Lots of CMOS gates and such are tolerant of pin voltages above Vcc, typically rated for 7 volts max. Does anybody know what sort of input circuits are typically used? Does it behave like a zener? To ground? Some parts specify an allowed input curent in the pullup direction, some don't. I'm designing a really tiny board, and an opamp can potentially overdrive an analog mux, an FSA3157. I can current limit it with a "free" resistor, in a pack, but a diode would take more room. -- 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
5-volt tolerant CMOS
Started by ●October 20, 2013
Reply by ●October 20, 20132013-10-20
On 10/20/2013 2:03 PM, John Larkin wrote:> > > Lots of CMOS gates and such are tolerant of pin voltages above Vcc, typically > rated for 7 volts max. Does anybody know what sort of input circuits are > typically used? Does it behave like a zener? To ground? > > Some parts specify an allowed input curent in the pullup direction, some don't. > > I'm designing a really tiny board, and an opamp can potentially overdrive an > analog mux, an FSA3157. I can current limit it with a "free" resistor, in a > pack, but a diode would take more room.Overdriving an analogue mux can have a lot of nasty effects without blowing up the part, e.g. connecting all the inputs together. How about a nice voltage divider to prevent the overdrive in the first place? Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 USA +1 845 480 2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●October 20, 20132013-10-20
On Sun, 20 Oct 2013 14:07:41 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 10/20/2013 2:03 PM, John Larkin wrote: >> >> >> Lots of CMOS gates and such are tolerant of pin voltages above Vcc, typically >> rated for 7 volts max. Does anybody know what sort of input circuits are >> typically used? Does it behave like a zener? To ground? >> >> Some parts specify an allowed input curent in the pullup direction, some don't. >> >> I'm designing a really tiny board, and an opamp can potentially overdrive an >> analog mux, an FSA3157. I can current limit it with a "free" resistor, in a >> pack, but a diode would take more room. > >Overdriving an analogue mux can have a lot of nasty effects without >blowing up the part, e.g. connecting all the inputs together. How about >a nice voltage divider to prevent the overdrive in the first place? >I don't mind the mux getting weird, I just don't want to fry it. If I absolutely have to add a part, it could be a tiny schottky diode to +5. I just begrudge every part on this schematic; the PCB will be under 2 square inches. I'll be using a dual opamp and the other half needs to be powered from +12, so the one that I'm gain-switching x1/x10 with the mux is +12 too. But I've been generally interested in finding out what's inside those "5 volt tolerant" cmos parts. The positive direction must have some sort of zener-y thing, for ESD. -- 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
Reply by ●October 20, 20132013-10-20
Den s=F8ndag den 20. oktober 2013 21.24.53 UTC+2 skrev John Larkin:> On Sun, 20 Oct 2013 14:07:41 -0400, Phil Hobbs >=20 > <pcdhSpamMeSenseless@electrooptical.net> wrote: >=20 >=20 >=20 > >On 10/20/2013 2:03 PM, John Larkin wrote: >=20 > >> >=20 > >> >=20 > >> Lots of CMOS gates and such are tolerant of pin voltages above Vcc, ty=pically>=20 > >> rated for 7 volts max. Does anybody know what sort of input circuits a=re>=20 > >> typically used? Does it behave like a zener? To ground? >=20 > >> >=20 > >> Some parts specify an allowed input curent in the pullup direction, so=me don't.>=20 > >> >=20 > >> I'm designing a really tiny board, and an opamp can potentially overdr=ive an>=20 > >> analog mux, an FSA3157. I can current limit it with a "free" resistor,=in a>=20 > >> pack, but a diode would take more room. >=20 > > >=20 > >Overdriving an analogue mux can have a lot of nasty effects without=20 >=20 > >blowing up the part, e.g. connecting all the inputs together. How about==20>=20 > >a nice voltage divider to prevent the overdrive in the first place? >=20 > > >=20 >=20 >=20 > I don't mind the mux getting weird, I just don't want to fry it. If I abs=olutely>=20 > have to add a part, it could be a tiny schottky diode to +5. I just begru=dge>=20 > every part on this schematic; the PCB will be under 2 square inches. I'll=be>=20 > using a dual opamp and the other half needs to be powered from +12, so th=e one>=20 > that I'm gain-switching x1/x10 with the mux is +12 too. >=20 >=20 >=20 > But I've been generally interested in finding out what's inside those "5 =volt>=20 > tolerant" cmos parts. The positive direction must have some sort of zener=-y>=20 > thing, for ESD. >=20the datasheet for the old xilinx fpgas that had 5V tolerant pins for 5V pci it said a zener like structure to ground afaict from the datasheet for that mux it is only the control input that=20 can handle 7V independent of supply, the analog is Vcc+0.5 -Lasse
