Surge Pulse Clamping with Ceramic Capacitors

[Klaus Kragelund]

Klaus Kragelund wrote:
Klaus Kragelund wrote:
[...]
Another idea is to use the Bourns CDSOT23-SM712, specifically designet
for surge protection for RS485 devices, but add ceramic caps in series
with each line connection so it can tolerate 30V without creating
wonderful smoke, but will be able to clamp surge pulses without
affecting the high speed bus.
Regards
Klaus
Like this:
www.electronicsdesign.dk/tmp/RS485_cap_protection.pdf
But that would not protect against a hard 30VDC applied because an
installer miswired something. It could cause your RS485 chip to go PHUT
unless it has internal protection against this.
The RS485 IC has +/-60V protection rating, so its ok
Which one do you use? The ones on mine (clients's choice) have abs max
ratings of -8V to +12V, and no internal circuitry given.

LTC2862, can take 60V indefinitely, but you need to add clamping

circuit to the VDD node since it will dump current into that node if

it is in transmit mode and is subjected to back fed voltage.

Wow, that sure is the Rolls-Royce of RS485 chips. With a corresponding

price tag



But isn't that bleed-through only an issue if the ground of the LTC2862

has come off? Otherwise this would really be a problem because you'd

just have moved the dissipation from one place to another. I think the

only real protection for 30V continuously is some sort of cut-off,

whether inside or outside a chip. At least a partial one where the

current becomes very small.

When the device is in transmit mode, it has no way of knowing if an appliedvoltage is just a load or a fault. So it has current limit on the outputs and if sourcing into the VDD is detected, the driver is shut off. If 5V is supplied to the device externally, only the current limit is active and thecurrent is steered to the GND or VDD rail.

Regards

Klaus

 

[Klaus Kragelund]

What's the source impedance of that 1KV pulse? Maybe a transzorb can

handle the energy.

Its 40ohms, 1kV, 8/20us, so approx 20A pulse.

If you use a diode, be careful about the diode's forward recovery

time.



Sometimes it works to use a series resistor to limit the current,

instead of sinking all that impulse energy.

We have no room for it, 0402 and 0603 resistors is only option a´nd they really cannot handle much

Cheers

Klaus

 

[John Devereux]

Klaus Kragelund wrote:
Klaus Kragelund wrote:
[...]
Another idea is to use the Bourns CDSOT23-SM712, specifically designet
for surge protection for RS485 devices, but add ceramic caps in series
with each line connection so it can tolerate 30V without creating
wonderful smoke, but will be able to clamp surge pulses without
affecting the high speed bus.
Regards
Klaus
Like this:
www.electronicsdesign.dk/tmp/RS485_cap_protection.pdf
But that would not protect against a hard 30VDC applied because an
installer miswired something. It could cause your RS485 chip to go PHUT
unless it has internal protection against this.
The RS485 IC has +/-60V protection rating, so its ok
Which one do you use? The ones on mine (clients's choice) have abs max
ratings of -8V to +12V, and no internal circuitry given.

LTC2862, can take 60V indefinitely, but you need to add clamping
circuit to the VDD node since it will dump current into that node if
it is in transmit mode and is subjected to back fed voltage.

Wow, that sure is the Rolls-Royce of RS485 chips. With a corresponding
price tag

But isn't that bleed-through only an issue if the ground of the LTC2862
has come off? Otherwise this would really be a problem because you'd
just have moved the dissipation from one place to another. I think the
only real protection for 30V continuously is some sort of cut-off,
whether inside or outside a chip. At least a partial one where the
current becomes very small.

TI do some fault protected ones, probably going to use some in a
redesign, where they keep blowing up the tranceiver.

<http://www.ti.com/lsds/ti/interface/rs-485-products.page>

<http://www.ti.com/product/sn65hvd1781>

 

[Joerg]

]

But isn't that bleed-through only an issue if the ground of the
LTC2862

has come off? Otherwise this would really be a problem because
you'd

just have moved the dissipation from one place to another. I think
the

only real protection for 30V continuously is some sort of cut-off,

whether inside or outside a chip. At least a partial one where the

current becomes very small.

