On Sun, 9 Apr 2023 00:29:17 -0400, bitrex <user@example.net> wrote:>On 4/8/2023 10:39 PM, John Larkin wrote: >> On Sat, 8 Apr 2023 22:22:44 -0400, bitrex <user@example.net> wrote: >> >>> On 4/8/2023 9:22 PM, Ricky wrote: >>>> I'm tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes. >>>> >>>> So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there's not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget. >>>> >>>> Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation. >>>> >>>> I'm thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors. >>>> >>>> Any thoughts? >>>> >>> >>> The common base shifter seems a good compromise between cost and >>> speed/power consumption: >>> >>> <https://next-hack.com/wp-content/uploads/2017/09/A5I9.png> >>> >>> And there's Jim Thompson's variant on it if the naked topology edges are >>> too slow: >>> >>> <https://electrooptical.net/static/oldsite/www.analog-innovations.com/SED/LogicLevelTranslator74HC14-RevA.pdf> >> >> Why not one resistor? >> >One resistor from the 5 volt output to the upper clamp diode of the >following 3.3 V CMOS seems OK for low speeds but RS-422 can go fast, do >we wanna go fast? Don't know.RS422 isn't fast! It can probably be done with a direct connection, if the driver is classic TTL and the load is modern CMOS. TTL doesn't pull up to +5. A 200 ohm resistor would limit the ESD diode current if that's a concern.
Minimilist Level Shifting
Started by ●April 8, 2023
Reply by ●April 9, 20232023-04-09
Reply by ●April 9, 20232023-04-09
On Sun, 9 Apr 2023 08:43:20 +0100, piglet <erichpwagner@hotmail.com> wrote:>On 09/04/2023 02:22, Ricky wrote: >> I'm tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes. >> >> So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there's not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget. >> >> Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation. >> >> I'm thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors. >> >> Any thoughts? >> > >If the 3.3V input has a 5V tolerant ESD structure then of course no >level shift is needed. If the 3.3V supply rail is stiff enough to accept >a few mA injection then a series resistor might be all you need. For >example 200 ohms would limit ESD diode current to 5mA and assuming 10pF >input capacity would only slowdown 2ns, > >piglet > >200 ohms was my suggestion!
Reply by ●April 9, 20232023-04-09
On Sunday, April 9, 2023 at 1:23:21 AM UTC-7, Ricky wrote:> On Sunday, April 9, 2023 at 3:44:30 AM UTC-4, whit3rd wrote: > > On Sunday, April 9, 2023 at 12:34:09 AM UTC-7, Ricky wrote: > > > On Sunday, April 9, 2023 at 3:08:46 AM UTC-4, whit3rd wrote: > > > > On Saturday, April 8, 2023 at 8:12:07 PM UTC-7, Ricky wrote: > > > > > On Saturday, April 8, 2023 at 10:22:51 PM UTC-4, bitrex wrote: > > > > > > On 4/8/2023 9:22 PM, Ricky wrote: > > > > > > > I'm tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS.> > > > If you want to power it from 5V, it's 5V tolerant; if you power it from 3.3V, it isn't. CD4049 and CD4050, > > > > powered from 3V takes 5V input, different input clamp diode structure.> > > That could work. But I'd need a circuit to shift from the 5V output of the CD4000 parts to the 3.3V device being driven. > > > > See the problem?> > No problem; 4000 series CMOS works at 3.3V just fine (not very fast, as logic goes, though). > > CMOS output isn't 5V if you don't power it from 5V.Does your 3.3V end of the problem even have a +5V supply?> But it's also not 5V tolerant.There's 5V tolerant inputs on the particular parts, CD4049 and CD4050, when running on 3.3V, was the point. If, by 'it' one means a general 4000 series part, or 74HC14, on 3.3V those 'it' items are not 5V tolerant.> If you are suggesting to use the entire circuit with the inverter, transistor and four resistors, that's just excessive.Yeah, it's a nuisance; some of the 'digital transistor' items can lower the resistor count, or if you have a 3.3V supply available that eliminates a resistor. The inverter isn't required to do the level translation, it just assures clean output (and inversion, of course). A common-base digital transistor like Rohm DTC014Y with emitter to the 3.3V logic input, base to +3.3V, and a pullup resistor from collector to +5V is the level-translator part of the JT design (for 3.3V in, 5V out) that you'd want. One resistor only.