Reply by ●October 20, 20132013-10-20
John Larkin wrote:> > Lots of CMOS gates and such are tolerant of pin voltages above Vcc, typically > rated for 7 volts max. Does anybody know what sort of input circuits are > typically used? Does it behave like a zener? To ground? >Usually there will be parasitic substrate diodes. One to the negative supply (mostly that would be ground) and one to VCC. There are usually no zeners involved on CMOS logic gates. If I deliberately bang into these I try to stay under 10% of abs max. But don't do that in multi-pack device for lots of sections at the same time because there will always be a total chip maximum to avoid latch-up or vaporizing a bond wire.> Some parts specify an allowed input curent in the pullup direction, some don't. >If in doubt, ask. I have use the SD5400 mux in unorthodox ways after they gave me some additional information.> I'm designing a really tiny board, and an opamp can potentially overdrive an > analog mux, an FSA3157. I can current limit it with a "free" resistor, in a > pack, but a diode would take more room. >Since they give an upper limit for Vs of VCC + 0.5V that kind of hints that a parasitic diode would come on. Strangely, then they don't give a parasirtic diode current rating for the signal path. For a 7ohms Rdson device that should be beefy but I'd ask. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 20, 20132013-10-20
On Sun, 20 Oct 2013 11:03:02 -0700, John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:> > >Lots of CMOS gates and such are tolerant of pin voltages above Vcc, typically >rated for 7 volts max. Does anybody know what sort of input circuits are >typically used? Does it behave like a zener? To ground? > >Some parts specify an allowed input curent in the pullup direction, some don't. > >I'm designing a really tiny board, and an opamp can potentially overdrive an >analog mux, an FSA3157. I can current limit it with a "free" resistor, in a >pack, but a diode would take more room."5V Tolerant" parts do not have an ESD diode to VDD. They instead have what is called a "snap" diode to ground... actually a tricky NMOS structure that avalanches and eats the ESD pulse. So... As long as you don't approach +7V with an ultra-fast pulse (*) you should be good to go. (*) A _really_ fast edge may be interpreted as an ESD event and trigger the "snap" diode... it's triggering is rate dependent. ...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 ●October 20, 20132013-10-20
Jim Thompson wrote:> On Sun, 20 Oct 2013 11:03:02 -0700, John Larkin > <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >> >> Lots of CMOS gates and such are tolerant of pin voltages above Vcc, typically >> rated for 7 volts max. Does anybody know what sort of input circuits are >> typically used? Does it behave like a zener? To ground? >> >> Some parts specify an allowed input curent in the pullup direction, some don't. >> >> I'm designing a really tiny board, and an opamp can potentially overdrive an >> analog mux, an FSA3157. I can current limit it with a "free" resistor, in a >> pack, but a diode would take more room. > > "5V Tolerant" parts do not have an ESD diode to VDD. They instead > have what is called a "snap" diode to ground... actually a tricky NMOS > structure that avalanches and eats the ESD pulse. > > So... As long as you don't approach +7V with an ultra-fast pulse (*) > you should be good to go. > > (*) A _really_ fast edge may be interpreted as an ESD event and > trigger the "snap" diode... it's triggering is rate dependent. >Are you sure this also goes for the signal path here? The datasheet looks like that may apply for the logic input but the signal range is stated 500mV beyond either rail abs max. Top of page 3: http://www.fairchildsemi.com/ds/FS/FSA3157.pdf -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 20, 20132013-10-20