When the device is in transmit mode, it has no way of knowing if an
applied voltage is just a load or a fault. So it has current limit on
the outputs and if sourcing into the VDD is detected, the driver is
shut off. If 5V is supplied to the device externally, only the
current limit is active and the current is steered to the GND or VDD
rail.

They probably should have built in an internal VDD run-up trigger that
then limits the current to zero. If it doesn't let go of the bus at that
point that could present a problem.

 

[Joerg]

Klaus Kragelund wrote:

[...]

Wow, that sure is the Rolls-Royce of RS485 chips. With a corresponding
price tag

But isn't that bleed-through only an issue if the ground of the LTC2862
has come off? Otherwise this would really be a problem because you'd
just have moved the dissipation from one place to another. I think the
only real protection for 30V continuously is some sort of cut-off,
whether inside or outside a chip. At least a partial one where the
current becomes very small.

TI do some fault protected ones, probably going to use some in a
redesign, where they keep blowing up the tranceiver.

<http://www.ti.com/lsds/ti/interface/rs-485-products.page>

<http://www.ti.com/product/sn65hvd1781>

Those are nice but one should read the disclaimers on page 12. Doesn't
sound like 100% useful protection to me.

 

[Joerg]

Its 40ohms, 1kV, 8/20us, so approx 20A pulse.

We have no room for it, 0402 and 0603 resistors is only option a´nd they really cannot handle much

I really do not like PTC fuses but they might be the only option here.

 

[John Devereux]

Klaus Kragelund wrote:

[...]

LTC2862, can take 60V indefinitely, but you need to add clamping
circuit to the VDD node since it will dump current into that node if
it is in transmit mode and is subjected to back fed voltage.

Wow, that sure is the Rolls-Royce of RS485 chips. With a corresponding
price tag

But isn't that bleed-through only an issue if the ground of the LTC2862
has come off? Otherwise this would really be a problem because you'd
just have moved the dissipation from one place to another. I think the
only real protection for 30V continuously is some sort of cut-off,
whether inside or outside a chip. At least a partial one where the
current becomes very small.

TI do some fault protected ones, probably going to use some in a
redesign, where they keep blowing up the tranceiver.

<http://www.ti.com/lsds/ti/interface/rs-485-products.page>

<http://www.ti.com/product/sn65hvd1781>

Those are nice but one should read the disclaimers on page 12. Doesn't
sound like 100% useful protection to me.

True, but in fact 30V is enough for my application. Basically they keep
connecting/shorting the data lines to a nominal 24VDC.

 

[Klaus Kragelund]

On Thursday, May 30, 2013 1:32:10 AM UTC+2, Joerg wrote:


[...]

But isn't that bleed-through only an issue if the ground of the
LTC2862

has come off? Otherwise this would really be a problem because
you'd

just have moved the dissipation from one place to another. I think
the

only real protection for 30V continuously is some sort of cut-off,

whether inside or outside a chip. At least a partial one where the

current becomes very small.

When the device is in transmit mode, it has no way of knowing if an

applied voltage is just a load or a fault. So it has current limit on

the outputs and if sourcing into the VDD is detected, the driver is

shut off. If 5V is supplied to the device externally, only the

current limit is active and the current is steered to the GND or VDD

They probably should have built in an internal VDD run-up trigger that

then limits the current to zero. If it doesn't let go of the bus at that

point that could present a problem.

It actually has that function, AFAIR the trigger level is 1-200 mV above rail

Regards

Klaus

 

[Klaus Kragelund]

You have room for power diodes and capacitors but not for resistors?

To drive the bus the impedance towards the line must be low, so the impedance is below 5 ohms anyway, and that wont matter much against a 20A pulse. Aresistor that takes 2000W for 20us is not easy to come by

A standard 1206 shows 10W for 10us...

Regards

Klaus

 

[Joerg]

On Thursday, May 30, 2013 1:32:10 AM UTC+2, Joerg wrote:

[...]