Reply by ●April 9, 20232023-04-09
On Sunday, April 9, 2023 at 7:32:48 AM UTC-4, upsid...@downunder.com wrote:> On Sun, 9 Apr 2023 00:29:17 -0400, bitrex <us...@example.net> wrote: > > >On 4/8/2023 10:39 PM, John Larkin wrote: > >> On Sat, 8 Apr 2023 22:22:44 -0400, bitrex <us...@example.net> wrote: > >> > >>> On 4/8/2023 9:22 PM, Ricky wrote: > >>>> I'm tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes. > >>>> > >>>> So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there's not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget. > >>>> > >>>> Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation. > >>>> > >>>> I'm thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors. > >>>> > >>>> Any thoughts? > >>>> > >>> > >>> The common base shifter seems a good compromise between cost and > >>> speed/power consumption: > >>> > >>> <https://next-hack.com/wp-content/uploads/2017/09/A5I9.png> > >>> > >>> And there's Jim Thompson's variant on it if the naked topology edges are > >>> too slow: > >>> > >>> <https://electrooptical.net/static/oldsite/www.analog-innovations.com/SED/LogicLevelTranslator74HC14-RevA.pdf> > >> > >> Why not one resistor? > >> > >One resistor from the 5 volt output to the upper clamp diode of the > >following 3.3 V CMOS seems OK for low speeds but RS-422 can go fast, do > >we wanna go fast? Don't know. > Put a small capacitance across the series resistor. This will speed up > the transitions. Keep the capacitance small so that it doesn't hurt > the CMOS ESD protection diodes during transitions. A suitable > capacitance would be in the same order as the CMOS input capacitance. > > If a two resistor voltage divider is used to drop from 5 V to 3.3 V, > put capacitors in parallel with both resistors so that the reactance > ratio matches the resistor ratio. This doesn't risk the CMOS ESD > diodes, but do not use too big capacitors that might limit the TTL > output swing. > > Regarding RS-422 transceivers, don't use full speed devices, unless it > is strictly required. The high speed devices connect a lot of high > speed line noise into the receivers. There are a lot of transceivers > that are speed limited to 250 kbit/s which is especially usable for > hundred of meters long lines. The RS-422 specifies up to 100 kbit/s up > to 1200 m long lines.As I've said, I have to support MHz on the RS-422 signal. Slow devices are not suitable. Two resistors and two capacitors would start to be significant real estate. Ii could use 0402 devices, with 1.6 x 0.7 mm pads. Just two such devices are the size of a 6-XSON and the eight I would need would be four times larger. I think using a LVC2G14 is a good way to go. I'd like to get the RS-422 device in a QFN, but supply is not clear at this point. But heck, this is the one part in the BoM that continues to be a problem in any package. Most of the parts I'm using seem to have settled down and I can get what I will need for this year. I may be stuck with the TSSOP package for this part. -- Rick C. --+ Get 1,000 miles of free Supercharging --+ Tesla referral code - https://ts.la/richard11209
Reply by ●April 9, 20232023-04-09
On Sunday, April 9, 2023 at 5:58:01 AM UTC-4, Fred Bloggs wrote:> On Saturday, April 8, 2023 at 9:22:56 PM UTC-4, Ricky wrote: > > I'm tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes. > > > > So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there's not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget. > > > > Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation. > > > > I'm thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors. > > > > Any thoughts? > There's tons of -422 level shifting transceivers for 3.3V in stock. The primitive kluges you have in mind don't come close to the same performance. Most of that junk with the MOSFETs, CB transistors, and HC14s, from the days when 1Mbps was considered lightning fast, are no longer suitable for a modern application."In stock"? Perhaps, but nothing suitable for this project. I think you are trying to design without knowing the requirements. Never a good idea. -- Rick C. -+- Get 1,000 miles of free Supercharging -+- Tesla referral code - https://ts.la/richard11209
