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design | Are 5v-tolerant inputs clamped?


There are 44 messages in this thread.

You are currently looking at messages 1 to 10.

Are 5v-tolerant inputs clamped? - 2010-12-29 16:07:00

I need a bunch of inputs, so I'm using an i2c port expander.  The one
I've chosen runs on 3.3v, but tolerates 5v inputs.  I don't see any
documentation on the operation of the inputs.

I'm trying to avoid wasting 32 clamp diodes if I don't need them.

Q: Generally speaking, if I feed a 2mA current-limited inductive
glitch into a +5v-tolerant input, Vdd=3.3v, is the glitch voltage
clamped by the device input?  At what voltage?  Most importantly, is
the device damaged?

The datasheet says inputs can handle +6v max and +/-20mA per input.

If needed I guess I could use a dummy logic device as a clamp-diode
array.  Hmmm.

Thoughts?

TIA,

James Arthur

Re: Are 5v-tolerant inputs clamped? - Jim Thompson - 2010-12-29 16:10:00

On Wed, 29 Dec 2010 13:07:31 -0800 (PST), d...@yahoo.com
wrote:

>I need a bunch of inputs, so I'm using an i2c port expander.  The one
>I've chosen runs on 3.3v, but tolerates 5v inputs.  I don't see any
>documentation on the operation of the inputs.
>
>I'm trying to avoid wasting 32 clamp diodes if I don't need them.
>
>Q: Generally speaking, if I feed a 2mA current-limited inductive
>glitch into a +5v-tolerant input, Vdd=3.3v, is the glitch voltage
>clamped by the device input?  At what voltage?  Most importantly, is
>the device damaged?
>
>The datasheet says inputs can handle +6v max and +/-20mA per input.
>
>If needed I guess I could use a dummy logic device as a clamp-diode
>array.  Hmmm.
>
>Thoughts?
>
>TIA,
>
>James Arthur

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
-- 
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

Re: Are 5v-tolerant inputs clamped? - 2010-12-29 16:39:00

On Dec 29, 4:10 pm, Jim Thompson <To-Email-Use-The-Envelope-I...@On-My-
Web-Site.com> wrote:
> On Wed, 29 Dec 2010 13:07:31 -0800 (PST), dagmargoodb...@yahoo.com
> wrote:
>
>
>
> >I need a bunch of inputs, so I'm using an i2c port expander.  The one
> >I've chosen runs on 3.3v, but tolerates 5v inputs.  I don't see any
> >documentation on the operation of the inputs.
>
> >I'm trying to avoid wasting 32 clamp diodes if I don't need them.
>
> >Q: Generally speaking, if I feed a 2mA current-limited inductive
> >glitch into a +5v-tolerant input, Vdd=3.3v, is the glitch voltage
> >clamped by the device input?  At what voltage?  Most importantly, is
> >the device damaged?
>
> >The datasheet says inputs can handle +6v max and +/-20mA per input.
>
> >If needed I guess I could use a dummy logic device as a clamp-diode
> >array.  Hmmm.
>
> >Thoughts?
>
> >TIA,
>
>
> As far as I know, "5V-tolerant" inputs are NOT clamped.  What would
> they be clamped to?

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.

>  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.

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

Re: Are 5v-tolerant inputs clamped? - 2010-12-29 16:43:00

On Dec 29, 4:39 pm, dagmargoodb...@yahoo.com wrote:
> On Dec 29, 4:10 pm, Jim Thompson wrote:

> > As far as I know, "5V-tolerant" inputs are NOT clamped.  What would
> > they be clamped to?
>
> 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.
>
> >  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.
>
> 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

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

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


--
Cheers,
James Arthur


Re: Are 5v-tolerant inputs clamped? - langwadt@fonz.dk - 2010-12-29 16:46:00

On 29 Dec., 22:10, Jim Thompson <To-Email-Use-The-Envelope-I...@On-My-
Web-Site.com> wrote:
> On Wed, 29 Dec 2010 13:07:31 -0800 (PST), dagmargoodb...@yahoo.com
> wrote:
>
>
>
> >I need a bunch of inputs, so I'm using an i2c port expander.  The one
> >I've chosen runs on 3.3v, but tolerates 5v inputs.  I don't see any
> >documentation on the operation of the inputs.
>
> >I'm trying to avoid wasting 32 clamp diodes if I don't need them.
>
> >Q: Generally speaking, if I feed a 2mA current-limited inductive
> >glitch into a +5v-tolerant input, Vdd=3.3v, is the glitch voltage
> >clamped by the device input?  At what voltage?  Most importantly, is
> >the device damaged?
>
> >The datasheet says inputs can handle +6v max and +/-20mA per input.
>
> >If needed I guess I could use a dummy logic device as a clamp-diode
> >array.  Hmmm.
>
> >Thoughts?
>
> >TIA,
>
> >James Arthur
>
> 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.
>

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
10mA

-Lasse

Re: Are 5v-tolerant inputs clamped? - Jim Thompson - 2010-12-29 16:51:00

On Wed, 29 Dec 2010 13:39:01 -0800 (PST), d...@yahoo.com
wrote:

>On Dec 29, 4:10 pm, Jim Thompson <To-Email-Use-The-Envelope-I...@On-My-
>Web-Site.com> wrote:
[snip]
>>
>> As far as I know, "5V-tolerant" inputs are NOT clamped.  What would
>> they be clamped to?
>
>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.
>
>>  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.
>
>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.

