Joerg <invalid@invalid.invalid> wrote:>> On Sat, 29 Jun 2013 08:00:01 -0700, Joerg <invalid@invalid.invalid>>>>>> Oops, sorry, your are right. At a 5V-tolerant pin it would work. >>>>> Problem is that we only have 5V in the area where the protectors >>>>> are. >>>> If you only have 5V around driving a 5V-tolerant pin... what are >>>> you fretting over? >>>> >>> The concern are surge events that make these lines carry nasty >>> transients, which in turn can lock up or damage the uC. >> >> So you have a "dirty" 5V supply? >> > > No, rock-stable. But clamping against it will lead to up to 7V spike > amplitude if it's a huge transient. Those small clamper arrays can > easily reach 2V Vf or more for a few usec.If you are dealing with 5V logic signals, isn't the threshold VCC/2, or around 2.5V? How about clamping with a 3.9V zener to ground with a small series resistor to limit the transient current? As long as the desired signal gets above the threshold, do you care if it goes all the way to 5V? For a BZX84C3V9, Fig. 2 on page 3 shows it doesn't start to conduct until you get above 3V, and 50mA doesn't even reach 5V: http://www.pridmore.com/datasheets/BZX84CA07.pdf Don't know what all the tolerances are, but so far nothing else seems to have any chance of working in your application. JK
8051F020 series, 5V tolerant input schematic?
Started by ●June 25, 2013
Reply by ●June 29, 20132013-06-29
Reply by ●June 29, 20132013-06-29
On Sat, 29 Jun 2013 15:49:52 GMT, John K <spam@me.not> wrote:>Joerg <invalid@invalid.invalid> wrote: > >>> On Sat, 29 Jun 2013 08:00:01 -0700, Joerg <invalid@invalid.invalid> > >>>>>> Oops, sorry, your are right. At a 5V-tolerant pin it would work. >>>>>> Problem is that we only have 5V in the area where the protectors >>>>>> are. >>>>> If you only have 5V around driving a 5V-tolerant pin... what are >>>>> you fretting over? >>>>> >>>> The concern are surge events that make these lines carry nasty >>>> transients, which in turn can lock up or damage the uC. >>> >>> So you have a "dirty" 5V supply? >>> >> >> No, rock-stable. But clamping against it will lead to up to 7V spike >> amplitude if it's a huge transient. Those small clamper arrays can >> easily reach 2V Vf or more for a few usec. > >If you are dealing with 5V logic signals, isn't the threshold VCC/2, or >around 2.5V? > >How about clamping with a 3.9V zener to ground with a small series >resistor to limit the transient current? As long as the desired signal >gets above the threshold, do you care if it goes all the way to 5V? > >For a BZX84C3V9, Fig. 2 on page 3 shows it doesn't start to conduct until >you get above 3V, and 50mA doesn't even reach 5V: > >http://www.pridmore.com/datasheets/BZX84CA07.pdf > >Don't know what all the tolerances are, but so far nothing else seems to >have any chance of working in your application. > >JKI still haven't seen described how a "rock solid" 5V supply produces such gross transients. As for thresholds... a 3.3V logic block, with 5V tolerant inputs, will threshold roughly at VDD/2, or 3.3V/2, or 1.65V. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●June 29, 20132013-06-29
Jim Thompson wrote:> On Sat, 29 Jun 2013 08:13:15 -0700, Joerg <invalid@invalid.invalid> > wrote: > >> Jim Thompson wrote: >>> On Sat, 29 Jun 2013 08:00:01 -0700, Joerg <invalid@invalid.invalid> >>> wrote: >>> >>>> Jim Thompson wrote: >>>>> On Fri, 28 Jun 2013 17:53:15 -0700, Joerg <invalid@invalid.invalid> >>>>> wrote: >>>>> >>>>>> Joerg wrote: >>>>>>> Jasen Betts wrote: >>>>>>>> On 2013-06-26, Joerg <invalid@invalid.invalid> wrote: >>>>>>>>> Spehro Pefhany wrote: >>>>>>>>>> I guess they assume you'll never exceed the absolute maximum voltage >>>>>>>>>> limits of +5.8/-0.3, so no significant current will ever flow. >>>>>>>>>> >>>>>>>>> Ok, but in real life that's almost unprotectable. The way I usually do >>>>>>>>> it, there is a diode against the 5V rail, followed by a resistor, then >>>>>>>>> the port pin. If the pulse from hell