What's the source impedance of that 1KV pulse? Maybe a transzorb can handle the energy.
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.
What's your data rate? A big series resistor or two will limit downstream current but slow down the data, too.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
gnet
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HUT
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.
Regards
Klaus
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
Cheers
Klaus
TI do some fault protected ones, probably going to use some in a redesign, where they keep blowing up the tranceiver.
-- John Devereux
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.
-- Regards, Joerg http://www.analogconsultants.com/
[...]
Those are nice but one should read the disclaimers on page 12. Doesn't sound like 100% useful protection to me.
-- Regards, Joerg http://www.analogconsultants.com/
really cannot handle much
I really do not like PTC fuses but they might be the only option here.
-- Regards, Joerg http://www.analogconsultants.com/
cannot handle much
You have room for power diodes and capacitors but not for resistors?
Anybody know how much voltage an 0603 resistor can stand? I'd guess 500 at least, assuming not a lot of joules were absorbed.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
True, but in fact 30V is enough for my application. Basically they keep connecting/shorting the data lines to a nominal 24VDC.
-- John Devereux
really cannot handle much
I think legally (datasheet) many are restricted to 50V. What they can really take depends on things such as the laser cut method for trimming.
-- Regards, Joerg http://www.analogconsultants.com/
It actually has that function, AFAIR the trigger level is 1-200 mV above rail
Regards
Klaus
3 .
ey really cannot handle much
To drive the bus the impedance towards the line must be low, so the impedan ce is below 5 ohms anyway, and that wont matter much against a 20A pulse. A resistor that takes 2000W for 20us is not easy to come by
A standard 1206 shows 10W for 10us...
Regards
Klaus
Interesting. Then you shouldn't have an issue with the rail floating up, or you only need a very light load on the rail.
-- Regards, Joerg http://www.analogconsultants.com/
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
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.
-- Regards, Joerg http://www.analogconsultants.com/
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
One approach I've used to protect sensitive analog circuitry is a pair of ordinary 1N4148 diodes from the input to the rails, and a filter capacitor between the rails, and a one watt zener diode in parallel with the capacitor. The ordinary diodes are normally reverse-biased and of low capacitance, and will shunt the surge to the zener and capacitor, which will absorb the surge.
Joe Gwinn
Assuming they mean the graph on page 7, middle right:
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.
-- Regards, Joerg http://www.analogconsultants.com/
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
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.
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?" :-)
-- Regards, Joerg http://www.analogconsultants.com/
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