Lets talk about input protection against ESD (human body).

[Ge0rge Marutz]

I have a question about modeling the level of ESD that might affect my
module and what type of low cost protection schemes might be used to
harden it.

My product is going to be an automotive assembly that has a remote push
button interface to a microprocessor controlled "black box" stashed in
the engine compartment. The push button can be whatever type my
particular customer decides to use in their vehicle for that model /
model year. It just needs to provide a momentary closure to ground to
activate my product.

1st question: How do I go about judging the level of static discharge
that can come from a persons finger, through the switch body, and onto
my wiring, when pushing this button? I assume the button will be in a
plastic housing with a plastic button surface. I know this will be
impossible for you to solve for me without knowing any details about
the switch. I don't even have details about the switch yet. But, what
steps do I need to go through once I do?

I do not have the equipment for testing in-house so need to do
everything on paper to the best of my ability first. I can then pay to
send a design out to a test lab. I'm trying to design this right up
front so I can minimize trial and error experimentation at an outside
lab. This takes time and money I don't have.

Another question:

Right now I have two capacitors in parallel connected between the
switch input and ground to help with EMC. One is a 1000pF, 0603, 100V
device and the other is 0.1uF, 1206, 100V. Supposedly these can also
help suppress ESD. However, I read an article saying that ESD pulses
will kill small multilayer chip caps and are not the best to use in
this application. Is there any merit to this statement? The article
was written by a guy trying to sell ESD suppression devices.

This same input also uses a thick film chip resistor (10K, 0603) / 5.1V
Zener clamp to limit input voltage to a level safe for a general
purpose micro input. On paper this appears to be fine. The resistor
will limit surge current to a safe level that can be handled by the
Zener. The zener clamps the voltage to safe levels.

Now, I am not familiar with high voltage design. I tinker with 12V.
Under some circumstances I need to worry about fast transients up to
300V. I'm under the impression that when you start to enter the KV
range things change considerably. You need to start worrying about
voltage arcing between stuff. Will my clamping scheme work or can high
voltage spikes from a ESD pulse jump across my 0603 current limiting
resistor a wreak havoc on my micro? All traces and land patterns on
the board are surrounded by solid copper ground plane with 10mil
spacing.

Last question: What methosd are used to controll ESD? I tried a spark
gap at one point but it was not possible to implement in copper on a
PCB with any sort of consistency or reliability. I am considering
trying a 0805 SMD package Multilayer Varistor but have no design
experience with these. I don't know how to properly specify the right
device for my application. I also don't know if these are a low cost
solution of if something else better exists.

What I said above may not sound right and/or may be full of errors in
logoc. Well, I'm trying to learn more about this topic so please allow
me a little leeway. The converstation will gell into something less
convoluted once I become more educated on the subject.

Thank you for any help you may offer.

Ge0rge

 

[Guy Macon]

Ge0rge Marutz wrote:

I have a question about modeling the level of ESD that might affect my
module and what type of low cost protection schemes might be used to
harden it.

My product is going to be an automotive assembly that has a remote push
button interface to a microprocessor controlled "black box" stashed in
the engine compartment. The push button can be whatever type my
particular customer decides to use in their vehicle for that model /
model year. It just needs to provide a momentary closure to ground to
activate my product.

1st question: How do I go about judging the level of static discharge
that can come from a persons finger, through the switch body, and onto
my wiring, when pushing this button? I assume the button will be in a
plastic housing with a plastic button surface. I know this will be
impossible for you to solve for me without knowing any details about
the switch. I don't even have details about the switch yet. But, what
steps do I need to go through once I do?

I do not have the equipment for testing in-house so need to do
everything on paper to the best of my ability first. I can then pay to
send a design out to a test lab. I'm trying to design this right up
front so I can minimize trial and error experimentation at an outside
lab. This takes time and money I don't have.

Here are some references that may help you in deciding which standard
to design for and test to.

http://www.esda.org/basics/part5.cfm
http://www.st.com/stonline/books/pdf/docs/6970.pdf
http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=guide_esd_005
http://standards.ieee.org/reading/ieee/std_public/description/emc/C63.16-1993_d
esc.html
http://www.semiconfareast.com/esdspecs.htm
http://www.semiconfareast.com/esdspecs2.htm

 

[Wolfgang Kemper]

Guy Macon wrote:

Ge0rge Marutz wrote:


I have a question about modeling the level of ESD that might affect my
module and what type of low cost protection schemes might be used to
harden it.

