Schottky diodes have higher V drop than PN?

Jim Thompson · Aug 1, 2005

On Sun, 31 Jul 2005 23:08:00 -0500, "jake of center"
<nospam@invalid.com> wrote:

Schottkys are built differently, more doped layers.
None of the idiots on this form apparently know that.

This idiot on this form (sic) does.

Would you care to enlighten us as to the "more doped layers"?

Or are YOU the "doped" one ?

480)460-2350 | |
| E-mail Address at Website Fax

Jim Thompson · Aug 1, 2005

On Mon, 01 Aug 2005 07:31:55 -0700, Chris Carlen
<crcarleRemoveThis@BOGUSsandia.gov> wrote:

Jim Thompson wrote:
[snip]

Except that 'HC' parts don't use Schottky diodes for ESD ;-)

Jim, the debate is about which is better to put external to the HC
device, or other device. My LTSpice model uses 4148 style diodes
internally. I might not mind if you had any inside tips on the models
of HC input diodes that you could share with us?

Typical ESD for 'HC'... can vary from part-to-part and whether the
device can stay on a bus with the power off...

Series resistor from input pin to circuit and ESD

Upper diode is the body diode of a PMOS.

Lower "diode" is an NMOS device, source AND gate grounded, drain to
circuit-side of input resistor.

There is often some fancy additions to help prevent latch-up.

I choose in my LTSpice
model whether to use Schottky vs. PN external protection diodes
depending on what the goals are (speed, etc.). The model also allows
varying the ESD charge and voltage, various resistances, inductances, etc.

For a situation where the input comes from "off-card" an additional
series resistor is all that is generally needed.

The debate has now shifted to whether the BAT54 models available are any
good or not. The ones that I have tried predict much higher voltage
drops than 4148 at the large albeit brief surge currents generated
during an ESD event using the IEC ESD models.

Thanks for the input.

Good day!

480)460-2350 | |
| E-mail Address at Website Fax

John Larkin · Aug 1, 2005

On Sun, 31 Jul 2005 23:08:00 -0500, "jake of center"
<nospam@invalid.com> wrote:

Schottkys are built differently, more doped layers.

Fewer, actually.

None of the idiots on this form apparently know that.

Good, because it ain't true.

John

RST Engineering (jw) · Aug 1, 2005

No, but the idiots here know how to spell 'forum'.

Jim

None of the idiots on this form apparently know that.

Joerg · Aug 1, 2005

Hello Chris,

It's >200mA with the BAT54 model, which is of course still in question.
Odd, I get only about 57mA peak with the 4148.

I was just eyeballing the 70mA. But 57mA is equally dicey to jam into an
HC input.

For the external, 3.9k limits to <2A RMS for 1us with 15kV. Would that
be safe enough?

Maybe but with currents of this magnitude I'd pick a large diode pair.

So the question becomes, as the time duration of the surge current
becomes smaller, how much more current can be tolerated.

Since I work mostly in med there either is mfg data in writing on that
or I can't do it. Similar to aeronautics and other critical apps.

I have used the Littlefuse SP724 and similar devices as well. Perhaps
they are truly superior in applications like this rather than
roll-your-own ESD protection schemes.

Pretty expensive. It may be better to select a beefier pair of regular
diodes. Slim pickings though and companies like ON Semi do not list
current ratings on their parametric tables, for whatever reason.

Regards, Joerg

http://www.analogconsultants.com

Joerg · Aug 1, 2005

Hello Jim,

No, but the idiots here know how to spell 'forum'.

Depends on where they live. In the mountains it's sometimes pronounced
"furrum" ;-)

Regards, Joerg

http://www.analogconsultants.com

Chris Carlen · Aug 1, 2005

Joerg wrote:

Since I work mostly in med there either is mfg data in writing on that
or I can't do it. Similar to aeronautics and other critical apps.

I have used the Littlefuse SP724 and similar devices as well. Perhaps
they are truly superior in applications like this rather than
roll-your-own ESD protection schemes.

Pretty expensive.

But at least the datasheets specifically state it can tolerate the IEC
ESD standards.

This leads me to wonder, since so many semi datasheets state that the
devices are not intended for use in life-support systems, etc., how do
you medical equipment designers find any parts at all? Or do you make
special legal arrangements with the manufacturers to absolve them of any
liability then they'll sell you whatever you want?

Good day!

--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
crcarleRemoveThis@BOGUSsandia.gov
NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply.