comes along that diode can briefly >>>>>>>>> lean in so badly that Vf goes slightly above 2V, meaning over 7V total. >>>>>>>>> Bigger diodes aren't an option because that messes up the signal, on >>>>>>>>> account of their large capacitance. >>>>>>>> How about this: series resistor, shunt diode to 3.3V , port pin >>>>>>>> >>>>>>> That doesn't protect very well because the parasitic substrate diode >>>>>>> will take a substantial portion of the hit. The only thing that works is: >>>>>>> >>>>>>> R1 -> shunt diodes -> R2 -> port pin. >>>>>>> >>>>>>> R2 can be small because there will only be 1-2V max across it and it >>>>>>> only has to make sure the substrate diode gets no more than a few mA. >>>>>>> >>>>>>> But ... too many parts in this case. >>>>>>> >>>>>> Oops, sorry, your are right. At a 5V-tolerant pin it would work. Problem >>>>>> is that we only have 5V in the area where the protectors are. >>>>> If you only have 5V around driving a 5V-tolerant pin... what are you >>>>> fretting over? >>>>> >>>> The concern are surge events that make these lines carry nasty >>>> transients, which in turn can lock up or damage the uC. >>> So you have a "dirty" 5V supply? >>> >> No, rock-stable. But clamping against it will lead to up to 7V spike >> amplitude if it's a huge transient. Those small clamper arrays can >> easily reach 2V Vf or more for a few usec. > > _Where_ are these "huge transients" coming from? >The usual, through outside cabling, induced by heavy machinery being turned on et cetera. Unit is plastic (has to be), a decent ground structure is difficult to impossible. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●June 29, 20132013-06-29
Jim Thompson wrote:> On Sat, 29 Jun 2013 15:49:52 GMT, John K <spam@me.not> wrote: > >> Joerg <invalid@invalid.invalid> wrote: >> >>>> On Sat, 29 Jun 2013 08:00:01 -0700, Joerg <invalid@invalid.invalid> >>>>>>> Oops, sorry, your are right. At a 5V-tolerant pin it would work. >>>>>>> Problem is that we only have 5V in the area where the protectors >>>>>>> are. >>>>>> If you only have 5V around driving a 5V-tolerant pin... what are >>>>>> you fretting over? >>>>>> >>>>> The concern are surge events that make these lines carry nasty >>>>> transients, which in turn can lock up or damage the uC. >>>> So you have a "dirty" 5V supply? >>>> >>> No, rock-stable. But clamping against it will lead to up to 7V spike >>> amplitude if it's a huge transient. Those small clamper arrays can >>> easily reach 2V Vf or more for a few usec. >> If you are dealing with 5V logic signals, isn't the threshold VCC/2, or >> around 2.5V? >> >> How about clamping with a 3.9V zener to ground with a small series >> resistor to limit the transient current? As long as the desired signal >> gets above the threshold, do you care if it goes all the way to 5V? >> >> For a BZX84C3V9, Fig. 2 on page 3 shows it doesn't start to conduct until >> you get above 3V, and 50mA doesn't even reach 5V: >> >> http://www.pridmore.com/datasheets/BZX84CA07.pdf >> >> Don't know what all the tolerances are, but so far nothing else seems to >> have any chance of working in your application. >> >> JK > > I still haven't seen described how a "rock solid" 5V supply produces > such gross transients. >It doesn't. It's the signal lines that do. [...] -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●June 29, 20132013-06-29
On Sat, 29 Jun 2013 10:22:13 -0700, Joerg <invalid@invalid.invalid> wrote:>Jim Thompson wrote: >> On Sat, 29 Jun 2013 15:49:52 GMT, John K <spam@me.not> wrote: >> >>> Joerg <invalid@invalid.invalid> wrote: >>> >>>>> On Sat, 29 Jun 2013 08:00:01 -0700, Joerg <invalid@invalid.invalid> >>>>>>>> Oops, sorry, your are right. At a 5V-tolerant pin it would work. >>>>>>>> Problem is that we only have 5V in the area where the protectors >>>>>>>> are. >>>>>>> If you only have 5V around driving a 5V-tolerant pin... what are >>>>>>> you fretting over? >>>>>>> >>>>>> The concern are surge events that make these lines carry