My product is going to be an automotive assembly that has a remote push
button interface to a microprocessor controlled "black box" stashed in
the engine compartment. The push button can be whatever type my
particular customer decides to use in their vehicle for that model /
model year. It just needs to provide a momentary closure to ground to
activate my product.

1st question: How do I go about judging the level of static discharge
that can come from a persons finger, through the switch body, and onto
my wiring, when pushing this button? I assume the button will be in a
plastic housing with a plastic button surface. I know this will be
impossible for you to solve for me without knowing any details about
the switch. I don't even have details about the switch yet. But, what
steps do I need to go through once I do?

I do not have the equipment for testing in-house so need to do
everything on paper to the best of my ability first. I can then pay to
send a design out to a test lab. I'm trying to design this right up
front so I can minimize trial and error experimentation at an outside
lab. This takes time and money I don't have.


Here are some references that may help you in deciding which standard
to design for and test to.

http://www.esda.org/basics/part5.cfm
http://www.st.com/stonline/books/pdf/docs/6970.pdf
http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=guide_esd_005
http://standards.ieee.org/reading/ieee/std_public/description/emc/C63.16-1993_d
esc.html
http://www.semiconfareast.com/esdspecs.htm
http://www.semiconfareast.com/esdspecs2.htm


Hi I am a ESD engineer dealing with ESD on IC level.

Please do not mix IC level and PCB level ESD.
PCB / Application level ESD is described in IEC 61000-4-2, which is
almost a worldwide standard now.
The Standards in the links above refers to IC level test which are not
applicable to board design.

I have only minor knowledge in PCB protection but I know that you could
add external ESD protections (Zener-clamps).

I guess the way of prevention is not that different at IC level and PCB
level.
Get the current dumped to GND , do not be scared about the 30A ESD
current during an ESD event. It is just for a short time (ns-us) try to
minimize the voltage drop due to parasitic resistors, this could cause
much more trouble.
And over all a mistake that all ESD "freshmen" do: Your device will
never,ever see thousands of Volts!!! (Except you done something terrible
wrong) It is just the capacitance inside the ESD gun which is charged to
this level, ESD is a current problem!
You may face high voltages if you force the ESD amps through some
resistance but this is what I mean with doing something terrible wrong.

..

 

[Joerg]

Hello Wolfgang,

I have only minor knowledge in PCB protection but I know that you could
add external ESD protections (Zener-clamps).

Zeners are not widely used here. The usual trick of the trade is to
employ double diodes a la BAV99 between IC pin and GND on one side, VCC
on the other. Then a few 10k of resistance up front from pin to where
the ESD gun can hit.

A BAV99 is a silicon diode so it has about the same U/I properties as
the internal substrate diodes, meaning that the substrate diodes still
share a large burden which could lead to latch-up or other grief. If
that's a problem there are Schottkys such as the BAT54S. However, two
compromises here. More leakage,plus they may cost an additional penny at
large quantities. But with Schottkys the lion's share of the ESD related
current pulse will flow through the external diodes because they start
their current ramp much earlier than the substrate diodes.

Regards, Joerg

http://www.analogconsultants.com

 

[Chris Carlen]

Joerg wrote:

Hello Wolfgang,

I have only minor knowledge in PCB protection but I know that you
could add external ESD protections (Zener-clamps).

Zeners are not widely used here. The usual trick of the trade is to
employ double diodes a la BAV99 between IC pin and GND on one side, VCC
on the other. Then a few 10k of resistance up front from pin to where
the ESD gun can hit.

A BAV99 is a silicon diode so it has about the same U/I properties as
the internal substrate diodes, meaning that the substrate diodes still
share a large burden which could lead to latch-up or other grief. If
that's a problem there are Schottkys such as the BAT54S. However, two
compromises here. More leakage,plus they may cost an additional penny at
large quantities. But with Schottkys the lion's share of the ESD related
current pulse will flow through the external diodes because they start
their current ramp much earlier than the substrate diodes.

Regards, Joerg

http://www.analogconsultants.com

I haven't tried actual measurements, since this sort of stuff is not
easy to measure. But I compared the IV curves for BAT54 vs. MMBD4148,
and realized that at higher currents, the voltage drop through the
Schottky exceeds that of the PN.