But isn't that bleed-through only an issue if the ground of the
LTC2862
has come off? Otherwise this would really be a problem because
you'd
just have moved the dissipation from one place to another. I think
the
only real protection for 30V continuously is some sort of cut-off,
whether inside or outside a chip. At least a partial one where the
current becomes very small.
When the device is in transmit mode, it has no way of knowing if an
applied voltage is just a load or a fault. So it has current limit on
the outputs and if sourcing into the VDD is detected, the driver is
shut off. If 5V is supplied to the device externally, only the
current limit is active and the current is steered to the GND or VDD
rail.

They probably should have built in an internal VDD run-up trigger that

then limits the current to zero. If it doesn't let go of the bus at that

point that could present a problem.

It actually has that function, AFAIR the trigger level is 1-200 mV above rail

Interesting. Then you shouldn't have an issue with the rail floating up,
or you only need a very light load on the rail.

 

[Klaus Kragelund]

Klaus Kragelund wrote:

On Thursday, May 30, 2013 1:32:10 AM UTC+2, Joerg wrote:


[...]





But isn't that bleed-through only an issue if the ground of the
LTC2862
has come off? Otherwise this would really be a problem because
you'd
just have moved the dissipation from one place to another. I think
the
only real protection for 30V continuously is some sort of cut-off,
whether inside or outside a chip. At least a partial one where the
current becomes very small.
When the device is in transmit mode, it has no way of knowing if an
applied voltage is just a load or a fault. So it has current limit on
the outputs and if sourcing into the VDD is detected, the driver is
shut off. If 5V is supplied to the device externally, only the
current limit is active and the current is steered to the GND or VDD
rail.


They probably should have built in an internal VDD run-up trigger that

then limits the current to zero. If it doesn't let go of the bus at that

point that could present a problem.

It actually has that function, AFAIR the trigger level is 1-200 mV above rail

Interesting. Then you shouldn't have an issue with the rail floating up,

or you only need a very light load on the rail.

The maximum rail current is 80mA as defines in the datasheet, so the shunt regulator on the VDD rail must be able to sink this current

Regards

Klaus

 

[Joerg]

]
But isn't that bleed-through only an issue if the ground of
the LTC2862 has come off? Otherwise this would really be a
problem because you'd just have moved the dissipation from
one place to another. I think the only real protection for
30V continuously is some sort of cut-off, whether inside or
outside a chip. At least a partial one where the current
becomes very small.
When the device is in transmit mode, it has no way of knowing
if an applied voltage is just a load or a fault. So it has
current limit on the outputs and if sourcing into the VDD is
detected, the driver is shut off. If 5V is supplied to the
device externally, only the current limit is active and the
current is steered to the GND or VDD rail.
They probably should have built in an internal VDD run-up
trigger that then limits the current to zero. If it doesn't let
go of the bus at that point that could present a problem.
It actually has that function, AFAIR the trigger level is 1-200
mV above rail

Interesting. Then you shouldn't have an issue with the rail
floating up,

or you only need a very light load on the rail.

The maximum rail current is 80mA as defines in the datasheet, so the
shunt regulator on the VDD rail must be able to sink this current

Just took a look at the datasheet again. I can't see that 200mV
above-rail cut off anywhere. Just an entry on page 3 that the thing can
sink up to 250mA into its supply. Because it says -60V to 60V in the
conditions box. Then a graph on page 7 that says something different.
I'd really contact LTC app support before using it, this datasheet looks
too ambiguous to me.

 

[Klaus Kragelund]

Klaus Kragelund wrote:

]
But isn't that bleed-through only an issue if the ground of
the LTC2862 has come off? Otherwise this would really be a
problem because you'd just have moved the dissipation from
one place to another. I think the only real protection for
30V continuously is some sort of cut-off, whether inside or
outside a chip. At least a partial one where the current
becomes very small.
When the device is in transmit mode, it has no way of knowing
if an applied voltage is just a load or a fault. So it has
current limit on the outputs and if sourcing into the VDD is
detected, the driver is shut off. If 5V is supplied to the
device externally, only the current limit is active and the
current is steered to the GND or VDD rail.
They probably should have built in an internal VDD run-up
trigger that then limits the current to zero. If it doesn't let
go of the bus at that point that could present a problem.
It actually has that function, AFAIR the trigger level is 1-200
mV above rail


Interesting. Then you shouldn't have an issue with the rail
floating up,

or you only need a very light load on the rail.

