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Query regarding rise/fall time calculation

Sreelekshmi Unnithan
Prodigy 120 points

Hi,

We want to clarify two things from the TI application note.

1. The minimum transition rate, dt/dv of SN74LVC is 5/10 as per the table given below. Does this mean 5ns rise rate and 10ns fall rate? What does the minimum required transition rate mean? Does this mean that the input signal should not go faster than that, for eg, if the input clock transition rise rate is 1ns/V which is faster than 5ns/V, does this affect the functionality of the IC? 

2. Regarding dt/dv calculation, as mentioned in the application note, it is better to define rise and fall times over the range between VIL(max) and VIH(min). Is it correct if the equation is modified as shown below: Please confirm.

 

Modified equation:

Regards

Sreelekshmi.

  • +1
    Guru 316510 points

    1. 5 ns/V at 5 V, 10 ns/V at 3.3 V.

    2. You must use the two voltage for which the time was measured. The modified equation is correct only if the time between VIL and VIH is 400 ns. In the example above, the 400 ns value was measured between 10% and 90% of VCC, so this equation would not be correct.