This was confirmed by my LTSpice models of ESD input circuits for HC logic.

I assume here that the input diodes look similar to 4148, but it is
possible they are little lighter. In any case, I look to meet the IEC
standards while also not exceeding datasheet specs even for static
overload conditions.

That way I make few assumptions about the surge capabilities of the
input diodes.

Check out this comparison of PN vs. Schottky (you will need LTSpice to
use this file):


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

Since I did this I use MMBD4148 for input protections instead of BAT54
anymore.


Good day!

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

 

[Joerg]

Hello Chris,

I haven't tried actual measurements, since this sort of stuff is not
easy to measure. But I compared the IV curves for BAT54 vs. MMBD4148,
and realized that at higher currents, the voltage drop through the
Schottky exceeds that of the PN.

Can't agreed there. Ok, I only have the data sheet of the MMSD4148 but
it should be pretty similar. At 100mA the BAT54 sits at 450mV. At that
same current the 4148 is already above 700mV, meaning the substrate
diodes are beginning to get punched.

This was confirmed by my LTSpice models of ESD input circuits for HC logic.

I assume here that the input diodes look similar to 4148, but it is
possible they are little lighter. In any case, I look to meet the IEC
standards while also not exceeding datasheet specs even for static
overload conditions.

That way I make few assumptions about the surge capabilities of the
input diodes.

Check out this comparison of PN vs. Schottky (you will need LTSpice to
use this file):

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

Since I did this I use MMBD4148 for input protections instead of BAT54
anymore.

I don't have LTSpice on this PC but are you sure the models are ok?

Regards, Joerg

http://www.analogconsultants.com

 

[Chris Carlen]

Joerg wrote:

Hello Chris,

I haven't tried actual measurements, since this sort of stuff is not
easy to measure. But I compared the IV curves for BAT54 vs. MMBD4148,
and realized that at higher currents, the voltage drop through the
Schottky exceeds that of the PN.

Can't agreed there. Ok, I only have the data sheet of the MMSD4148 but
it should be pretty similar. At 100mA the BAT54 sits at 450mV. At that
same current the 4148 is already above 700mV, meaning the substrate
diodes are beginning to get punched.

This was confirmed by my LTSpice models of ESD input circuits for HC
logic.

I assume here that the input diodes look similar to 4148, but it is
possible they are little lighter. In any case, I look to meet the IEC
standards while also not exceeding datasheet specs even for static
overload conditions.

That way I make few assumptions about the surge capabilities of the
input diodes.

Check out this comparison of PN vs. Schottky (you will need LTSpice to
use this file):

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

Since I did this I use MMBD4148 for input protections instead of BAT54
anymore.

I don't have LTSpice on this PC but are you sure the models are ok?

Hmm, actually the model is the main question mark. I just did a DC
sweep. There seems to be something wrong with the BAT54 model.

Here's a sim comparing 3 diodes including Fairchild's model parameters:

http://web.newsguy.com/crcarl/diode-compare.asc

Hmm, their BAT54 also has more voltage drop at high currents, but the
crossover isn't the same as the LTSpice built0in model.

Ok, anyway whatever the case, my ESD input protections use a resistor of
about 100-220R between my external protection diodes, and before the
device pin. That way the external protection diodes even if they drop a
couple volts (I get a 10A, 6.4V spike across my diode with the 15kV IEC
1000-4-2 conditions) then the internal device diodes can be kept to
within absolute maximum ratings.


I started another thread about this:

"Schottky diodes have higher V drop than PN?"

Maybe you can add some comments there too.


Good day!


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

 

[Ge0rge Marutz]

Sorry about my absence from the conversation over the last few days. I
had another pressing issue to attend to. It appears the conversation
is buzzing along without me. All of you bring up interesting points.

I don't know how to do that fancy ASCII art so I decided to post an
image of my circuits input structure. Please visit the following URL
and critique the design.

[img:89f3f34d10]http://img166.imageshack.us/img166/6913/oemswitchinputmodel1ne.jpg[/img:89f3f34d10]

I'd like to know how robust you believe this will be. Will it stand up
to a 8KV or higher direct contact discharge using the standard ESD gun?

 

[Joerg]

Hello George,

I'd like to know how robust you believe this will be. Will it stand up
to a 8KV or higher direct contact discharge using the standard ESD gun?

What's the reason for pulling against +12V where your logic runs on 5V?
Also, I don't trust zeners for this application. Too much tolerance and
they cost too much.

