74<magic>157 -- 3.3V supply, 5V tolerant inputs?

I've run into a few interesting facts. I did a project a few years ago, and used a single-funtion line of chips, the SN74LVC1Gxxx family, where you mostly replace the xxx with the equivalent 7400 part. Good datasheet performance, lots of drive current. But, when we put a board together, the horrors started. I finally discoverd that there was a 3 ns shoot-through when the output transitioned, that ran in the area of 500 mA per chip! This was a mixed-signal board with low-level signals under 100 mV that comparators had to trigger (correctly) on.

I redid the whole design, moving to all 3.3 V chips, and selected the 74AUP family, because they had a stated equivalent switching capacitance that was VERY small. Worked like a charm, they must have shaved their transistors JUST SO to minimize shoot-through. It went from 500 mA ++ with the other family to unmeasurable.

Yeah, I know, this doesn't really answer the original question.


Reply to
Jon Elson
Loading thread data ...

Good enough!

Reply to
Robert Baer

Am 14.10.15 um 19:00 schrieb Tim Wescott:

74LVC157, maybe 74LVT157.


With this series resistor, you are on the safe side, even if the

74AHC157 is not 5V-tolerant. Calculate the difference between the two supply voltages, subtract the forward voltage of the input clamping diode of the 74AHC157 and divide theresult by the series resistance. Thats a current going into the input of your multiplexer of around 1mA. In the NXP datasheet of the 74AHC157, it is stated, that it's clamping diodes withstand 20mA.

But if you want to safe energy, particularly the current through the clamping diode, use the 74LVC157.

No surprise.

Concerning energy consumption, you are right. But concerning functionality and safety, it's OK.

If there's a better part that I could drop in I'm all ears.

See the LVC type.

Best regards,

Reply to
Günther Dietrich

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.