Are 5v-tolerant inputs clamped?

As far as I know, "5V-tolerant" inputs are NOT clamped. What would they be clamped to? There's no 5V VDD on the chip to strap a diode to. All that is there is a so-called "snap-diode" (to ground) for ESD pulses. ...Jim Thompson

Some protected MOSFETs clamp overvoltages by incorporating a zener from drain-to-gate. I didn't expect zeners en masse in a digital part, but possibly some equivalent function.

Yeah, crap. Reading app notes for the device family, I'm not confident the device survives >6v inputs, current-limited or not.

I'm thinking I'll use a dummy logic part for the clamps. An 'HC244 (outputs disabled) gives me 18 clamps to Vdd and Vss. Does that horrify you? :-) It's only a mA or two, worst case.

-- Cheers, James Arthur

Oops---------------------------^^^^^^

Make that 16 clamps--the enables have to be hardwired '1'.

-- Cheers, James Arthur

seem to remember Xilinx describing the clamping on their old 5V tolerant fpga/cplds as a ~6V zener like diode to ground

but if the input signal are current limited well below the max limits and there's no risk of input current lifting the supply is should be fine, Xilinx specificly spec that inputs should be limited to Vcco + 0.5V OR

-Lasse

Doesn't horrify me at all... a clever solution to a problem that's been mentioned here before!

Only caution... output devices can be big and CAPACITIVE, so you'll take a hit if you're trying for high speed. ...Jim Thompson

I _was_ going to get the data sheet and count ;-) ...Jim Thompson

is

On some designs I was involved in we used to use 74133, 13-input nand gates for the same function. 13 clamp inputs in a 16-pin package.

kevin

Naa, they're solenoids and such--slow as dirt. I'm reading back the driver state at its output to make sure it really did what it was supposed to do. "Trust, but verify."

Thanks for the feedback.

James Arthur

is

A chip could have one 5v zener and a bunch of internal ESD diodes to clamp positive spikes--that works.

If the input isn't clamped at all, ever, that would seem to make it unacceptably fragile--how would it survive minor ESD?

FWIW: The most ESD-sensitive part I've ever used is the 2n7002. I can't remember ever killing another chip with static, but I've fried half a dozen 2n7002's just soldering them in, carelessly.

-- Cheers, James Arthur

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Thanks for the suggestion. As one alternate I was thinking of the 'HC30 (8-input NAND, a jellybean), but 13 inputs feels luckier ;-)

-- Cheers, James Arthur

So that begs the question: how are such inputs protected internally against ESD? There's got to be something, possibly a zener after all?

-- Cheers, James Arthur

If all you're doing is reading back the state of the relays, why not use the '244s as buffers. They tend to be more robust than the inputs of higher integration parts.

A big-ass N-channel with its gate grounded thru a resistor. A fast edge turns it on. ...Jim Thompson

I think different companies are going to do it differently.

I can't remember whether it was 3.3V tolerant inputs on a 2.5V device, or 5V tolerant inputs on a 3.3V device, but I do remember that there was one Xilinx part that would sorta kinda turn on a body diode (maybe they used two in series???) when you gave it the full rated input, leaking a bit of current _into_ the chips main supply.

This was no problem, except that in the board we used it at startup when there wasn't much current draw on that supply and all the inputs were high, and it lifted the FPGA's supply rail. _That_ took us a while to figure out.

I'll bet that your data sheet says whats what.

Sounds like the LV inputs weren't "tolerant", but had classic ESD diodes.

James Arthur is using the dummy device to +5V, so high outputs can't lift that supply.

And the 3.3V parts are spec'd as 5V "tolerant", thus no classic diodes to VDD. ...Jim Thompson

I think there has to be some kind of ESD protection. Motor^H^H^HFreescale apparently use a "Thick field" n-channel on the inputs that turns on around 7-11V typically (gate and drain tied to input, source to Vss).

Would I _depend_ on that? hmm...

Best regards, Spehro Pefhany

it is in the datasheet for several xilinx parts. Some of the config pins are powered by 2.5V so to use a 3.3V device for configuration there should be series resistor to limit the input current to less than 10mA

And they also recommend you add resistor in parallel with the supply sinking the max combined current into the inputs

-Lasse

I don't know if this is what you were thinking of, but the Virtex-2 (I think it was -2) parts were *5V tolerant. When you looked for the '*' there was a little note stating that they were "5V tolerant" if you added a series limiting resistor. The inputs were clamped to 3.3V but you could use the clamp as long as you didn't exceed some current.

That sounds like it.

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I believe the first virtex had real 5V tolerance with a zener like esd diode to ground so it could do 5V pci

everything after that just says stay under Vcco+500mV or ~10mA continuous

snip

-Lasse