nasty >>>>>> transients, which in turn can lock up or damage the uC. >>>>> So you have a "dirty" 5V supply? >>>>> >>>> No, rock-stable. But clamping against it will lead to up to 7V spike >>>> amplitude if it's a huge transient. Those small clamper arrays can >>>> easily reach 2V Vf or more for a few usec. >>> If you are dealing with 5V logic signals, isn't the threshold VCC/2, or >>> around 2.5V? >>> >>> How about clamping with a 3.9V zener to ground with a small series >>> resistor to limit the transient current? As long as the desired signal >>> gets above the threshold, do you care if it goes all the way to 5V? >>> >>> For a BZX84C3V9, Fig. 2 on page 3 shows it doesn't start to conduct until >>> you get above 3V, and 50mA doesn't even reach 5V: >>> >>> http://www.pridmore.com/datasheets/BZX84CA07.pdf >>> >>> Don't know what all the tolerances are, but so far nothing else seems to >>> have any chance of working in your application. >>> >>> JK >> >> I still haven't seen described how a "rock solid" 5V supply produces >> such gross transients. >> > >It doesn't. It's the signal lines that do. > >[...]So "induced" transients? How much actual energy? ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●June 29, 20132013-06-29
Jim Thompson wrote:> On Sat, 29 Jun 2013 10:22:13 -0700, Joerg <invalid@invalid.invalid> > wrote: > >> Jim Thompson wrote: >>> On Sat, 29 Jun 2013 15:49:52 GMT, John K <spam@me.not> wrote: >>> >>>> Joerg <invalid@invalid.invalid> wrote: >>>> >>>>>> On Sat, 29 Jun 2013 08:00:01 -0700, Joerg <invalid@invalid.invalid> >>>>>>>>> Oops, sorry, your are right. At a 5V-tolerant pin it would work. >>>>>>>>> Problem is that we only have 5V in the area where the protectors >>>>>>>>> are. >>>>>>>> If you only have 5V around driving a 5V-tolerant pin... what are >>>>>>>> you fretting over? >>>>>>>> >>>>>>> The concern are surge events that make these lines carry nasty >>>>>>> transients, which in turn can lock up or damage the uC. >>>>>> So you have a "dirty" 5V supply? >>>>>> >>>>> No, rock-stable. But clamping against it will lead to up to 7V spike >>>>> amplitude if it's a huge transient. Those small clamper arrays can >>>>> easily reach 2V Vf or more for a few usec. >>>> If you are dealing with 5V logic signals, isn't the threshold VCC/2, or >>>> around 2.5V? >>>> >>>> How about clamping with a 3.9V zener to ground with a small series >>>> resistor to limit the transient current? As long as the desired signal >>>> gets above the threshold, do you care if it goes all the way to 5V? >>>> >>>> For a BZX84C3V9, Fig. 2 on page 3 shows it doesn't start to conduct until >>>> you get above 3V, and 50mA doesn't even reach 5V: >>>> >>>> http://www.pridmore.com/datasheets/BZX84CA07.pdf >>>> >>>> Don't know what all the tolerances are, but so far nothing else seems to >>>> have any chance of working in your application. >>>> >>>> JK >>> I still haven't seen described how a "rock solid" 5V supply produces >>> such gross transients. >>> >> It doesn't. It's the signal lines that do. >> >> [...] > > So "induced" transients? How much actual energy? >Hard to say how much arrives, all I know from reports is that it has frozen the uC at times. So we had to do something about it. We should be good with those clamps. I was just wondering how far things can go above abs max and for how long. Of course we won't be sending in 7V for hours. When I redesign stuff I like to make it not just good enoug to pass a test, but ideally almost bullet-proof. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●June 29, 20132013-06-29
