not to exceed -0.5V on the low side and 7.0V on the high side. So this means you should not try -1.8V regardless of the current flow at the input.

    When you try to go below 0 Volts at the input, based on the "Iik, Input clamp current", you could potentially forward bias the input clamp and the current from the diodes might vary from part to part based on process variation as well as based on how much forward bias voltage you put on that input clamp. The current can go as little as nothing to as large as 20mA.

    You stated "The datasheet states that Vin can go below -0.5V as long as the input current isn't > -20mA. The input appears to be limited to less than 1mA." Based on datasheet and explanation above, this is rather misinterpretation of the datasheet information.

    Basically, the datasheet says, "the moment input voltage is below 0V, you will automatically take risk of forward biasing the input clamp diode and the current from that diode might vary based on how much forward bias voltage as well as process variation."

    Furthermore, the datasheet puts additional constraint that the user is not recommended to go below -0.5V ever. This is because it is an absolute maximum ratings.

    From practical perspective, basically the datasheet says you can only float from -0.5V to 0V if you were to go below 0V, but then you're still risking forward biasing the input clamp with unknown current that can go as large as 20mA.

    Hope this helps and let me know if you have further questions,
    Herman