Digital Output Protection

a) Is the output push-pull, or open collector? b) What, and how much do you have to drive?

James Arthur

Hi Klaus, What are you using for the "15V digital outputs that are pretty fast"?

A diode in the 15V VDD line would keep the 30V from back-feeding into the +15V.

Maybe a current limiter in the ground of the "15V digital outputs that are pretty fast"?

...Jim Thompson

--
| James E.Thompson, P.E.                           |    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  |
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With all this hope and change, all you need is a dab of mayonaisse
and you\'ll have a tasty lunch on which you will choke to death.

The output is push-pull because we need to re-charge EMC capacitors quickly on the receiving end. The receiving end impedance is 5k resistor with 1n in parallel to ground

Thanks

Klaus

The TC4468 CMOS driver

Yes, but it would toast the stuff on the 15V VDD line. I was thinking about using a crowbar on the VDD line triggered when the power supply for the VDD line went out of regulation (due to a surge on the VDD line).

Do you mean seperate current sensing in the ground line reference (won't work if multible grounds are used due to several output with numerous grounds)

Regards

Klaus

Probably something can be concocted around larger depletion mode FETs so the path starves when the outside wants to drive it past the rail. This doesn't take care of a short to GND though, that would have to be sensed, possibly meaning too many parts.

If your outputs are CMOS with body diodes in there, how about PTC resistors? Just make sure there is a shunt regulator on the 15V rail which kicks in when the outside wants to drive it past, say, 16V.

--
Regards, Joerg

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Use unique connectors on your cables, so they can't be switched.

Nice. Will try that idea. Actually I have not be told you if we need to survive a short to ground.

Yes, that was my first thought. Simple and provide dual protection of the power supply also.

Another trick could be to use keep our low ohmic resistor in the output along with the protection diode.

Then using a micro or whatever circuit to monitor the voltage on both sides of the resistor. In case of overvoltage the supply clamping diodes will do the trick and the protection circuit would kick in before the supply would rise to a too high level or the resistor would be destroyed. It would then also report if the output is malfunctioned in some way. Of course this add some failure modes and needs to be considered.

Regards

Klaus

Yes, thats one of the things we have looked at, but there is so many connectors that it would be pricey to have different ones. Did look into mating pins also, but thats also troublesome with price and production

Thanks

Klaus

I meant the diode between the driver and the +15V rail.

Maybe a disconnect, such as a variation on...

There are CMOS bus drivers which go tri-state when VDD is dropped OR if the output is forced above rail. I don't have a part number off the top of my head, but I've designed them for Moto/ON-Semi around 10 years ago.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    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     |
             
With all this hope and change, all you need is a dab of mayonaisse
and you\'ll have a tasty lunch on which you will choke to death.

e

Will try to see if I can find one, thanks for the hint :-)

Regards

Klaus

Might this help?

John

I was looking for a device that would protect an output, this on protects an input. But that will certainly help me, that perhaps analog devices has a part that can do the trick.

I found this one from NXP:

But, its a 3.3V part :-(

Regards

Klaus

First you must do the same overvoltage analysis for both common mode and differential mode. Your definition implies low capacitance devices are required. Two possible sources for solutions to both differential mode and common mode overvoltages may be low capacitance devices from Sidactor and low capacitance avalanche diodes from

It will protect an output if you can stand the ON resistance.

John

However, that would dissipate more because you must select a low ohms resistor so it'll keep burning those 4W. A PTC would rise in resistance when it becomes hot and the total dissipated power would be much less. It would just sit there at an elevated temperature. In medical we often aren't allowed to do that because a local hotspot is not allowed.

As long as you can guarantee that the uC will not hang. Because a smelly/smokey exit is frowned upon by customers and it would trigger a service case.

--
Regards, Joerg

http://www.analogconsultants.com/

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Failing that, color-code the cables and connectors.

4W @ 30V says your series resistor is already 220 ohms, so why not John's idea (Analog Devices part)?--it's only 85 ohms. 85 ohms * 1nF = 85 nS.

Is that fast enough?

Cheers, James Arthur

Some idiot will grumble about the "non-standard" connector(s), clip them out and install "decent" connectors...and the next idiot will do the dirty switching.........................

Have Fun! Rich