On Sat, 29 Jun 2013 11:08:02 -0700, Joerg <invalid@invalid.invalid> wrote:>Jim Thompson wrote: >> On Sat, 29 Jun 2013 10:22:13 -0700, Joerg <invalid@invalid.invalid> >> wrote: >> >>> Jim Thompson wrote: >>>> On Sat, 29 Jun 2013 15:49:52 GMT, John K <spam@me.not> wrote: >>>> >>>>> Joerg <invalid@invalid.invalid> wrote: >>>>> >>>>>>> On Sat, 29 Jun 2013 08:00:01 -0700, Joerg <invalid@invalid.invalid> >>>>>>>>>> Oops, sorry, your are right. At a 5V-tolerant pin it would work. >>>>>>>>>> Problem is that we only have 5V in the area where the protectors >>>>>>>>>> are. >>>>>>>>> If you only have 5V around driving a 5V-tolerant pin... what are >>>>>>>>> you fretting over? >>>>>>>>> >>>>>>>> The concern are surge events that make these lines carry nasty >>>>>>>> transients, which in turn can lock up or damage the uC. >>>>>>> So you have a "dirty" 5V supply? >>>>>>> >>>>>> No, rock-stable. But clamping against it will lead to up to 7V spike >>>>>> amplitude if it's a huge transient. Those small clamper arrays can >>>>>> easily reach 2V Vf or more for a few usec. >>>>> If you are dealing with 5V logic signals, isn't the threshold VCC/2, or >>>>> around 2.5V? >>>>> >>>>> How about clamping with a 3.9V zener to ground with a small series >>>>> resistor to limit the transient current? As long as the desired signal >>>>> gets above the threshold, do you care if it goes all the way to 5V? >>>>> >>>>> For a BZX84C3V9, Fig. 2 on page 3 shows it doesn't start to conduct until >>>>> you get above 3V, and 50mA doesn't even reach 5V: >>>>> >>>>> http://www.pridmore.com/datasheets/BZX84CA07.pdf >>>>> >>>>> Don't know what all the tolerances are, but so far nothing else seems to >>>>> have any chance of working in your application. >>>>> >>>>> JK >>>> I still haven't seen described how a "rock solid" 5V supply produces >>>> such gross transients. >>>> >>> It doesn't. It's the signal lines that do. >>> >>> [...] >> >> So "induced" transients? How much actual energy? >> > >Hard to say how much arrives, all I know from reports is that it has >frozen the uC at times. So we had to do something about it. We should be > good with those clamps. I was just wondering how far things can go >above abs max and for how long. Of course we won't be sending in 7V for >hours. When I redesign stuff I like to make it not just good enoug to >pass a test, but ideally almost bullet-proof.The 7V limit only applies to those pins which are configurable I/O. If it's an input-only pin, it can stand a whole lot more... some processes can be as high as 50V. But you said uC... they're probably ALL I/O :-( ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●June 29, 20132013-06-29
Jim Thompson wrote:> On Sat, 29 Jun 2013 11:08:02 -0700, Joerg <invalid@invalid.invalid> > wrote: > >> Jim Thompson wrote: >>> On Sat, 29 Jun 2013 10:22:13 -0700, Joerg <invalid@invalid.invalid> >>> wrote: >>> >>>> Jim Thompson wrote: >>>>> On Sat, 29 Jun 2013 15:49:52 GMT, John K <spam@me.not> wrote: >>>>> >>>>>> Joerg <invalid@invalid.invalid> wrote: >>>>>> >>>>>>>> On Sat, 29 Jun 2013 08:00:01 -0700, Joerg <invalid@invalid.invalid> >>>>>>>>>>> Oops, sorry, your are right. At a 5V-tolerant pin it would work. >>>>>>>>>>> Problem is that we only have 5V in the area where the protectors >>>>>>>>>>> are. >>>>>>>>>> If you only have 5V around driving a 5V-tolerant pin... what are >>>>>>>>>> you fretting over? >>>>>>>>>> >>>>>>>>> The concern are surge events that make these lines carry nasty >>>>>>>>> transients, which in turn can lock up or damage the uC. >>>>>>>> So you have a "dirty" 5V supply? >>>>>>>> >>>>>>> No, rock-stable. But clamping against it will lead to up to 7V spike >>>>>>> amplitude if it's a huge transient. Those small clamper arrays can >>>>>>> easily reach 2V Vf or more for a few usec. >>>>>> If you are dealing with 5V logic signals, isn't the threshold VCC/2, or >>>>>> around 2.5V? >>>>>> >>>>>> How about clamping with a 3.9V zener to ground with a small series >>>>>> resistor to limit the transient current? As long as the desired signal >>>>>> gets above the threshold, do you care if it goes all the way to 5V? >>>>>> >>>>>> For a BZX84C3V9, Fig. 2 on page 3 shows it doesn't start to conduct until >>>>>> you get above 3V, and 50mA doesn't even reach 5V: >>>>>> >>>>>> http://www.pridmore.com/datasheets/BZX84CA07.pdf >>>>>> >>>>>> Don't know what all the tolerances are, but so far