The maximum rail current is 80mA as defines in the datasheet, so the

shunt regulator on the VDD rail must be able to sink this current

Just took a look at the datasheet again. I can't see that 200mV

above-rail cut off anywhere. Just an entry on page 3 that the thing can

sink up to 250mA into its supply. Because it says -60V to 60V in the

conditions box. Then a graph on page 7 that says something different.

I'd really contact LTC app support before using it, this datasheet looks

too ambiguous to me.

The 200mV number is a number from the engineers from Linear Technology, I contacted them since I saw the same thing you did.

From LTC:

Qoute

The answers to the questions regarding LT2862 is the following.
In receive and shutdown, the lines are high ohmic, 112k, meaning no problem if connected forever to +/-60V.
In transmit mode there is the graph showing pin current for output low and output high as function of voltage, assume worst case 1.66 x nominal.
The 80mA worst case current going to Vcc is happening at the narrow spike (output high, a few volts positive, peaking at about 60mA typ) in diagram below, at higher voltages the current goes to GND.

Unqoute

Cheers

Klaus

 

[Joerg]

]
But isn't that bleed-through only an issue if the
ground of the LTC2862 has come off? Otherwise this
would really be a problem because you'd just have moved
the dissipation from one place to another. I think the
only real protection for 30V continuously is some sort
of cut-off, whether inside or outside a chip. At least
a partial one where the current becomes very small.
When the device is in transmit mode, it has no way of
knowing if an applied voltage is just a load or a fault.
So it has current limit on the outputs and if sourcing
into the VDD is detected, the driver is shut off. If 5V
is supplied to the device externally, only the current
limit is active and the current is steered to the GND or
VDD rail.
They probably should have built in an internal VDD run-up
trigger that then limits the current to zero. If it doesn't
let go of the bus at that point that could present a
problem.
It actually has that function, AFAIR the trigger level is
1-200 mV above rail
Interesting. Then you shouldn't have an issue with the rail
floating up, or you only need a very light load on the rail.
The maximum rail current is 80mA as defines in the datasheet, so
the shunt regulator on the VDD rail must be able to sink this
current

Just took a look at the datasheet again. I can't see that 200mV

above-rail cut off anywhere. Just an entry on page 3 that the thing
can

sink up to 250mA into its supply. Because it says -60V to 60V in
the

conditions box. Then a graph on page 7 that says something
different.

I'd really contact LTC app support before using it, this datasheet
looks

too ambiguous to me.

The 200mV number is a number from the engineers from Linear
Technology, I contacted them since I saw the same thing you did.

From LTC:

Qoute

The answers to the questions regarding LT2862 is the following. In
receive and shutdown, the lines are high ohmic, 112k, meaning no
problem if connected forever to +/-60V. In transmit mode there is the
graph showing pin current for output low and output high as function
of voltage, assume worst case 1.66 x nominal. The 80mA worst case
current going to Vcc is happening at the narrow spike (output high, a
few volts positive, peaking at about 60mA typ) in diagram below, at
higher voltages the current goes to GND.

Unqoute

Assuming they mean the graph on page 7, middle right:

http://www.linear.com/docs/40761

That shows that if someone would apply +30V it'll still draw 50mA times
1.66 for worst case. That's 2.5 watts worst case ... phsssst ... *POOF*
.... I'd say then it's not safe.

It says it has thermal shutdown that disables the driver but from the
way I understand it that all means you'd still have a big fat current
dumping into the logic rail. Which would need to go somewhere so it
won't float up.

