Level Shift 1.8v to 5v  - Page 1
Forum section: electronics / beginners

author: Jbliss
author_rank: Regular Contributor
timestamp: March 13, 2017, 08:00:21 am
content: Hi all,I'm looking for the easiest way to convert 1.8v volts to 3.5v (or higher up to 5v) to trigger the input of a CMOS And gateThanks Joshua Bliss

author: Ian.M
author_rank: Super Contributor
timestamp: March 13, 2017, 08:15:36 am
content: Errrr...... Use a level translator, e.g.74LVC1T45

author: Jbliss
author_rank: Regular Contributor
timestamp: March 13, 2017, 08:36:17 am
content: Hi Thanks for you replay.just had a quick search is there an a IC that will do 8 or so channels?Would this work74LVC245https://core-electronics.com.au/74lvc245-breadboard-friendly-8-bit-logic-level-shifter.html?utm_source=google_shopping&gclid=Cj0KEQjwhpnGBRDKpY-My9rdutABEiQAWNcslPyuqTAy9tG4ZTgr2KyoZW5FrrwGa0-45BoFFykCIr0aAvBL8P8HAQ

author: Ian.M
author_rank: Super Contributor
timestamp: March 13, 2017, 09:02:38 am
content: Octal translators between <2V and >>3.3V are pretty rare. You may do better looking for quad ones. OTOH implementing eight lines as four of the two line74LVC2T45probably takes less board area because the higher pin count packages required for octal translators are wider.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 13, 2017, 09:27:03 am
content: 74HCT245.

author: Jbliss
author_rank: Regular Contributor
timestamp: March 13, 2017, 10:07:18 am
content: Quote from: mikeselectricstuff on March 13, 2017, 09:27:03 amHi MikeThanks for your reply.So the with the 74HCT245 I will be able to covert the 1.8 volts to 5V using a 5v Power rail. Reading the data sheet I believe it will convert as low at 1.5v to 5vhttp://au.element14.com/texas-instruments/sn74hct245n/ic-bus-transceiver-octal-74hct245/dp/9591931

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 13, 2017, 10:36:06 am
content: VIHmin is actually 2V but I think in practice you'd get away with it.

author: Ian.M
author_rank: Super Contributor
timestamp: March 13, 2017, 11:03:38 am
content: Its probably OK for a one-off but there's minimal or no noise margin for logic '1', and if you are unlucky it may never go high. That's what you get for designing a circuit outside of the datasheet limits.

author: Jbliss
author_rank: Regular Contributor
timestamp: March 13, 2017, 01:08:25 pm
content: Quote from: Ian.M on March 13, 2017, 11:03:38 amwhat would be a more suitable option?

author: Zero999
author_rank: Super Contributor
timestamp: March 13, 2017, 01:47:12 pm
content: 74LVC245https://cdn-shop.adafruit.com/datasheets/sn74lvc245a.pdf

author: Ian.M
author_rank: Super Contributor
timestamp: March 13, 2017, 01:59:29 pm
content: I've already given you a part number for the two channel (line) version - 74LVC2T45. Just use as many as you need.There are a lot of single chip options if you are starting at 3.3V, going up to 5V or if you are starting a1 1.8V-2V, going up to 3.3V but octal single chip solutions for the full range that aren't open drain don't seem to exist.e.g. Hero999's suggestion of 74LVC245 is only suitable as a down-translator or if you are driving 5V 74HCT logic, as although all its inputs are 5V tolerant, it only has one Vcc pin rated for 3.6V abs. max so cant drive higher than that.