If ESD is serious I'd take the circuit that Chris had suggested
somewhere. Basically a large resistor value like 100K from the connector
to the center of antiserial diodes, such as the BAV99. The diode clamps
to GND and +5V. Then from that center point to the HC chip with another
resistor which could be lower (maybe 1K).

You could also pull up with about 1M if needed (but to +5V, not +12V). A
cap from the center of the diode pair to GND can slow down transients.
However, then your HC should be a Schmitt input.

Also, keep in mind that physically small resistors might arc over when
hit with a few kV. I have seen one turn from the usual off-white ceramic
to green bubbly glass. You may have to place several in series.

Regards, Joerg

http://www.analogconsultants.com

 

[Ge0rge Marutz]

"What's the reason for pulling against +12V where your logic runs on
5V?"

This is not so straight forward. I'm using a low dropout linear
voltage regulator. I'm using a limited amount of capacitance at its
output to conserve board space. If I draw too much current from the
regulator with a low amount of output capacitance the regulator can
become unstable. I need to keep branch current at 12mA to 20mA
(depending on customer requirement) to act as wetting current for
in-line connector pins and switch contacts. The proper amount of
wetting current helps keep contacts clean, and therefore low
resistance.

"If ESD is serious I'd take the circuit that Chris had suggested
somewhere. Basically a large resistor value like 100K from the
connector to the center of antiserial diodes, such as the BAV99. The
diode clamps
to GND and +5V. Then from that center point to the HC chip with another
resistor which could be lower (maybe 1K)."

I'm considering doing this with other general purpose 5V logic inputs.
Although I'll probably use 10K and 1K resistance.

However, for reasons listed about, I can't do this with my switch
input. Branch current would be too low for proper wetting.

A question about using regular PN double diode clamps to protect the
input to my micro. Something that was mentioned earlier in the thread.
Say I had a 10K resistance limiting current to a BAV99 double diode
clamp. Then 1K limiting current to the micro pin after that. The
micro's internal clamps are Schottky barrier diodes. Thier forward
voltage drop is much less than the BAV99's. Wouldn't they start
conducting first and soak up the transient? Shouldn't I use an
external Schottky clamp instead of a standard PN part like the Bav99?

"Also, keep in mind that physically small resistors might arc over when

hit with a few kV. I have seen one turn from the usual off-white
ceramic
to green bubbly glass. You may have to place several in series."

Something Wolfgang Kemper mentioned earlier. ESD is a current based
phenominon, not voltage. That is as long as you keep the series
resistance of this current path to a minimum. If you place a 100K
resistance in series, the resulting voltage drop will be substantial,
possibly enough to arc. Now, what about the caps I have right at the
input pins? Will these absorb some of the transient current so the
injection current into the proposed series input resistor can be
limited, therefore lowering the voltage drop across the chip resistor?

Trying to picture exactly what happens.

Thank you so much.

Gerb

 

[Joerg]

Hello George,

"What's the reason for pulling against +12V where your logic runs on
5V?"

This is not so straight forward. I'm using a low dropout linear
voltage regulator. I'm using a limited amount of capacitance at its
output to conserve board space. If I draw too much current from the
regulator with a low amount of output capacitance the regulator can
become unstable. I need to keep branch current at 12mA to 20mA
(depending on customer requirement) to act as wetting current for
in-line connector pins and switch contacts. The proper amount of
wetting current helps keep contacts clean, and therefore low
resistance.

Maybe use a switcher instead of the LDO?

You could apply the wetting current right to the connector and from a
source that has enough caps to weather the ESD surge. And a resistor
that doesn't fry from the 8kV. Then go from there with 100K -> double
diode -> 1k -> HC logic.

I'm considering doing this with other general purpose 5V logic inputs.
Although I'll probably use 10K and 1K resistance.

10k is a bit of a stretch for a BAV99 and an 8kV ESD pulse.

A question about using regular PN double diode clamps to protect the
input to my micro. Something that was mentioned earlier in the thread.
Say I had a 10K resistance limiting current to a BAV99 double diode
clamp. Then 1K limiting current to the micro pin after that. The
micro's internal clamps are Schottky barrier diodes. Thier forward
voltage drop is much less than the BAV99's. Wouldn't they start
conducting first and soak up the transient? Shouldn't I use an
external Schottky clamp instead of a standard PN part like the Bav99?