Reply by ●April 9, 20232023-04-09
Am 09.04.23 um 20:16 schrieb whit3rd:> On Sunday, April 9, 2023 at 1:23:21 AM UTC-7, Ricky wrote:> Does your 3.3V end of the problem even have a +5V supply? > >> But it's also not 5V tolerant. > > There's 5V tolerant inputs on the particular parts, CD4049 and CD4050, when running on 3.3V, was the point. > If, by 'it' one means a general 4000 series part, or 74HC14, on 3.3V those 'it' items are not 5V tolerant. > >> If you are suggesting to use the entire circuit with the inverter, transistor and four resistors, that's just excessive. > > Yeah, it's a nuisance; some of the 'digital transistor' items can lower the resistor count, or if > you have a 3.3V supply available that eliminates a resistor. The inverter isn't required to do > the level translation, it just assures clean output (and inversion, of course). > > A common-base digital transistor like Rohm DTC014Y with emitter to the 3.3V > logic input, base to +3.3V, and a pullup resistor from collector to +5V is the > level-translator part of the JT design (for 3.3V in, 5V out) that you'd want. One resistor only.< https://assets.nexperia.com/documents/data-sheet/74LVC1G125.pdf > There is also an 1G04. Gerhard
Reply by ●April 9, 20232023-04-09
On Sunday, April 9, 2023 at 8:31:46 AM UTC-4, Lasse Langwadt Christensen wrote:> søndag den 9. april 2023 kl. 03.22.56 UTC+2 skrev Ricky: > > I'm tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes. > > > > So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there's not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget. > > > > Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation. > > > > I'm thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors. > > > > Any thoughts? > plenty of parts are specified for injecting several mA into the ESD diodes so all you need going from 5V to 3.3V is single resistorWhat value? Do you guarantee this for the part I'm using? Does it matter how many lines are terminated this way? That is the sort of short cut that I don't like to do. Injecting current into the ESD diodes always has risk, from unexpected events. Two resistors for two channels, is about the same board space as a single X2-DFN1410-6 which is very similar to the SOT886, 1 x 1.5 mm. I think the 74LVC2G14 is probably the best solution without playing games with ESD diodes. If the chip maker of the receiving part had an app note for interfacing to 5V this way, I might do it. But I have no interest in taking liability for replacing thousands of boards that start failing in the field after some months or years. So other than one less resistor, how is the series resistor better than a voltage divider? The 200 ohm value I see recommended, would result in around 10 mA of high level current. I'd like to use much less than this. -- Rick C. -++ Get 1,000 miles of free Supercharging -++ Tesla referral code - https://ts.la/richard11209
Reply by ●April 9, 20232023-04-09
On Sunday, April 9, 2023 at 2:16:38 PM UTC-4, whit3rd wrote:> On Sunday, April 9, 2023 at 1:23:21 AM UTC-7, Ricky wrote: > > On Sunday, April 9, 2023 at 3:44:30 AM UTC-4, whit3rd wrote: > > > On Sunday, April 9, 2023 at 12:34:09 AM UTC-7, Ricky wrote: > > > > On Sunday, April 9, 2023 at 3:08:46 AM UTC-4, whit3rd wrote: > > > > > On Saturday, April 8, 2023 at 8:12:07 PM UTC-7, Ricky wrote: > > > > > > On Saturday, April 8, 2023 at 10:22:51 PM UTC-4, bitrex wrote: > > > > > > > On 4/8/2023 9:22 PM, Ricky wrote: > > > > > > > > I'm tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. > > > > > If you want to power it from 5V, it's 5V tolerant; if you power it from 3.3V, it isn't. CD4049 and CD4050, > > > > > powered from 3V takes 5V input, different input clamp diode structure. > > > > > That could work. But I'd need a circuit to shift from the 5V output of the CD4000 parts to the 3.3V device being driven. > > > > See the problem? > > > > No problem; 4000 series CMOS works at 3.3V just fine (not very fast, as logic goes, though). > > > CMOS output isn't 5V if you don't power it from 5V. > Does your 3.3V end of the problem even have a +5V supply? > > But it's also not 5V tolerant. > There's 5V tolerant inputs on the particular parts, CD4049 and CD4050, when running on 3.3V, was the point.Ah, I was not aware that any parts in the CD4000 series were 5V tolerant when run from 3.3V. So that's like a 74LVC2G14 then, which is the part I'm thinking of using, but in a MUCH smaller package.