Harry Dellamano · Aug 1, 2005

"Chris Carlen" <crobc@BOGUSFIELD.sbcglobal.net> wrote in message
news:dcjpm51mlj@news1.newsguy.com...

John Larkin wrote:
On Sun, 31 Jul 2005 12:16:30 -0700, Chris Carlen
crobc@BOGUSFIELD.sbcglobal.net> wrote:

The Fairchild BAT54 model also has more voltage drop above about 0.61A.

I hope you're talking pulsed current here. Fairchild specs theta at
430k/w.

John

It's about ESD.

See these:

http://web.newsguy.com/crcarl/esd-HC-input-pn-vs-Shottky.asc
http://web.newsguy.com/crcarl/diode-compare.asc

--
_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5

Hey Chris,
Funny, I'm using 74HC14 gates as input line receiver in an avionics job.
The inputs are from 28V/GND relays so I use a 100K input series protection
resistors and a lot of filtering. The customer does not believe it is rugged
enough to handle 32V inputs. I say +/- 2KV or until the 100K resistor smokes
but he's not buying it. What a life!

cheers,
Harry

Joerg · Aug 1, 2005

Hello Chris,

I have used the Littlefuse SP724 and similar devices as well.
Perhaps they are truly superior in applications like this rather than
roll-your-own ESD protection schemes.

Pretty expensive.

But at least the datasheets specifically state it can tolerate the IEC
ESD standards.

Well, yes, but there are less expensive ways (big diodes).

This leads me to wonder, since so many semi datasheets state that the
devices are not intended for use in life-support systems, etc., how do
you medical equipment designers find any parts at all? Or do you make
special legal arrangements with the manufacturers to absolve them of any
liability then they'll sell you whatever you want?

We don't do any life support designs without a waiver. Companies that do
often produce their own semiconductors. Seriously, I have seen a company
run their own transistor and logic chip production. They test the
dickens out of every single part. Of course, a 10c chip will then become
a $10 chip. Now you know why pacemakers are so freaking expensive ;-)

Regards, Joerg

http://www.analogconsultants.com

Joerg · Aug 2, 2005

Hello Harry,

Funny, I'm using 74HC14 gates as input line receiver in an avionics job.
The inputs are from 28V/GND relays so I use a 100K input series protection
resistors and a lot of filtering. The customer does not believe it is rugged
enough to handle 32V inputs. I say +/- 2KV or until the 100K resistor smokes
but he's not buying it. What a life!

If the customer doesn't know electronics that will require a lot of
s'plainin'. Just like my EMI sermons. Takes forever to sink in. It's
like talking to marketing folks although the results can be very
different there. The first time I had to do that in my career resulted
in marriage ;-)

Oh, and we are still married.

Regards, Joerg

http://www.analogconsultants.com

The Phantom · Aug 2, 2005

On Sun, 31 Jul 2005 17:16:27 -0700, Chris Carlen <crobc@BOGUSFIELD.sbcglobal.net> wrote:

<Snip>

That's why you should look at the LTSpice file I posted which models a
typical ESD scenario. It gives 9-10A surge currents through the diodes.
What can you do to protect from that except make some assumptions
about the surge capability of the diodes at very short pulse widths.

So the question becomes, as the time duration of the surge current
becomes smaller, how much more current can be tolerated.

I was given some very old Philips databook images for HC logic, which
showed a graph of this. Now I don't know where it got to.

With any sort of input resistance that still allows fast data rates, one
has little choice but to exceed the 1 second ratings of the diode.
Although at least for the MMBD4148, they give a 1us rating of 2A.

But by putting a resistor in between the external diodes and the
internal, as I said before, one can ensure that the chip's ratings
aren't exceeded.

I sure wish the diode datasheets included more detailed surge ratings,
such as a graph.

I looked at all the 1N4148 datasheets I have, and only one, the original GE one, shows
any information about surge capability. The GE sheet has a transient thermal impedance
curve, from which you can derive the surge capability. Shall I post it over on ABSE?

Good day!

Ban · Aug 2, 2005

Joerg wrote:

Hello Chris,

It's >200mA with the BAT54 model, which is of course still in
question. Odd, I get only about 57mA peak with the 4148.

I was just eyeballing the 70mA. But 57mA is equally dicey to jam into
an HC input.

For the external, 3.9k limits to <2A RMS for 1us with 15kV. Would
that be safe enough?

Maybe but with currents of this magnitude I'd pick a large diode pair.

