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

I wouldn't worry about it if the draw from the 3.3V supply is >>

Since each input has it's own ESD diodes you're safe. ...Jim Thompson

Tack on a 3V3 zener to the input ? double safe with the 1K...

Where do you get a 3.3V zener that does not have a soft knee? ...Jim Thompson

Or a Schottky diode to +3.3. That'll prevent the ESD diodes from turning on, which I think is a good policy.

Cheers

Phil Hobbs

Should be fine as-is. One mA into an ESD diode won't cause problems.

I guess since you're presently using the 74AHC157, your 3.3V inputs are probably OK. The other thing to check, however, is if the increased output drive of the LVC (+/- 24mA) going to cause any problems.

Den onsdag den 14. oktober 2015 kl. 19.00.17 UTC+2 skrev Tim Wescott:

The datasheet says you that any voltage is allowed as long as you stay below abs max +/-20mA into the input

just watch out for the minimum load on the supply if it is source only

an old Xilinx app note suggested putting a load resistor on the supply

-Lasse

I'm trying to do this with NO BOARD CHANGES, guys! I didn't mention that the "+5V domain through a 1k" is actually an open collector pulling down a 1k whose hot end is at +5V -- if I cared that deeply I'd have them do a cut and jump to +3.3V.

But -- they wouldn't want to do that, so my alternative is to suggest a different part number for one chip.

then all you need as an R shunt to ground to make a divider...

thats a pretty minor change that can probable be done without board spin.

Mark

You could make it a 2k resistor, if that would let you sleep better. It sounds like it ain't broke, so why fix it?

George H.

no answer to the question, but I have another one.

Are they still making latest/greatest "magic" series of chips?

My junk box stops in about the mid 90s with the "als" stuff. Any new 40xxx or 74xxx stuff introduced in the last decade?

from the NXP datasheet for 74ahc/ahct157

"Inputs accepts voltages higher than VCC"

And it doesn't mention a max positive clamp current for inputs, just abs. max 7V, recommended 0-5.5V with no mention of Vcc

i.e. you don't need to change anything

-Lasse

There is a bewildering variety. Go to DigiKey and type "7400" into the search bar for an idea.

It's not necessary. Leave it alone and go pour a cool one ;-) ...Jim Thompson

There's also several families of "tiny logic" CMOS, single-gate functions mostly. Many allow logic inputs past Vcc, to 7 volts maybe, and some are blindingly fast, like 600 ps rise/fall times.

There's a 1 ns flipflop, NC7SV74, for 16 cents.

And there is SiGe ECL and CML logic, edges way below 100 ps. Like $40 to $150 per gate.

At the hospital, after the titanium implant surgery?

I have a Titanium hip joint ;-) ...Jim Thompson

TI's SN74LVC157A is 5v tolerant, Or you can go with Jim's suggestion.

Cheers