> If, by 'it' one means a general 4000 series part, or 74HC14, on 3.3V those 'it' items are not 5V tolerant. > > If you are suggesting to use the entire circuit with the inverter, transistor and four resistors, that's just excessive. > Yeah, it's a nuisance; some of the 'digital transistor' items can lower the resistor count, or if > you have a 3.3V supply available that eliminates a resistor. The inverter isn't required to do > the level translation, it just assures clean output (and inversion, of course).The circuit with the transistor, inverter and four resistors, is a bit absurd. If the design had an unused inverter and lots of room, fine. I have neither.> A common-base digital transistor like Rohm DTC014Y with emitter to the 3.3V > logic input, base to +3.3V, and a pullup resistor from collector to +5V is the > level-translator part of the JT design (for 3.3V in, 5V out) that you'd want. One resistor only.But the pullup is either slow, or consumes excess current, or both. The 74LVC2G14 is a much better solution for the two channels I need (I can even buy them). Part of my frustration is the incredible plethora of logic families. I find it hard to track what is what and why one is better than another. Maybe I'm just getting old. I remember when 74LS was the hot thing and then they came out with 74ALS! WOW! I think we had one design that used a 74S part for speed! Thanks for the comments. -- Rick C. +-- Get 1,000 miles of free Supercharging +-- Tesla referral code - https://ts.la/richard11209
Reply by ●April 9, 20232023-04-09
søndag den 9. april 2023 kl. 20.45.32 UTC+2 skrev Ricky:> On Sunday, April 9, 2023 at 8:31:46 AM UTC-4, Lasse Langwadt Christensen wrote: > > søndag den 9. april 2023 kl. 03.22.56 UTC+2 skrev Ricky: > > > I'm tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes. > > > > > > So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there's not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget. > > > > > > Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation. > > > > > > I'm thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors. > > > > > > Any thoughts? > > plenty of parts are specified for injecting several mA into the ESD diodes so all you need going from 5V to 3.3V is single resistor > What value? Do you guarantee this for the part I'm using? Does it matter how many lines are terminated this way?you already said two lines ...
Reply by ●April 9, 20232023-04-09
On Sunday, April 9, 2023 at 3:00:33 PM UTC-4, Lasse Langwadt Christensen wrote:> søndag den 9. april 2023 kl. 20.45.32 UTC+2 skrev Ricky: > > On Sunday, April 9, 2023 at 8:31:46 AM UTC-4, Lasse Langwadt Christensen wrote: > > > søndag den 9. april 2023 kl. 03.22.56 UTC+2 skrev Ricky: > > > > I'm tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes. > > > > > > > > So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there's not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget. > > > > > > > > Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation. > > > > > > > > I'm thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors. > > > > > > > > Any thoughts? > > > plenty of parts are specified for injecting several mA into the ESD diodes so all you need going from 5V to 3.3V is single resistor > > What value? Do you guarantee this for the part I'm using? Does it matter how many lines are terminated this way? > you already said two lines ...And... -- Rick C. +-+ Get 1,000 miles of free Supercharging +-+ Tesla referral code - https://ts.la/richard11209