nothing else seems to >>>>>> have any chance of working in your application. >>>>>> >>>>>> JK >>>>> I still haven't seen described how a "rock solid" 5V supply produces >>>>> such gross transients. >>>>> >>>> It doesn't. It's the signal lines that do. >>>> >>>> [...] >>> So "induced" transients? How much actual energy? >>> >> Hard to say how much arrives, all I know from reports is that it has >> frozen the uC at times. So we had to do something about it. We should be >> good with those clamps. I was just wondering how far things can go >> above abs max and for how long. Of course we won't be sending in 7V for >> hours. When I redesign stuff I like to make it not just good enoug to >> pass a test, but ideally almost bullet-proof. > > The 7V limit only applies to those pins which are configurable I/O. If > it's an input-only pin, it can stand a whole lot more... some > processes can be as high as 50V. >They state a 5.8V abs max, 7V is just what one would typically see if a monster pulse comes along and leans into the clamp diodes.> But you said uC... they're probably ALL I/O :-( >Yup, all I/O :-( -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●June 29, 20132013-06-29
On Sat, 29 Jun 2013 11:18:00 -0700, Joerg <invalid@invalid.invalid> wrote:>Jim Thompson wrote: >> On Sat, 29 Jun 2013 11:08:02 -0700, Joerg <invalid@invalid.invalid> >> wrote: >> >>> Jim Thompson wrote: >>>> On Sat, 29 Jun 2013 10:22:13 -0700, Joerg <invalid@invalid.invalid> >>>> wrote: >>>> >>>>> Jim Thompson wrote: >>>>>> On Sat, 29 Jun 2013 15:49:52 GMT, John K <spam@me.not> wrote: >>>>>> >>>>>>> Joerg <invalid@invalid.invalid> wrote: >>>>>>> >>>>>>>>> On Sat, 29 Jun 2013 08:00:01 -0700, Joerg <invalid@invalid.invalid> >>>>>>>>>>>> Oops, sorry, your are right. At a 5V-tolerant pin it would work. >>>>>>>>>>>> Problem is that we only have 5V in the area where the protectors >>>>>>>>>>>> are. >>>>>>>>>>> If you only have 5V around driving a 5V-tolerant pin... what are >>>>>>>>>>> you fretting over? >>>>>>>>>>> >>>>>>>>>> The concern are surge events that make these lines carry nasty >>>>>>>>>> transients, which in turn can lock up or damage the uC. >>>>>>>>> So you have a "dirty" 5V supply? >>>>>>>>> >>>>>>>> No, rock-stable. But clamping against it will lead to up to 7V spike >>>>>>>> amplitude if it's a huge transient. Those small clamper arrays can >>>>>>>> easily reach 2V Vf or more for a few usec. >>>>>>> If you are dealing with 5V logic signals, isn't the threshold VCC/2, or >>>>>>> around 2.5V? >>>>>>> >>>>>>> How about clamping with a 3.9V zener to ground with a small series >>>>>>> resistor to limit the transient current? As long as the desired signal >>>>>>> gets above the threshold, do you care if it goes all the way to 5V? >>>>>>> >>>>>>> For a BZX84C3V9, Fig. 2 on page 3 shows it doesn't start to conduct until >>>>>>> you get above 3V, and 50mA doesn't even reach 5V: >>>>>>> >>>>>>> http://www.pridmore.com/datasheets/BZX84CA07.pdf >>>>>>> >>>>>>> Don't know what all the tolerances are, but so far nothing else seems to >>>>>>> have any chance of working in your application. >>>>>>> >>>>>>> JK >>>>>> I still haven't seen described how a "rock solid" 5V supply produces >>>>>> such gross transients. >>>>>> >>>>> It doesn't. It's the signal lines that do. >>>>> >>>>> [...] >>>> So "induced" transients? How much actual energy? >>>> >>> Hard to say how much arrives, all I know from reports is that it has >>> frozen the uC at times. So we had to do something about it. We should be >>> good with those clamps. I was just wondering how far things can go >>> above abs max and for how long. Of course we won't be sending in 7V for >>> hours. When I redesign stuff I like to make it not just good enoug to >>> pass a test, but ideally almost bullet-proof. >> >> The 7V limit only applies to those pins which are configurable I/O. If >> it's an input-only pin, it can stand a whole lot more... some >> processes can be as high as 50V. >> > >They state a 5.8V abs max, 7V is just what one would typically see if a >monster pulse comes along and leans into the clamp diodes. > > >> But you said uC... they're probably ALL I/O :-( >> > >Yup, all I/O :-(If you can get a handle on the energy involved (*), I can show you some simple clamps that are reliable and tight on the limits. (*) Just patch in a diode to +5V and current probe the data line. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●June 29, 20132013-06-29
