SN74AVCB164245: Output rise/fall time

Jacob Maljian

Part Number: SN74AVCB164245

On my project, we need to translate between 1.8V (input) and 3.3V (output) using the SN74AVCB164245, and i need to make sure the output rise/fall time meets our requirements. However, i have been having issues finding this. There is no reference to output rise/fall time in the datasheet, only input.

I was given this forum showing how to calculate the rise/fall time given the Voh, Vol, Ioh, and Iol: (+) [FAQ] What is the output transition rate for a logic device? - Logic forum - Logic - TI E2E support forums

However, i do not know exactly which values to use to calculate it. When i tried to calculate it, i got that the Roh and Rol were exactly the same (16.67 ohms), but the attached forum seems to show that there are two different resistance values (though they were pretty close).

Also, i am not sure exactly what capacitance i should use. The forum seemed to use an arbitrary 50 pF load capacitance.

Some clarification on this would be greatly appreciated.

Thanks.

Screenshots of the electrical characteristics & recommended operating conditions for reference:

over 3 years ago

0 Clemens Ladisch over 3 years ago

Guru 318050 points

Some devices have different R_OH/R_OL, some are symmetric.

The load capacitance depends on the input capacitance of all devices connected to the trace and on the trace/board geometry. You could try to estimate that, but it might be easier to just measure the waveforms.

0 Jacob Maljian over 3 years ago in reply to Clemens Ladisch

Prodigy 10 points

Ok. Good to know.

The issue is that this is in the design phase, so we do not have the hardware right now. I am trying to make sure that the parts I had picked out will meet our timing requirements.

To find the Roh and Rol, i used the following values:

Voh = 3.1V

Vol = 0.2V

Ioh = 12mA

Iol = 12mA

Our Vcca = 1.8V and Vccb = 3.3V

Am i using the wrong values?

0 Clemens Ladisch over 3 years ago in reply to Jacob Maljian

Guru 318050 points

You are mixing different test conditions (100 µA and 12 mA); the voltage drop for 12 mA is 0.7 V.

What rise/fall times do you need?

0 Jacob Maljian over 3 years ago in reply to Clemens Ladisch

Prodigy 10 points

Ok. So, the R would be about 58.3 ohms, right?

Our requirement is it has to be less than 40 ns.

+1 Clemens Ladisch over 3 years ago in reply to Jacob Maljian

Guru 318050 points

Yes, the worst-case DC output resistance is 58.3 Ω. But the edges will be faster because this device has "dynamic output control" (see page 8 of the datasheet).

Anyway, the worst-case propagation delay is 4.3 ns. The propagation delay measure internal delays and the time to reach V_CCO/2, so the rise/fall time will certainly be much shorter than 40 ns.

0 Jacob Maljian over 3 years ago in reply to Clemens Ladisch

Prodigy 10 points

I see. Thank you Clemens. I appreciate it.