On Sun, 20 Oct 2013 12:47:23 -0700 (PDT), Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:>Den s�ndag den 20. oktober 2013 21.24.53 UTC+2 skrev John Larkin: >> On Sun, 20 Oct 2013 14:07:41 -0400, Phil Hobbs >> >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >> >> >> >On 10/20/2013 2:03 PM, John Larkin wrote: >> >> >> >> >> >> >> >> >> Lots of CMOS gates and such are tolerant of pin voltages above Vcc, typically >> >> >> rated for 7 volts max. Does anybody know what sort of input circuits are >> >> >> typically used? Does it behave like a zener? To ground? >> >> >> >> >> >> Some parts specify an allowed input curent in the pullup direction, some don't. >> >> >> >> >> >> I'm designing a really tiny board, and an opamp can potentially overdrive an >> >> >> analog mux, an FSA3157. I can current limit it with a "free" resistor, in a >> >> >> pack, but a diode would take more room. >> >> > >> >> >Overdriving an analogue mux can have a lot of nasty effects without >> >> >blowing up the part, e.g. connecting all the inputs together. How about >> >> >a nice voltage divider to prevent the overdrive in the first place? >> >> > >> >> >> >> I don't mind the mux getting weird, I just don't want to fry it. If I absolutely >> >> have to add a part, it could be a tiny schottky diode to +5. I just begrudge >> >> every part on this schematic; the PCB will be under 2 square inches. I'll be >> >> using a dual opamp and the other half needs to be powered from +12, so the one >> >> that I'm gain-switching x1/x10 with the mux is +12 too. >> >> >> >> But I've been generally interested in finding out what's inside those "5 volt >> >> tolerant" cmos parts. The positive direction must have some sort of zener-y >> >> thing, for ESD. >> > >the datasheet for the old xilinx fpgas that had 5V tolerant pins for 5V pci >it said a zener like structure to ground > >afaict from the datasheet for that mux it is only the control input that >can handle 7V independent of supply, the analog is Vcc+0.5 > >-LasseGood point, but the footnote is still ambiguous. Maybe the analog pins have esd diodes to Vcc; I can test for that. -- 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
Reply by ●October 20, 20132013-10-20
On Sun, 20 Oct 2013 15:39:11 -0700, Joerg <invalid@invalid.invalid> wrote:>Jim Thompson wrote: >> On Sun, 20 Oct 2013 11:03:02 -0700, John Larkin >> <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: >> >>> >>> Lots of CMOS gates and such are tolerant of pin voltages above Vcc, typically >>> rated for 7 volts max. Does anybody know what sort of input circuits are >>> typically used? Does it behave like a zener? To ground? >>> >>> Some parts specify an allowed input curent in the pullup direction, some don't. >>> >>> I'm designing a really tiny board, and an opamp can potentially overdrive an >>> analog mux, an FSA3157. I can current limit it with a "free" resistor, in a >>> pack, but a diode would take more room. >> >> "5V Tolerant" parts do not have an ESD diode to VDD. They instead >> have what is called a "snap" diode to ground... actually a tricky NMOS >> structure that avalanches and eats the ESD pulse. >> >> So... As long as you don't approach +7V with an ultra-fast pulse (*) >> you should be good to go. >> >> (*) A _really_ fast edge may be interpreted as an ESD event and >> trigger the "snap" diode... it's triggering is rate dependent. >> > >Are you sure this also goes for the signal path here? The datasheet >looks like that may apply for the logic input but the signal range is >stated 500mV beyond either rail abs max. Top of page 3: > >http://www.fairchildsemi.com/ds/FS/FSA3157.pdfCould be. Looks like the "5V Tolerant" only applies to the logic pins They could have made the signal path "tolerant" as well. Maybe the Fairchild engineers didn't study those old chip designs I did for them>:-}Body floating is downright trivial to implement on the VDD side. Signal mangling will occur above VDD. I doubt that physical harm will occur... it says 50mA max. Who knows. The more I read of the data sheet, the more forked tongue I see. ...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 ●October 20, 20132013-10-20
On Sunday, October 20, 2013 3:24:53 PM UTC-4, John Larkin wrote:> > have to add a part, it could be a tiny schottky diode to +5. I just begrudge > > every part on this schematic; the PCB will be under 2 square inches. I'll be > > using a dual opamp and the other half needs to be powered from +12, so the one > > that I'm gain-switching x1/x10 with the mux is +12 too.What are you switching, the feedback resistor in a basic inverting configuration, or what is it?
