On Monday, April 10, 2023 at 12:33:08 PM UTC-4, piglet wrote:> On 10/04/2023 3:32 pm, Ricky wrote: > > On Monday, April 10, 2023 at 8:03:54 AM UTC-4, piglet wrote: > >> On 09/04/2023 15:44, John Larkin wrote: > >>> On Sun, 9 Apr 2023 08:43:20 +0100, piglet <erichp...@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! > >>> > >> Great minds think alike! :) > > > > Too bad they come up with the wrong answer. What will be the current when the TTL output is high? Does anyone consider the requirements? > > > > Parameter TEST CONDITIONS MIN TYP(1) MAX UNIT > > VOH High-level output voltage VID = 200 mV, IOH = –6 mA 3.8 4.2 V > > > > We'll see if that makes it through the usenet filter. Likely not. > > > I don't get your point? If the TTL Voh is 4.2V then the current in 200R > would be 1mA (assuming 0.7V drop for the input clamp diode to 3.3Vdd) > > If even 1mA is too much then one could increase the resistor until RC > delay becomes significant.The spec on most chips is Vdd+0.3V abs max. The 4.2V is at 6 mA. 1 mA would be fine, if I believed that's the number it would be. What point will the RC delay be significant? If this part actually is only passing 1 mA, it isn't doing much regulating of the current. A small change in either power supply voltage and the current will easily double or triple, or more. Every approach I looked at has issues like this, which make it a bit awkward, or even potentially unreliable. I know people talk about injecting current into the IO pins and into the power supplies this way, but a bit too much current and it goes into latchup... fatally. There's just not enough utility in the single resistor approach to justify using it. The alternative is a single IC (a very small one) for the two lines. I started out looking for a two channel version of the 10 bit chip I had been using. It's not an easy thing to look for on Digikey or Mouser. This voltage conversion switches don't have a category of their own. So I got frustrated and started looking for a short cut. Not finding any of those that I liked, I searched for a digital buffer type part and they are small, cheap and plentiful. That's the route I will be going. The only thing that bugs me is adding another line item to the BOM, but the 200 ohm resistor does that as well. Thanks for your suggestions. -- Rick C. ---- Get 1,000 miles of free Supercharging ---- Tesla referral code - https://ts.la/richard11209
Minimilist Level Shifting
Started by ●April 8, 2023
Reply by ●April 10, 20232023-04-10
Reply by ●April 11, 20232023-04-11
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? >Series diode or two, pull down resistor. -- Cheers Clive
Reply by ●April 11, 20232023-04-11
On 11/04/2023 12:18, Clive Arthur 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? >> > > Series diode or two, pull down resistor. >Or single 1.8V series zener, if your 0V doesn't mind being ~0.7V or so. -- Cheers Clive
Reply by ●April 11, 20232023-04-11
On Tuesday, April 11, 2023 at 9:30:56 AM UTC-4, Clive Arthur wrote:> On 11/04/2023 12:18, Clive Arthur 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? > >> > > > > Series diode or two, pull down resistor. > > > Or single 1.8V series zener, if your 0V doesn't mind being ~0.7V or so.You mean +0.7V, no? -- Rick C. ---+ Get 1,000 miles of free Supercharging ---+ Tesla referral code - https://ts.la/richard11209
Reply by ●April 11, 20232023-04-11
On 11/04/2023 14:53, Ricky wrote:> On Tuesday, April 11, 2023 at 9:30:56 AM UTC-4, Clive Arthur wrote: >> On 11/04/2023 12:18, Clive Arthur 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? >>>> >>> >>> Series diode or two, pull down resistor. >>> >> Or single 1.8V series zener, if your 0V doesn't mind being ~0.7V or so. > > You mean +0.7V, no? >Yeah, bad habit, I tend to use the tilde to mean approx. -- Cheers Clive
Reply by ●April 11, 20232023-04-11
On Tuesday, April 11, 2023 at 10:12:35 AM UTC-4, Clive Arthur wrote:> On 11/04/2023 14:53, Ricky wrote: > > On Tuesday, April 11, 2023 at 9:30:56 AM UTC-4, Clive Arthur wrote: > >> On 11/04/2023 12:18, Clive Arthur 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? > >>>> > >>> > >>> Series diode or two, pull down resistor. > >>> > >> Or single 1.8V series zener, if your 0V doesn't mind being ~0.7V or so. > > > > You mean +0.7V, no? > > > Yeah, bad habit, I tend to use the tilde to mean approx.Sorry. I'm having a poor vision morning and thought it was a -. Tilde I understand. Thanks for your suggestions. -- Rick C. --+- Get 1,000 miles of free Supercharging --+- Tesla referral code - https://ts.la/richard11209
Reply by ●April 11, 20232023-04-11
On Tue, 11 Apr 2023 14:30:47 +0100, Clive Arthur <clive@nowaytoday.co.uk> wrote:>On 11/04/2023 12:18, Clive Arthur 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? >>> >> >> Series diode or two, pull down resistor. >> > >Or single 1.8V series zener, if your 0V doesn't mind being ~0.7V or so.What's the zener drop? What's the zener current? Where can I buy a 1.8v zener?
