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SN74AHC1G04: about over stress of SN74AHC1G04QDCKRQ1

Part Number: SN74AHC1G04
Other Parts Discussed in Thread: SN74LVC1G57, SN74LVC1G14

hi dear supporting team,

customer found PIN2 of SN74AHC1G04QDCKRQ1 has a short term -0.8V voltage, while in the d/s, it mentioned the min voltage is -0.5V, the waveform is as below, is it possible damage the chip? tks a lot!

  • Hi Vera,
    It's unlikely that the -0.8V signal will cause any problems -- while this does violate the datasheet's absolute maximum rating (Vi (min) = -0.5V), there is also a current rating for the diode on the input for 20mA max. As long as the current does not exceed 20mA , the device will not be damaged, and I have never seen an undershoot like this causing that much current draw.

    I would be more concerned with the slow edge rates of the signal. This device has a recommended input edge rate of 20ns/V (max), while the above signal has an edge rate of about 1us/V. This can cause excessive supply current, output oscillations, and damage to the device.

    If it were my system, I would replace it with a Schmitt-trigger inverter such as the SN74LVC1G14, or I would go for a different solution. It looks like they are going for an edge detect circuit here - I could recommend a simpler circuit using the SN74LVC1G57 that would eliminate all their problems. Let me know if they are still at a design stage where a board change is possible.
  • hi Maier,

    thank you very much for the reply!

    as for -0.8V, customer's max current is 10mA, so it should be no problem, right?

    and for the slew rate, as for my understanding, for higher slew rate, the current will be bigger, why slower slew rate will cause the problem? (I=dV/dt)

    tks a lot!

  • Hi Vera,

    It is desirable to have input transition rates as fast as possible with CMOS logic. There is an application note that goes into details here: Implications of Slow or Floating CMOS Inputs

    This is common to all CMOS devices - and TI sells a lot of them.

    10mA is no problem as the max is 20mA and 10mA < 20mA