So the question becomes, as the time duration of the surge current
becomes smaller, how much more current can be tolerated.

Since I work mostly in med there either is mfg data in writing on that
or I can't do it. Similar to aeronautics and other critical apps.

I have used the Littlefuse SP724 and similar devices as well. Perhaps
they are truly superior in applications like this rather than
roll-your-own ESD protection schemes.

Pretty expensive. It may be better to select a beefier pair of regular
diodes. Slim pickings though and companies like ON Semi do not list
current ratings on their parametric tables, for whatever reason.

Regards, Joerg

For audio balanced line inputs I use a 470R followed by a BAT720S to the
rails. There are 12VMOVs across the rails to absorb the injected current
before the 22V rated opamps get destroyed. I also put a 1n cap after the
470R, so the 120p of the combined diodes do not hurt in this application.
This way the semiconductors survive even an accidental mains on the input.
The 470R resistor acts like a fuse.
The BAT720S is 1A rated, cheap and in SOT223. 1A/460mV; 5A/ 0.9V

--
ciao Ban
Bordighera, Italy

Ban · Aug 2, 2005

Ban wrote:

For audio balanced line inputs I use a 470R followed by a BAT720S to
the rails. There are 12VMOVs across the rails to absorb the injected
current before the 22V rated opamps get destroyed. I also put a 1n
cap after the 470R, so the 120p of the combined diodes do not hurt in
this application. This way the semiconductors survive even an
accidental mains on the input. The 470R resistor acts like a fuse.
The BAT720S is 1A rated, cheap and in SOT223. 1A/460mV; 5A/ 0.9V

Sorry I mixed up the number
*BAT140S*

--
ciao Ban
Bordighera, Italy

RST Engineering (jw) · Aug 2, 2005

Joerg ...

I did the same thing, only mine was a forum after which this little Chicago
blonde and I "disagreed" on a technical point.

Today is our 8th anniversary.

Jim

The first time I had to do that in my career resulted

in marriage ;-)

Oh, and we are still married.

Regards, Joerg

Jim Thompson · Aug 2, 2005

On Tue, 2 Aug 2005 08:37:43 -0700, "RST Engineering $jw$"
<jim@rstengineering.com> wrote:

Joerg ...

I did the same thing, only mine was a forum after which this little Chicago
blonde and I "disagreed" on a technical point.

Today is our 8th anniversary.

Jim

The first time I had to do that in my career resulted
in marriage ;-)

Oh, and we are still married.

Regards, Joerg

480)460-2350 | |
| E-mail Address at Website Fax

Joerg · Aug 2, 2005

Hello Ban,

For audio balanced line inputs I use a 470R followed by a BAT720S to
the rails. There are 12VMOVs across the rails to absorb the injected
current before the 22V rated opamps get destroyed. I also put a 1n
cap after the 470R, so the 120p of the combined diodes do not hurt in
this application. This way the semiconductors survive even an
accidental mains on the input. The 470R resistor acts like a fuse.
The BAT720S is 1A rated, cheap and in SOT223. 1A/460mV; 5A/ 0.9V

Sorry I mixed up the number
*BAT140S*

That is a good diode but 45c isn't exactly cheap. I could only see it at

Philips (I wish they would stop that stupid stock quote wait on their
site, when do they learn?). Single sourced?

It does cave in around 2amps but that may be ok if there is another
resistor between the BAT140S and the device pin.

Regards, Joerg

http://www.analogconsultants.com

Harry Dellamano · Aug 2, 2005

"Jim Thompson" <thegreatone@example.com> wrote in message
news:kh5ve1lpccq5musltmafbjmmvoabag2olq@4ax.com...

On Tue, 2 Aug 2005 08:37:43 -0700, "RST Engineering $jw$"
jim@rstengineering.com> wrote:

Joerg ...

I did the same thing, only mine was a forum after which this little
Chicago
blonde and I "disagreed" on a technical point.

Today is our 8th anniversary.

Jim

The first time I had to do that in my career resulted
in marriage ;-)

Oh, and we are still married.

Regards, Joerg

REALLY bright women are the preferred embodiment !-)

...Jim Thompson
--
Either that or a good bar-be-que. Man can't have both.

Cheers,
Harry

Terry Given · Aug 2, 2005

jake of center wrote:

Schottkys are built differently, more doped layers.
None of the idiots on this form apparently know that.

ROTFLMAO!

Bwahahahahahaha

Cheers
Terry

Schottky diodes have higher V drop than PN?

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