Jim Thompson wrote:> On Sat, 29 Jun 2013 11:18:00 -0700, Joerg <invalid@invalid.invalid> > wrote: > >> Jim Thompson wrote: >>> On Sat, 29 Jun 2013 11:08:02 -0700, Joerg <invalid@invalid.invalid> >>> wrote: >>> >>>> Jim Thompson wrote: >>>>> On Sat, 29 Jun 2013 10:22:13 -0700, Joerg <invalid@invalid.invalid> >>>>> wrote: >>>>> >>>>>> Jim Thompson wrote: >>>>>>> On Sat, 29 Jun 2013 15:49:52 GMT, John K <spam@me.not> wrote: >>>>>>> >>>>>>>> Joerg <invalid@invalid.invalid> wrote: >>>>>>>> >>>>>>>>>> On Sat, 29 Jun 2013 08:00:01 -0700, Joerg <invalid@invalid.invalid> >>>>>>>>>>>>> Oops, sorry, your are right. At a 5V-tolerant pin it would work. >>>>>>>>>>>>> Problem is that we only have 5V in the area where the protectors >>>>>>>>>>>>> are. >>>>>>>>>>>> If you only have 5V around driving a 5V-tolerant pin... what are >>>>>>>>>>>> you fretting over? >>>>>>>>>>>> >>>>>>>>>>> The concern are surge events that make these lines carry nasty >>>>>>>>>>> transients, which in turn can lock up or damage the uC. >>>>>>>>>> So you have a "dirty" 5V supply? >>>>>>>>>> >>>>>>>>> No, rock-stable. But clamping against it will lead to up to 7V spike >>>>>>>>> amplitude if it's a huge transient. Those small clamper arrays can >>>>>>>>> easily reach 2V Vf or more for a few usec. >>>>>>>> If you are dealing with 5V logic signals, isn't the threshold VCC/2, or >>>>>>>> around 2.5V? >>>>>>>> >>>>>>>> How about clamping with a 3.9V zener to ground with a small series >>>>>>>> resistor to limit the transient current? As long as the desired signal >>>>>>>> gets above the threshold, do you care if it goes all the way to 5V? >>>>>>>> >>>>>>>> For a BZX84C3V9, Fig. 2 on page 3 shows it doesn't start to conduct until >>>>>>>> you get above 3V, and 50mA doesn't even reach 5V: >>>>>>>> >>>>>>>> http://www.pridmore.com/datasheets/BZX84CA07.pdf >>>>>>>> >>>>>>>> Don't know what all the tolerances are, but so far nothing else seems to >>>>>>>> have any chance of working in your application. >>>>>>>> >>>>>>>> JK >>>>>>> I still haven't seen described how a "rock solid" 5V supply produces >>>>>>> such gross transients. >>>>>>> >>>>>> It doesn't. It's the signal lines that do. >>>>>> >>>>>> [...] >>>>> So "induced" transients? How much actual energy? >>>>> >>>> Hard to say how much arrives, all I know from reports is that it has >>>> frozen the uC at times. So we had to do something about it. We should be >>>> good with those clamps. I was just wondering how far things can go >>>> above abs max and for how long. Of course we won't be sending in 7V for >>>> hours. When I redesign stuff I like to make it not just good enoug to >>>> pass a test, but ideally almost bullet-proof. >>> The 7V limit only applies to those pins which are configurable I/O. If >>> it's an input-only pin, it can stand a whole lot more... some >>> processes can be as high as 50V. >>> >> They state a 5.8V abs max, 7V is just what one would typically see if a >> monster pulse comes along and leans into the clamp diodes. >> >> >>> But you said uC... they're probably ALL I/O :-( >>> >> Yup, all I/O :-( > > If you can get a handle on the energy involved (*), I can show you > some simple clamps that are reliable and tight on the limits. > > (*) Just patch in a diode to +5V and current probe the data line. >I could design some clamps that are more precise but there is so little space that just adding the teeny clamp arrays was already a challenge. Also, it's thousands of miles away. -- Regards, Joerg http://www.analogconsultants.com/