Reply by ●April 11, 20232023-04-11
tirsdag den 11. april 2023 kl. 16.27.42 UTC+2 skrev John Larkin:> On Tue, 11 Apr 2023 14:30:47 +0100, Clive Arthur > <cl...@nowaytoday.co.uk> wrote: > > >On 11/04/2023 12:18, Clive Arthur 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? > >>> > >> > >> Series diode or two, pull down resistor. > >> > > > >Or single 1.8V series zener, if your 0V doesn't mind being ~0.7V or so. > What's the zener drop? > > What's the zener current? > > Where can I buy a 1.8v zener?https://www.mouser.dk/ProductDetail/Nexperia/BZX8850S-C1V8-QYL?qs=7D1LtPJG0i3BPC7qWB9brw%3D%3D
Reply by ●April 11, 20232023-04-11
On Tuesday, April 11, 2023 at 10:34:29 AM UTC-4, Lasse Langwadt Christensen wrote:> tirsdag den 11. april 2023 kl. 16.27.42 UTC+2 skrev John Larkin: > > On Tue, 11 Apr 2023 14:30:47 +0100, Clive Arthur > > <cl...@nowaytoday.co.uk> wrote: > > > > >On 11/04/2023 12:18, Clive Arthur 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? > > >>> > > >> > > >> Series diode or two, pull down resistor. > > >> > > > > > >Or single 1.8V series zener, if your 0V doesn't mind being ~0.7V or so. > > What's the zener drop? > > > > What's the zener current? > > > > Where can I buy a 1.8v zener? > https://www.mouser.dk/ProductDetail/Nexperia/BZX8850S-C1V8-QYL?qs=7D1LtPJG0i3BPC7qWB9brw%3D%3DI actually did that once. I had a board in production where I had forgotten about the voltage difference. I added a zener in series and let it play against the protection diode. It was a really ugly solution, but could be done without a board spin. Maybe that's why I don't like the series resistor idea. This is all too kludgy for me. Is 200 ohms enough? Is 600 ohms too many? The logic parts are tiny and easy to buy. 5V tolerant regardless of Vcc. Yeah, that works great for me! -- Rick C. --++ Get 1,000 miles of free Supercharging --++ Tesla referral code - https://ts.la/richard11209
Reply by ●April 11, 20232023-04-11
On Tue, 11 Apr 2023 07:34:25 -0700 (PDT), Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:>tirsdag den 11. april 2023 kl. 16.27.42 UTC+2 skrev John Larkin: >> On Tue, 11 Apr 2023 14:30:47 +0100, Clive Arthur >> <cl...@nowaytoday.co.uk> wrote: >> >> >On 11/04/2023 12:18, Clive Arthur 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? >> >>> >> >> >> >> Series diode or two, pull down resistor. >> >> >> > >> >Or single 1.8V series zener, if your 0V doesn't mind being ~0.7V or so. >> What's the zener drop? >> >> What's the zener current? >> >> Where can I buy a 1.8v zener? > >https://www.mouser.dk/ProductDetail/Nexperia/BZX8850S-C1V8-QYL?qs=7D1LtPJG0i3BPC7qWB9brw%3D%3DWhat will the zener voltage drop be in the proposed circuit?