HC behaves like normal diodes. Anyway, the BAV99 will have cut it down
to, say, a volt or two beyond the rails when pushed hard (depends on
what resistor is in front). Now there will be less than 2V across the 1k
resistor which results in a small current surge that HC can easily tolerate.

Something Wolfgang Kemper mentioned earlier. ESD is a current based
phenominon, not voltage. That is as long as you keep the series
resistance of this current path to a minimum. If you place a 100K
resistance in series, the resulting voltage drop will be substantial,
possibly enough to arc. Now, what about the caps I have right at the
input pins? Will these absorb some of the transient current so the
injection current into the proposed series input resistor can be
limited, therefore lowering the voltage drop across the chip resistor?

It's still a voltage thing when an external cap however small is charged
to 8kV and then discharged. Input caps directly across an input may not
be so good since they could fail. A resistor may see an initial 8kV
across its body. ESD tests like this are often degrading component
performance over the number of tests conducted on the same circuit.

It might be worth to invest in a book about ESD. Can't give you a hint
here since I kind of grew up with this stuff and don't have a book. In
our case it's medical where Hipot and defibrillator testing are done. A
defibrillator test is nothing to sneeze at. 5kV roaring down an inductor
bank from a 32uF cap can literally disintegrate things. Once the TUEV
guy wanted to see it live. When I was ready to press the big button he
moved his chair several feet back ;-)

Regards, Joerg

http://www.analogconsultants.com

 

[Ge0rge Marutz]

"Maybe use a switcher instead of the LDO?"

The word "switcher" is taboo in the greater Detroit area automotive
electrical design community. EMC nightmares... Also, a switcher
sounds expensive compared to my $0.17 iron clad vreg.

"You could apply the wetting current right to the connector and from a
source that has enough caps to weather the ESD surge."

Could you please re-phrase this thought? I'm having a hard time
following you. Are you talking about adding a second 5V source to
provide the pull-up for my external switch? Why not use 12V, its free?

"And a resistor that doesn't fry from the 8kV."

OK, are we talking about peak pulse power limitations or adjusting
package type to prevent arcing? Remember, I'm not real familiar with
what happens above 100V.

"HC behaves like normal diodes."

That is what I always thought. However, Freescale Semi insists that
their internal clamps behave like Shottky barrier diodes and have a
lower Vf.

Please look at this updated drawing. I added some more detail about
the 12V side of the circuit to see how ESD transient current might
affect things. I also changed the uC pin protection to something more
along the lines of what we have been talking about. As long as the
input impedance of the micro is >> than the 100k series resistance I
should be OK.

http://img235.imageshack.us/img235/2200/proposedswitchinputmodel9rq.jpg

Thanks a bunch,

Ge0rge

 

[Joerg]

Hello George,

"Maybe use a switcher instead of the LDO?"

The word "switcher" is taboo in the greater Detroit area automotive
electrical design community. EMC nightmares... Also, a switcher
sounds expensive compared to my $0.17 iron clad vreg.

At which qties? 17c is a lot of money if you are talking about a few
million units a year. Also, make sure that whatever LDO you are using
will not go on allocation. Or at least that your company is high up in
the pecking order should that happen. And check it out for stability
inside out. These things can be rather fickle.

I'd have reliability concerns with any heat dissipating semis in cars.
EMC nightmares? No pun intended but maybe they should worry a little
more about the keyless entry systems or ignitions computers ;-)

I remember someone keying an FM radio mike and a little car next to them
slowed down everytime they did that. They could render it unable to pass
them.

"You could apply the wetting current right to the connector and from a
source that has enough caps to weather the ESD surge."

Could you please re-phrase this thought? I'm having a hard time
following you. Are you talking about adding a second 5V source to
provide the pull-up for my external switch? Why not use 12V, its free?

Ok, you can use 12V if it's before the 100K like you have in your
drawing. But when the switch is open you'll see 7mA or so flowing at all
times, more if the switch is pressed, of course. If the circuit is
powered down when not in use that may be ok. Else the valuable customer
sits there one winter day in his fancy vehicle and the engine won't turn
over :-(

"And a resistor that doesn't fry from the 8kV."

OK, are we talking about peak pulse power limitations or adjusting
package type to prevent arcing? Remember, I'm not real familiar with
what happens above 100V.

