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SN74LVC2G74-Q1: Why does this device has such a fast input transition requirement?

Emrys Maier
Guru* 96591 points
Part Number: SN74LVC2G74-Q1
Other Parts Discussed in Thread: SN74HCS74-Q1

Posting on behalf of Balazs Kelenvolgyi:

A customer asked me why the input transition rise or fall rate is defined so short (5nS) in the datasheet of SN74LVC2G74QDCURQ1? They want to filter the input with an RC filter and they want to size the filter in a way to have the filtering maximized but still fulfil the minimum slew rate. They successfully tested the device with much slower slew rates e.g.: 100nS/V (ok it was only tested at room temp and this was tried on 1 sample only).

Is it possible to get more background about this parameter? (maybe it can be slower if supply voltage is 5V or 3v3 …I am looking for such dependences if there is any).

It is also a question what happens if they do not fulfil the 5nS/V requirement?

  • 0
    TI__Guru* 96591 points

    Hi Balazs,

    Emrys Maier said:
    A customer asked me why the input transition rise or fall rate is defined so short (5nS) in the datasheet of SN74LVC2G74QDCURQ1?

    The specifications in the datasheet are selected based on the worst case operation from weak, nominal, strong process corners, low, room, and high temperature corners, and across the operating voltage range (usually just the worst-case for voltage). For any specification, it can be said that it was selected to cover all possible cases within the expected operation of the device.

    Emrys Maier said:
    They want to filter the input with an RC filter and they want to size the filter in a way to have the filtering maximized but still fulfil the minimum slew rate.

    Do you know why they want to filter this signal? They may need a different solution if they expect to see significant noise on the input signal. For example, the SN74HCS74-Q1 that include Schmitt-trigger inputs and can handle any level of input transition rate.

    Emrys Maier said:
    They successfully tested the device with much slower slew rates e.g.: 100nS/V (ok it was only tested at room temp and this was tried on 1 sample only).

    Our devices are very high quality and often can survive punishment by customers. I have seen devices work in similar circumstances for years, however if any one fails, we will not accept a return since it was being operated outside the recommended operating conditions.

    Emrys Maier said:
    Is it possible to get more background about this parameter? (maybe it can be slower if supply voltage is 5V or 3v3 …I am looking for such dependences if there is any).

    Yes, the spec is supply dependent - I assume you didn't try just looking in the datasheet, since the across-voltage specs are provided very clearly:

    Emrys Maier said:
    It is also a question what happens if they do not fulfil the 5nS/V requirement?

    If they have a slow input, it can create oscillations, both internal and external, and can produce damaging current through the input stage of the device (although damage is very rare from this alone). We have an FAQ on this topic since it is very frequently asked. Please see the very first question on our FAQ post here: [FAQ] [H] Frequently Asked Questions: Logic and Voltage Translation