Are 5v-tolerant inputs clamped?

Started by December 29, 2010
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
On Wed, 29 Dec 2010 13:07:31 -0800 (PST), dagmargoodboat@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.
On Dec 29, 4:10=A0pm, 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. =A0The one > >I've chosen runs on 3.3v, but tolerates 5v inputs. =A0I 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=3D3.3v, is the glitch voltage > >clamped by the device input? =A0At what voltage? =A0Most 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. =A0Hmmm. > > >Thoughts? > > >TIA, > > > As far as I know, "5V-tolerant" inputs are NOT clamped. =A0What 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.
> =A0There's no 5V VDD on the chip to strap a diode > to. =A0All 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 Dec 29, 4:39=A0pm, dagmargoodb...@yahoo.com wrote:
> On Dec 29, 4:10=A0pm, Jim Thompson wrote:
> > As far as I know, "5V-tolerant" inputs are NOT clamped. =A0What would > > they be clamped to? > > Some protected MOSFETs clamp overvoltages by incorporating a zener > from drain-to-gate. =A0I didn't expect zeners en masse in a digital > part, but possibly some equivalent function. > > > =A0There's no 5V VDD on the chip to strap a diode > > to. =A0All that is there is a so-called "snap-diode" (to ground) for ES=
D
> > pulses. > > Yeah, crap. =A0Reading 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. =A0An 'HC244 > (outputs disabled) gives me 18 clamps to Vdd and Vss. =A0Does that
Oops---------------------------^^^^^^ Make that 16 clamps--the enables have to be hardwired '1'. -- Cheers, James Arthur
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. =A0The one > >I've chosen runs on 3.3v, but tolerates 5v inputs. =A0I 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=3D3.3v, is the glitch voltage > >clamped by the device input? =A0At what voltage? =A0Most 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. =A0Hmmm. > > >Thoughts? > > >TIA, > > >James Arthur > > As far as I know, "5V-tolerant" inputs are NOT clamped. =A0What would > they be clamped to? =A0There's no 5V VDD on the chip to strap a diode > to. =A0All 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
On Wed, 29 Dec 2010 13:39:01 -0800 (PST), dagmargoodboat@yahoo.com
wrote:

>On Dec 29, 4:10&#2013266080;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. &#2013266080;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. > >> &#2013266080;There's no 5V VDD on the chip to strap a diode >> to. &#2013266080;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.
On Wed, 29 Dec 2010 13:43:03 -0800 (PST), dagmargoodboat@yahoo.com
wrote:

>On Dec 29, 4:39&#2013266080;pm, dagmargoodb...@yahoo.com wrote: >> On Dec 29, 4:10&#2013266080;pm, Jim Thompson wrote: > >> > As far as I know, "5V-tolerant" inputs are NOT clamped. &#2013266080;What
would
>> > they be clamped to? >> >> Some protected MOSFETs clamp overvoltages by incorporating a zener >> from drain-to-gate. &#2013266080;I didn't expect zeners en masse in a digital >> part, but possibly some equivalent function. >> >> > &#2013266080;There's no 5V VDD on the chip to strap a diode >> > to. &#2013266080;All that is there is a so-called "snap-diode" (to ground) for
ESD
>> > pulses. >> >> Yeah, crap. &#2013266080;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. &#2013266080;An 'HC244 >> (outputs disabled) gives me 18 clamps to Vdd and Vss. &#2013266080;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.
On Dec 29, 1:39=A0pm, dagmargoodb...@yahoo.com wrote:
> On Dec 29, 4:10=A0pm, 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. =A0The on=
e
> > >I've chosen runs on 3.3v, but tolerates 5v inputs. =A0I 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=3D3.3v, is the glitch voltage > > >clamped by the device input? =A0At what voltage? =A0Most 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. =A0Hmmm. > > > >Thoughts? > > > >TIA, > > > As far as I know, "5V-tolerant" inputs are NOT clamped. =A0What would > > they be clamped to? > > Some protected MOSFETs clamp overvoltages by incorporating a zener > from drain-to-gate. =A0I didn't expect zeners en masse in a digital > part, but possibly some equivalent function. > > > =A0There's no 5V VDD on the chip to strap a diode > > to. =A0All that is there is a so-called "snap-diode" (to ground) for ES=
D
> > pulses. > > Yeah, crap. =A0Reading 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. =A0An 'HC244 > (outputs disabled) gives me 18 clamps to Vdd and Vss. =A0Does that > horrify you? :-) =A0It'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
On Dec 29, 4:51=A0pm, 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. =A0I 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. =A0Reading 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. =A0An 'HC244 > >(outputs disabled) gives me 18 clamps to Vdd and Vss. =A0Does that > >horrify you? :-) =A0It'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
On Dec 29, 4:46=A0pm, "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. =A0The on=
e
> > >I've chosen runs on 3.3v, but tolerates 5v inputs. =A0I 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=3D3.3v, is the glitch voltage > > >clamped by the device input? =A0At what voltage? =A0Most 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. =A0Hmmm. > > > >Thoughts? > > > >TIA, > > > >James Arthur > > > As far as I know, "5V-tolerant" inputs are NOT clamped. =A0What would > > they be clamped to? =A0There's no 5V VDD on the chip to strap a diode > > to. =A0All that is there is a so-called "snap-diode" (to ground) for ES=
D
> > pulses. > > seem to =A0remember 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