That is something you need to talk about with the resistor manufacturer.
They are all different.

"HC behaves like normal diodes."

That is what I always thought. However, Freescale Semi insists that
their internal clamps behave like Shottky barrier diodes and have a
lower Vf.

Freescale? HC parts? Mine are spec'd from ONSemi, not Freescale. I'd be
surprised if they sold any. From a specsmanship point of view (familiy
specs, ESD etc.) Philips is really good but their web site is a pain.

Anyway, HC is a regular plain clothes CMOS process. Look at the family
specs.

Please look at this updated drawing. I added some more detail about
the 12V side of the circuit to see how ESD transient current might
affect things. I also changed the uC pin protection to something more
along the lines of what we have been talking about. As long as the
input impedance of the micro is >> than the 100k series resistance I
should be OK.

http://img235.imageshack.us/img235/2200/proposedswitchinputmodel9rq.jpg

That MLV cap is suspicious. Could fry on ESD. Also, the 0.01uF and 100K
create a 1msec time constant. HC doesn't really like that, you should
use a Schmitt such as the HC14.

Regards, Joerg

http://www.analogconsultants.com

 

[KoKlust]

affect things. I also changed the uC pin protection to something > more
along the lines of what we have been talking about. As long as the
input impedance of the micro is >> than the 100k series resistance > I
should be OK.

http://img235.imageshack.us/img235/2200/proposedswitchinputmodel9rq.jpg

- Are you planning to do early ESD prototype tests? If not, I would suggest
to do early ESD tests on a prototype as soon as possible, because this story
might cost a relayout! Not only the circuit and components but also the
layout, connector and front are critical here.

- The 0.01 uF cap never sees voltages higher than say 8V so if it is cheaper
you can replace it with a lower voltage type down to 8V. Ceramic caps can
withstand quite a lot of overvoltage without breakdown.

- I'ld guess that the 1K / tantalum 35V cap combination may be the weakest
link in the ESD test.

- I was wondering about the wetting current, and the reason why you cannot
pull it from the 5V voltage regulator. If instability at 20 mA extra loads
is a problem, you could probably solve that either by adding a capacitor in
the regulators feedback network, or by using the right type of output cap. I
mean you may get it stable without needing more boardspace.
By pulling from 5V, you save the large 1210 resistor, and you don't stress
the 12V caps so you make the ESD story lots easier to guarantee.

- You mention that switchers are an EMC nightmare in Detroit. Well here in
the netherlands we did a couple of switchers, even on data acquisition
systems, and were ALWAYS able to get EMC right. Early prototyping, as well
as involving the component application engineers is essential. btw sometimes
the digital guys screwed it up with their clock oscillators ;-) . Anyway,
the currents you need for the other 5V clients seem fairly small so that
makes building a low-radiation switcher easier.

Anyway if you'd succeed with a switcher, you can also save a lot of power
dissipation compared to a 5V regulator that draws 20mA from 12V. Especially
if you need to interface more multiple momentary switches.

Best regards,

Marco

 

[Joerg]

Hello Marco,

- The 0.01 uF cap never sees voltages higher than say 8V so if it is cheaper
you can replace it with a lower voltage type down to 8V. Ceramic caps can
withstand quite a lot of overvoltage without breakdown.

Thing is it slows down the transition into the HC chip. Without a

Schmitt input that is a concern.

- I'ld guess that the 1K / tantalum 35V cap combination may be the weakest
link in the ESD test.

I am more worried about his cap from the connector to GND. If it's big

there won't be much ESD coming through but the switch will suffer.
Bzzzzt. If it's small then the cap may go kapoof.

- You mention that switchers are an EMC nightmare in Detroit. Well here in
the netherlands we did a couple of switchers, even on data acquisition
systems, and were ALWAYS able to get EMC right. ...

Well, yes. Some of the more luxurious sport utility vehicles in the US
even have some hefty 115V outlets in the passenger cabin and baggage
area. They pass EMC.

Then there are pull-down LCD monitors with back light converters in the
ceiling, navigation aid screens etc. Just imagine, cruising down Highway
101 while someone else is doing the driving and you are watching your
favorite Western. Not that mankind needs all this stuff (I don't have it
in mine) but it must pass EMC.