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
-- 
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

Re: Are 5v-tolerant inputs clamped? - Jim Thompson - 2010-12-29 16:52:00

On Wed, 29 Dec 2010 13:43:03 -0800 (PST), d...@yahoo.com
wrote:

>On Dec 29, 4:39 pm, dagmargoodb...@yahoo.com wrote:
>> On Dec 29, 4:10 pm, Jim Thompson wrote:
>
>> > As far as I know, "5V-tolerant" inputs are NOT clamped.  What would
>> > they be clamped to?
>>
>> 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.
>>
>> >  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.
>>
>> 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
>
>Oops---------------------------^^^^^^
>
>Make that 16 clamps--the enables have to be hardwired '1'.

I _was_ going to get the data sheet and count ;-)
		
                                        ...Jim Thompson
-- 
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

Re: Are 5v-tolerant inputs clamped? - kevin93 - 2010-12-29 16:54:00

On Dec 29, 1:39 pm, dagmargoodb...@yahoo.com wrote:
> On Dec 29, 4:10 pm, Jim Thompson <To-Email-Use-The-Envelope-I...@On-My-
>
>
>
>
>
>
>
>
>
> Web-Site.com> wrote:
> > On Wed, 29 Dec 2010 13:07:31 -0800 (PST), dagmargoodb...@yahoo.com
> > wrote:
>
> > >I need a bunch of inputs, so I'm using an i2c port expander.  The one
> > >I've chosen runs on 3.3v, but tolerates 5v inputs.  I don't see any
> > >documentation on the operation of the inputs.
>
> > >I'm trying to avoid wasting 32 clamp diodes if I don't need them.
>
> > >Q: Generally speaking, if I feed a 2mA current-limited inductive
> > >glitch into a +5v-tolerant input, Vdd=3.3v, is the glitch voltage
> > >clamped by the device input?  At what voltage?  Most importantly, is
> > >the device damaged?
>
> > >The datasheet says inputs can handle +6v max and +/-20mA per input.
>
> > >If needed I guess I could use a dummy logic device as a clamp-diode
> > >array.  Hmmm.
>
> > >Thoughts?
>
> > >TIA,
>
> > As far as I know, "5V-tolerant" inputs are NOT clamped.  What would
> > they be clamped to?
>
> 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.
>
> >  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.
>
> 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

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

Re: Are 5v-tolerant inputs clamped? - 2010-12-29 17:04:00

On Dec 29, 4:51 pm, Jim Thompson <To-Email-Use-The-Envelope-I...@On-My-
Web-Site.com> wrote:
> On Wed, 29 Dec 2010 13:39:01 -0800 (PST), dagmargoodb...@yahoo.com
> wrote:
>
>
>
>
>
> >On Dec 29, 4:10 pm, Jim Thompson <To-Email-Use-The-Envelope-I...@On-My-
> >Web-Site.com> wrote:
> [snip]
>
> >> As far as I know, "5V-tolerant" inputs are NOT clamped. What would
> >> they be clamped to?
>
> >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.
>
> >> 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.
>
> >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.
>
> 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.

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

Re: Are 5v-tolerant inputs clamped? - 2010-12-29 17:12:00

On Dec 29, 4:46 pm, "langw...@fonz.dk" <langw...@fonz.dk> wrote:
> On 29 Dec., 22:10, Jim Thompson <To-Email-Use-The-Envelope-I...@On-My-
>
>
>
> Web-Site.com> wrote:
> > On Wed, 29 Dec 2010 13:07:31 -0800 (PST), dagmargoodb...@yahoo.com
> > wrote:
>
> > >I need a bunch of inputs, so I'm using an i2c port expander.  The one
> > >I've chosen runs on 3.3v, but tolerates 5v inputs.  I don't see any
> > >documentation on the operation of the inputs.
>
> > >I'm trying to avoid wasting 32 clamp diodes if I don't need them.
>
> > >Q: Generally speaking, if I feed a 2mA current-limited inductive
> > >glitch into a +5v-tolerant input, Vdd=3.3v, is the glitch voltage
> > >clamped by the device input?  At what voltage?  Most importantly, is
> > >the device damaged?
>
> > >The datasheet says inputs can handle +6v max and +/-20mA per input.
>
> > >If needed I guess I could use a dummy logic device as a clamp-diode
> > >array.  Hmmm.
>
> > >Thoughts?
>
> > >TIA,
>
> > >James Arthur
>
> > 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.
>
> 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
> 10mA
>
> -Lasse

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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