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Surge Pulse Clamping with Ceramic Capacitors

Started by Klaus Kragelund May 29, 2013
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 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 Regards Klaus
On Thursday, May 30, 2013 5:09:47 PM UTC+2, John Larkin wrote:
> On Thu, 30 May 2013 00:17:43 -0700 (PDT), Klaus Kragelund >=20 > <klauskvik@hotmail.com> wrote: >=20 >=20 >=20 > >On Thursday, May 30, 2013 1:51:03 AM UTC+2, John Larkin wrote: >=20 > >> On Wed, 29 May 2013 14:15:58 -0700 (PDT), Klaus Kragelund >=20 > >>=20 >=20 > >> <klauskvik@hotmail.com> wrote: >=20 > >>=20 >=20 > >>=20 >=20 > >>=20 >=20 > >> >Hi >=20 > >>=20 >=20 > >> > >=20 > >>=20 >=20 > >> >For an application I need to protect the electronics of an RS485 >=20 > >>=20 >=20 > >> >interface from surges (8/20us 1kV pulse) >=20 > >>=20 >=20 > >> > >=20 > >>=20 >=20 > >> >The normal way be to use a transzorb, in a SMA housing or even a SOT2=
3
>=20 > >>=20 >=20 > >> >device. >=20 > >>=20 >=20 > >> > >=20 > >>=20 >=20 > >> >That is not possible since we have to be able to withstand up to 30V >=20 > >>=20 >=20 > >> >DC on the bus also (that is handled by a special RS485 IC) >=20 > >>=20 >=20 > >> > >=20 > >>=20 >=20 > >> >The problem is that the selected breakdown voltage of the transzorb >=20 > >>=20 >=20 > >> >therefore is high (>30V) and a lot of energy is dissipated into the >=20 > >>=20 >=20 > >> >transzorb >=20 > >>=20 >=20 > >> > >=20 > >>=20 >=20 > >> >In another product I have used a diode from the affected node to a >=20 > >>=20 >=20 > >> >ceramic capacitor with a bleeder resistor in parallel to clamp the >=20 > >>=20 >=20 > >> >energy and dissipate the energy into the bleeder and that worked fine=
.
>=20 > >>=20 >=20 > >> >The diode sees very little energy and the capacitor is just charged >=20 > >>=20 >=20 > >> >during the pulse >=20 > >>=20 >=20 > >> > >=20 > >>=20 >=20 > >> >I never saw any problems doing that, but I would like to know if >=20 > >>=20 >=20 > >> >anyone here has tried the same and has any inputs into failure cases >=20 > >>=20 >=20 > >> >or even a better way to clamp the pulse? >=20 > >>=20 >=20 > >> > >=20 > >>=20 >=20 > >> >One "feature" of the diode-capacitor clamp is that closely spaced >=20 > >>=20 >=20 > >> >pulses will eventually destroy the capacitor, but anyhow closely >=20 > >>=20 >=20 > >> >spaced pulses in a tranzorb will also destroy that one.... >=20 > >>=20 >=20 > >> > >=20 > >>=20 >=20 > >> >Thanks >=20 > >>=20 >=20 > >> > >=20 > >>=20 >=20 > >> >Klaus >=20 > >>=20 >=20 > >>=20 >=20 > >>=20 >=20 > >> What's the source impedance of that 1KV pulse? Maybe a transzorb can >=20 > >>=20 >=20 > >> handle the energy. >=20 > >>=20 >=20 > >>=20 >=20 > >Its 40ohms, 1kV, 8/20us, so approx 20A pulse. >=20 > > >=20 > >>=20 >=20 > >> If you use a diode, be careful about the diode's forward recovery >=20 > >>=20 >=20 > >> time. >=20 > >>=20 >=20 > >>=20 >=20 > >>=20 >=20 > >> Sometimes it works to use a series resistor to limit the current, >=20 > >>=20 >=20 > >> instead of sinking all that impulse energy.=20 >=20 > >>=20 >=20 > >>=20 >=20 > >>=20 >=20 > >We have no room for it, 0402 and 0603 resistors is only option a=B4nd th=
ey really cannot handle much
>=20 > > >=20 > >Cheers >=20 > > >=20 > >Klaus >=20 >=20 >=20 > You have room for power diodes and capacitors but not for resistors? >=20
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
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 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. -- Regards, Joerg http://www.analogconsultants.com/
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 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
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 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. -- Regards, Joerg http://www.analogconsultants.com/
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 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
In article
<9269edc5-9fae-4f7e-bd35-ed7b02c4fa02@k4g2000vba.googlegroups.com>,
Klaus Kragelund <klauskvik@hotmail.com> wrote:

> Hi > > For an application I need to protect the electronics of an RS485 > interface from surges (8/20us 1kV pulse) > > The normal way be to use a transzorb, in a SMA housing or even a SOT23 > device. > > That is not possible since we have to be able to withstand up to 30V > DC on the bus also (that is handled by a special RS485 IC) > > The problem is that the selected breakdown voltage of the transzorb > therefore is high (>30V) and a lot of energy is dissipated into the > transzorb > > In another product I have used a diode from the affected node to a > ceramic capacitor with a bleeder resistor in parallel to clamp the > energy and dissipate the energy into the bleeder and that worked fine. > The diode sees very little energy and the capacitor is just charged > during the pulse > > I never saw any problems doing that, but I would like to know if > anyone here has tried the same and has any inputs into failure cases > or even a better way to clamp the pulse? > > One "feature" of the diode-capacitor clamp is that closely spaced > pulses will eventually destroy the capacitor, but anyhow closely > spaced pulses in a tranzorb will also destroy that one....
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
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 >>>>>>> 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. -- Regards, Joerg http://www.analogconsultants.com/
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 > > >>>>>>> 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* :-) 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
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 >>>>>>>>> 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?" :-) -- Regards, Joerg http://www.analogconsultants.com/