 

[Klaus Kragelund]

Klaus Kragelund wrote:

]
But isn't that bleed-through only an issue if the
ground of the LTC2862 has come off? Otherwise this
would really be a problem because you'd just have moved
the dissipation from one place to another. I think the
only real protection for 30V continuously is some sort
of cut-off, whether inside or outside a chip. At least
a partial one where the current becomes very small.
When the device is in transmit mode, it has no way of
knowing if an applied voltage is just a load or a fault.
So it has current limit on the outputs and if sourcing
into the VDD is detected, the driver is shut off. If 5V
is supplied to the device externally, only the current
limit is active and the current is steered to the GND or
VDD rail.
They probably should have built in an internal VDD run-up
trigger that then limits the current to zero. If it doesn't
let go of the bus at that point that could present a
problem.
It actually has that function, AFAIR the trigger level is
1-200 mV above rail
Interesting. Then you shouldn't have an issue with the rail
floating up, or you only need a very light load on the rail.
The maximum rail current is 80mA as defines in the datasheet, so
the shunt regulator on the VDD rail must be able to sink this
current


Just took a look at the datasheet again. I can't see that 200mV

above-rail cut off anywhere. Just an entry on page 3 that the thing
can

sink up to 250mA into its supply. Because it says -60V to 60V in
the

conditions box. Then a graph on page 7 that says something
different.

I'd really contact LTC app support before using it, this datasheet
looks

too ambiguous to me.

The 200mV number is a number from the engineers from Linear

Technology, I contacted them since I saw the same thing you did.

From LTC:



The answers to the questions regarding LT2862 is the following. In

receive and shutdown, the lines are high ohmic, 112k, meaning no

problem if connected forever to +/-60V. In transmit mode there is the

graph showing pin current for output low and output high as function

of voltage, assume worst case 1.66 x nominal. The 80mA worst case

current going to Vcc is happening at the narrow spike (output high, a

few volts positive, peaking at about 60mA typ) in diagram below, at

higher voltages the current goes to GND.

Assuming they mean the graph on page 7, middle right:



http://www.linear.com/docs/40761



That shows that if someone would apply +30V it'll still draw 50mA times

1.66 for worst case. That's 2.5 watts worst case ... phsssst ... *POOF*

... I'd say then it's not safe.

That's the correct figure, but only valid if the device is in transmit mode, during receive mode the currents are zero.

The only way the device can be in transmit mode is if a telegram is send to it and that's impossible if 30V DC is applied to the line. So no phssssst...... *POOF*

Anyway, the discussion about the DC is a little theoretical, since if 30V DC is applied to the line, what happens to all those other slaves and the master that has no DC protection? collective phssssst...... *POOF* ;-)

Regards

Klaus

 

[Joerg]

[...]
But isn't that bleed-through only an issue if the
ground of the LTC2862 has come off? Otherwise this
would really be a problem because you'd just have
moved the dissipation from one place to another. I
think the only real protection for 30V continuously
is some sort of cut-off, whether inside or outside
a chip. At least a partial one where the current
becomes very small.
When the device is in transmit mode, it has no way of
knowing if an applied voltage is just a load or a
fault. So it has current limit on the outputs and if
sourcing into the VDD is detected, the driver is shut
off. If 5V is supplied to the device externally, only
the current limit is active and the current is
steered to the GND or VDD rail.
They probably should have built in an internal VDD
run-up trigger that then limits the current to zero. If
it doesn't let go of the bus at that point that could
present a problem.
It actually has that function, AFAIR the trigger level is
1-200 mV above rail
Interesting. Then you shouldn't have an issue with the rail
floating up, or you only need a very light load on the
rail.
The maximum rail current is 80mA as defines in the datasheet,
so the shunt regulator on the VDD rail must be able to sink
this current
Just took a look at the datasheet again. I can't see that 200mV
above-rail cut off anywhere. Just an entry on page 3 that the
thing can sink up to 250mA into its supply. Because it says
-60V to 60V in the conditions box. Then a graph on page 7 that
says something different. I'd really contact LTC app support
before using it, this datasheet looks too ambiguous to me.
The 200mV number is a number from the engineers from Linear
Technology, I contacted them since I saw the same thing you did.
From LTC: Qoute The answers to the questions regarding LT2862 is
the following. In receive and shutdown, the lines are high ohmic,
112k, meaning no problem if connected forever to +/-60V. In
transmit mode there is the graph showing pin current for output
low and output high as function of voltage, assume worst case
1.66 x nominal. The 80mA worst case current going to Vcc is
happening at the narrow spike (output high, a few volts positive,
peaking at about 60mA typ) in diagram below, at higher voltages
the current goes to GND. Unqoute

Assuming they mean the graph on page 7, middle right:



http://www.linear.com/docs/40761



That shows that if someone would apply +30V it'll still draw 50mA
times

1.66 for worst case. That's 2.5 watts worst case ... phsssst ...
*POOF*

... I'd say then it's not safe.

