Surge Pulse Clamping with Ceramic Capacitors

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.

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.

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

really cannot handle much

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

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.

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

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.

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

Regards

Klaus

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.

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.

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

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.

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......

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?" :-)