Regards, Joerg

http://www.analogconsultants.com

 

[Ge0rge Marutz]

"At which qties? 17c is a lot of money if you are talking about a few
million units a year. Also, make sure that whatever LDO you are using
will not go on allocation. Or at least that your company is high up in
the pecking order should that happen. And check it out for stability
inside out. These things can be rather fickle."

Qty = 800K (for now)

You think this part is expensive? Geesh, I looked long and hard for
this regulator. It's very precise, has tons of built in stuff that
eliminates external circuitry, etc...

Do you think a switcher could be implemented for anywhere near this
price? I don't think so. But then again, I've never really looked .

I picked a device that can be sourced from 3 different vendors if need
be.

The regulator is stable given the loading vs. capacitance used on its
output. I've tested it to death and have never had a failure.

"Ok, you can use 12V if it's before the 100K like you have in your
drawing. But when the switch is open you'll see 7mA or so flowing at
all
times, more if the switch is pressed, of course. If the circuit is
powered down when not in use that may be ok. Else the valuable customer

sits there one winter day in his fancy vehicle and the engine won't
turn
over :-("

The 12V source switches off with vehicle IGN. Key off load current is
about 90uA. The acceptable limit is 300uA. I'm safe.

"Freescale? HC parts? Mine are spec'd from ONSemi, not Freescale. I'd
be
surprised if they sold any. From a specsmanship point of view (familiy
specs, ESD etc.) Philips is really good but their web site is a pain.

Anyway, HC is a regular plain clothes CMOS process. Look at the family
specs."

I think there is a little confusion here. The device is a
microcontroller manufactured by Freescale (Motorola). Not a common
logic gate. True, the HC in the part number MC68HC908kX8does stand for
CMOS, but the microcontroller implementation goes a little further with
its input scheme than a general logic gate. Or so I am lead to
believe.

"That MLV cap is suspicious. Could fry on ESD."

This multi layered varistor (MLV) is made to absorb transient engergy
from ESD. I would only hope it can stand up to a strike without
frying. Take a look at its data sheet here:

http://www.epcos.com/inf/70/db/var_01/01170130.pdf

 

[Joerg]

Hello George,

"At which qties? 17c is a lot of money if you are talking about a few
million units a year. Also, make sure that whatever LDO you are using
will not go on allocation. Or at least that your company is high up in
the pecking order should that happen. And check it out for stability
inside out. These things can be rather fickle."

Qty = 800K (for now)

Well, that's a lot but not a whole lot ;-)

You think this part is expensive? Geesh, I looked long and hard for
this regulator. It's very precise, has tons of built in stuff that
eliminates external circuitry, etc...

If you can save other parts with it you may have the optimal solution. I
just did that with a TPS77001 by "mooching" the reference off of it for
other stuff. It is also an LDO but quite picky about the output cap ESR.
It's like in a food recipe, not too much but also not too little.

Do you think a switcher could be implemented for anywhere near this
price? I don't think so. But then again, I've never really looked .

Probably but it would require quite a bit of engineering and pricing
research. It might not be feasible at 800K qties since you have to
factor in the engineering costs per unit sold. Anyway, you also have to
look at the caps and their prices. Switchers can sometimes live with
lesser parts here. But they do need an inductor which adds cost.

The 12V source switches off with vehicle IGN. Key off load current is
about 90uA. The acceptable limit is 300uA. I'm safe.

That sounds good.

I think there is a little confusion here. The device is a
microcontroller manufactured by Freescale (Motorola). Not a common
logic gate. True, the HC in the part number MC68HC908kX8does stand for
CMOS, but the microcontroller implementation goes a little further with
its input scheme than a general logic gate. Or so I am lead to
believe.

Sorry, I thought the HC08 in your schematic meant 74HC08. I would still
be quite surprised if the uC would feature Schottky limiters. That would
be like the golden faucets that royalty has in their castles.

I am surprised that a micro in the $3-4 class makes it into cars. Some
of my friends from college work in automotive today. They always say
that every penny gets turned around and around before it enter into the
cost of goods. Sometimes it then has to be taken out again in a cost
reduction effort.

"That MLV cap is suspicious. Could fry on ESD."

This multi layered varistor (MLV) is made to absorb transient engergy
from ESD. I would only hope it can stand up to a strike without
frying. Take a look at its data sheet here:

http://www.epcos.com/inf/70/db/var_01/01170130.pdf

Didn't see that since you drew a cap on your schematic, not a MOV symbol.

Regards, Joerg

http://www.analogconsultants.com