Which 74xx logic families are overvoltage tolerant on their outputs?

Question

Here's my dilemma: I have a 3.3v active-low logic line, and I want to use it to drive a 5v active-low logic line. The 5v line already has a pullup, so my plan was to use an open-collector buffer IC like the 74LCX07, with its VCC at 3.3v, as a buffer.

This seems fine, but I've read that many 74xx logic families have ESD protection diodes on the output, meaning that applying >VCC to their output pins - even open drain ones - will feed current into the chip's VCC via the ESD protection diode.

Is this the case? If so, which if any of the 74xx families are safe to use like this?

Answer 1

The 74LCX07 will not clamp the output to \$V_{DD}\$ (the datasheet talks about \$V_{CC}\$), since its particularly targeted at interfacing between different supply voltages. Note that the inputs are 5V tolerant, also with \$V_{DD}\$ = 3.3V.

The datasheet says maximum output voltage is 5.5V (never use the 7V mentioned under Absolute Maximum Ratings), but should have mentioned for what supply voltage, for instance also for 3.3V.

The only characteristic I could find which actually indicates that a higher output voltage is allowed is off state current on page 4, which gives a value of 10\$\mu\$A at \$V_{CC}\$ = 2 to 5.5V and \$V_O\$ = 5.5V.

edit
That could have been clearer. Especially since it contradicts what it says under Absolute Maximum Ratings: "DC Output Diode Current, for \$V_O > V_{CC}\$: 50mA."

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The NXP 74LVC1G07 seems to be a solution.

To level shift up from a 3.3V system to drive 5V CMOS input levels, simply connect the LVC output to a 5V termination voltage through a pullup resistor as shown below. The outputs of these devices are 5V tolerant and provide a simple solution to drive 5V CMOS input levels.

From here.

It also only mentions output clamping current for \$V_O < 0\$, so it won't clamp 5V.

Answer 2

I fully realize you specifically wanted to use a 74xx family device, but if you're not tightly constrained (why?), a member of the optoisolator family might be the better solution. Your grammar was all "a" so if you only need one I/O an indestructible 4N25 could be the better solution. A 74LCX07 provides six io ports... but for about the same price you can get a 4N25 optoisolator and a resistor or two and get 7500 volts of isolation on one port...

There is also something to be said for the simplicity of design of an opto solution... nothing you do involving less than 7500 volts or so on the output has any effect on the input devices... makes things a little simpler, time is money, etc.

Finally a peculiar logic family could disappear in the future, leaving you with a future design problem. On the other hand the venerable 4N25 opto will be around as long as 555s 741s 8051s who knows. LCX limits you to, what, Fairchild and STM? Not that there's anything wrong with that, but there must be at least a dozen generic opto makers. Opto is pretty well future proof, they'll never be a day when you can't buy a 4N25 or similar off the shelf.

Answer 3

As long as you give some margin below the MAX SAFE LEVELS , you are safe.

that means .. +/- 50mA for output diode current and -.5 to 7.0 Vdc for Vout as long as above is observed,

Running at any Vdd (that) is allowed is ok, even 3.3V (this is a tuff little part)

no worries for this part,same for other parts.. observe all MAX SAFE LEVELS but they may be lower.

. Good question tho...