That's the correct figure, but only valid if the device is in
transmit mode, during receive mode the currents are zero.

The only way the device can be in transmit mode is if a telegram is
send to it and that's impossible if 30V DC is applied to the line. So
no phssssst...... *POOF*

Sure, if there is this sort of "natural" protection via protocol and
your system will never be sending unsolicited half-hour sermons (or be
the communications initiator) then you should be safe.

Anyway, the discussion about the DC is a little theoretical, since if
30V DC is applied to the line, what happens to all those other slaves
and the master that has no DC protection? collective phssssst......
*POOF* ;-)

Yeah but ... imagine what the end customer would notice: "Now why is it
that every time things go kablouie only the device from the Vikings
survives?"

 

[[email protected]]

Klaus Kragelund wrote:
[...]
But isn't that bleed-through only an issue if the
ground of the LTC2862 has come off? Otherwise this
would really be a problem because you'd just have
moved the dissipation from one place to another. I
think the only real protection for 30V continuously
is some sort of cut-off, whether inside or outside
a chip. At least a partial one where the current
becomes very small.
When the device is in transmit mode, it has no way of
 knowing if an applied voltage is just a load or a
fault. So it has current limit on the outputs and if
sourcing into the VDD is detected, the driver is shut
off. If 5V is supplied to the device externally, only
the current limit is active and the current is
steered to the GND or VDD rail.
They probably should have built in an internal VDD
run-up trigger that then limits the current to zero. If
it doesn't let go of the bus at that point that could
present a problem.
It actually has that function, AFAIR the trigger level is
 1-200 mV above rail
Interesting. Then you shouldn't have an issue with the rail
 floating up, or you only need a very light load on the
rail.
The maximum rail current is 80mA as defines in the datasheet,
so the shunt regulator on the VDD rail must be able to sink
this current
Just took a look at the datasheet again. I can't see that 200mV
 above-rail cut off anywhere. Just an entry on page 3 that the
thing can sink up to 250mA into its supply. Because it says
-60V to 60V in the conditions box. Then a graph on page 7 that
says something different. I'd really contact LTC app support
before using it, this datasheet looks too ambiguous to me.
The 200mV number is a number from the engineers from Linear
Technology, I contacted them since I saw the same thing you did.
From LTC: Qoute The answers to the questions regarding LT2862 is
the following. In receive and shutdown, the lines are high ohmic,
112k, meaning no problem if connected forever to +/-60V. In
transmit mode there is the graph showing pin current for output
low and output high as function of voltage, assume worst case
1.66 x nominal. The 80mA worst case current going to Vcc is
happening at the narrow spike (output high, a few volts positive,
peaking at about 60mA typ) in diagram below, at higher voltages
the current goes to GND. Unqoute
Assuming they mean the graph on page 7, middle right:
http://www.linear.com/docs/40761
That shows that if someone would apply +30V it'll still draw 50mA
times
1.66 for worst case. That's 2.5 watts worst case ... phsssst ...
*POOF*
... I'd say then it's not safe.

That's the correct figure, but only valid if the device is in
transmit mode, during receive mode the currents are zero.

The only way the device can be in transmit mode is if a telegram is
send to it and that's impossible if 30V DC is applied to the line. So
no phssssst...... *POOF*

Sure, if there is this sort of "natural" protection via protocol and
your system will never be sending unsolicited half-hour sermons (or be
the communications initiator) then you should be safe.

you could put some form of inhibit on tx_enable when you see the
fault condition


-Lasse