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Surge Pulse Clamping with Ceramic Capacitors

Started by Klaus Kragelund May 29, 2013
On Jun 1, 4:41=A0pm, Joerg <inva...@invalid.invalid> wrote:
> Klaus Kragelund wrote: > > On Friday, May 31, 2013 7:54:16 PM UTC+2, Joerg wrote: > >> Klaus Kragelund wrote: > > >>> On Friday, May 31, 2013 1:16:42 AM UTC+2, Joerg wrote: > >>>> Klaus Kragelund wrote: > >>>>> On Thursday, May 30, 2013 9:32:19 PM UTC+2, Joerg wrote: > >>>>>> Klaus Kragelund wrote: > >>>>>>> On Thursday, May 30, 2013 4:43:00 PM UTC+2, Joerg wrote: > >>>>>>>> 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 > >>>>>>>>> =A0knowing 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 > >>>>>>> =A01-200 mV above rail > >>>>>> Interesting. Then you shouldn't have an issue with the rail > >>>>>> =A0floating 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 > >>>> =A0above-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