author: Zero999
author_rank: Super Contributor
timestamp: March 13, 2017, 02:56:11 pm
content: Quote from: Ian.M on March 13, 2017, 01:59:29 pmActually the absolute maximum input voltage rating of the 74LVC2T45 is 6.5V, although operation over 3.6V is not recommended.I think it will probably work fine of 5V. If you don't want to take that risk, then run if from 3.6V, which should be compatible with most 5V logic anyway.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: March 13, 2017, 03:29:12 pm
content: Quote from: Hero999 on March 13, 2017, 02:56:11 pmHuh?Literally every part of the datasheet that mentions voltage says it's fine up to 5.5V:http://assets.nexperia.com/documents/data-sheet/74LVC_LVCH2T45.pdfWhere are you seeing that operation over 3.6V is not recommended?

author: Zero999
author_rank: Super Contributor
timestamp: March 13, 2017, 03:33:52 pm
content: Quote from: suicidaleggroll on March 13, 2017, 03:29:12 pmSorry, I meant 74LVC245. It was a typo.

author: dimmich
author_rank: Newbie
timestamp: March 13, 2017, 06:41:26 pm
content: TXB0108- 8bit bidirectional voltage-level translator from 1.2...3.6V to 1.65...5.5V. Example inthis article.Also I recommend readingsimilar articleswith mosfet circuits. But use transistors with low Vgs.

author: mbless
author_rank: Regular Contributor
timestamp: March 13, 2017, 08:12:28 pm
content: Quote from: dimmich on March 13, 2017, 06:41:26 pmIt is more expensive, but I do like the TXB0108 as well since it is 8 channel, bidirectional, and autosensing. Here's thedatasheet on TI's websiteif you don't want to click a dropbox link.

author: Ian.M
author_rank: Super Contributor
timestamp: March 13, 2017, 08:22:37 pm
content: Bidirectional autosensing = weak drive capabilities and potential trouble. You have to be very careful of capacitive loading in both power domains and it seriously restricts use of pullup or pulldown resistors. Exception: I2C fast level translating buffers with active edge enhancement, but they get there by offsetting their output logic '0' level on the bus they can detect whether its being externally driven low or by their transceiver so they can break the loop to avoid latching low, and that can play merry hob with your margins for noise immunity.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 15, 2017, 01:38:30 am
content: Unless you need bidirectionality steer well clear of those bidirectional isolators, they're evil little bastards. You barely need to look at the 'output' to make them oscillate.I once had to make 200 bodge boards in a week to fix an issue in a big installation where someone had used them to drive LED pixels via 2 feet of cable

author: Jbliss
author_rank: Regular Contributor
timestamp: March 15, 2017, 12:08:54 pm
content: Thanks everyone. I was able to pick up some 74HCT245's. And they work fantastically!!! very useful chip!Thanks for all your help.

author: Ian.M
author_rank: Super Contributor
timestamp: March 15, 2017, 12:37:48 pm
content: It's a gamble using them - with a 2.0V minimum guaranteed logic '1' threshold, and 1.8V logic driving it, although this batch appears to work, the next may not, or maybe it will fail at extreme temperatures.Assuming this is a one-off project, take the chip up to about 85 deg C with a hot air pencil and down to a bit below freezing with freezer spray, and check it still works at those extremes. Also try varying its nominal 5V Vcc +/- 0.5V.If its for production, you are just plumb crazy unless you've bought 1000 from different suppliers and done your own statistical testing.

author: Jbliss
author_rank: Regular Contributor
timestamp: March 15, 2017, 12:50:50 pm
content: No not for production!! I'm not that crazy. Just one off but I will definitely do the tests as you have recommended.

author: Ian.M
author_rank: Super Contributor
timestamp: March 15, 2017, 01:03:10 pm
content: One bit of advice I was given a good many years ago when designing something properly was to go through a copy of the datasheet with a marker pen blacking out the 'Typ.' column. You can only rely on the limiting values.

author: Jbliss
author_rank: Regular Contributor
timestamp: March 15, 2017, 11:56:25 pm
content: Just did a bit of searching to find a more suitable chip but struggling to find one in a DIP package that goes down to 1.5v or 1.8vIf anyone knowns of any.Also great Advice Ian Thanks