A high-speed digital voltage level shifter design? - Page 1
Forum section: electronics / beginners

author: Syntax Error
author_rank: Frequent Contributor
timestamp: November 16, 2020, 10:15:28 pm
content: Dear community...The objective is to create a circuit that shifts digital signals from a voltage of 2V5 thru 3V3, to a stable 5Vbut, does not degrade (or destroy) the input which can be up to 1MHz. The problem is when using a simple MOSFET level shifter, the RC charge time of the MOSFET means this will only function from DC to 10KHz. A TXB0108 level shifter was tried, but this turned square waves into butcher's knives beyond 100KHz. Are there any clever (and simple) methods that use, for example, op-amps as the level shifter? The data only needs to flow in one direction.Thanks.

author: Ian.M
author_rank: Super Contributor
timestamp: November 16, 2020, 10:29:49 pm
content: 74LVC1T45, 74LVC2T45 or 74LVC8T245 depending on whether you want 1, 2 or 8 signals translated. Tie the DIR pin (and on '8T245 the /OE pin) for unidirectional operation.

author: langwadt
author_rank: Super Contributor
timestamp: November 16, 2020, 10:36:49 pm
content: Quote from: Syntax Error on November 16, 2020, 10:15:28 pmany 74HCT gate (needs 5V supply)

author: BrianHG
author_rank: Super Contributor
timestamp: November 17, 2020, 12:35:00 am
content: Quote from: langwadt on November 16, 2020, 10:36:49 pmWhere did you read that?

author: Syntax Error
author_rank: Frequent Contributor
timestamp: November 17, 2020, 01:02:51 am
content: Quote from: Ian.M on November 16, 2020, 10:29:49 pmThank you for that suggestionThe datasheet for the 74LVC1T45 looks promising. Plus the package is small and cheap. I'll add a few to my next 'stuff' order to have a play with

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 17, 2020, 01:11:30 am
content: What supply current is allowable?If you pick smaller transistors (including 74HC or LVC05 or 07) and smaller pull-up resistors, the pulse goes through just fine.Transistors like 2N7002 are rather bulky affairs (~30pF Cout) and go slowly as claimed, with large pullups (10k+). They're quite fast if you don't happen to mind drawing 10s or 100s of mA.Those automatic bidirectional level shifters are a hack job. They work only in very specific cases.Tim

author: langwadt
author_rank: Super Contributor
timestamp: November 17, 2020, 01:32:52 am
content: Quote from: BrianHG on November 17, 2020, 12:35:00 amthe datasheetHCT is only spec'd for 5V and Vih = 2V

author: JustMeHere
author_rank: Frequent Contributor
timestamp: November 17, 2020, 01:58:51 am
content: 3v3 is enough to drive a 5v input directly. 2v5 is real close. See this:https://www.allaboutcircuits.com/textbook/digital/chpt-3/logic-signal-voltage-levels/

author: langwadt
author_rank: Super Contributor
timestamp: November 17, 2020, 09:40:42 am
content: Quote from: JustMeHere on November 17, 2020, 01:58:51 amonly for 5V TTL, not 5V CMOS

author: Yansi
author_rank: Super Contributor
timestamp: November 17, 2020, 10:04:14 am
content: Quote from: JustMeHere on November 17, 2020, 01:58:51 amIt is definitely NOT.5V CMOS logic uses 0.7Vdd Vih and higher for logic 1, that is 3.5V. 3V3 is not enough.But what 3V3 could be directly interfaced with, is a TTL logic, as they have 2.4V Vih minimum. So 74HCTgates work well as a 3V3 to 5V converters.2V5 can not be interfaced directly, because 2V5 CMOS high is also 0.7 Vdd Voh, ie from 1.75V to 2.5V is high. Not enough for TTL.

author: Zero999
author_rank: Super Contributor
timestamp: November 17, 2020, 01:15:49 pm
content: Quote from: Yansi on November 17, 2020, 10:04:14 amHCT should work for 2.5V, although I wouldn't advise going any lower than that.Here are some data sheets of parts which will convert 2.5V logic to 5V.https://www.diodes.com/assets/Datasheets/74AHCT125.pdfhttps://assets.nexperia.com/documents/data-sheet/74AHC_AHCT1G17.pdfhttps://assets.nexperia.com/documents/data-sheet/74HC_HCT2G17.pdfhttps://assets.nexperia.com/documents/data-sheet/74HC_HCT7541.pdf

author: Yansi
author_rank: Super Contributor
timestamp: November 17, 2020, 02:46:39 pm
content: Okay, I admit these will work. But I recall the HCT family was originally for TTL compatibility, which means required Vih(min) of 2.4V, so 2.5V supplied CMOS just barely fits.

author: langwadt
author_rank: Super Contributor
timestamp: November 17, 2020, 02:52:01 pm
content: Quote from: Yansi on November 17, 2020, 02:46:39 pmafair it is 2.0Vih 2.4Voh

author: Yansi
author_rank: Super Contributor
timestamp: November 17, 2020, 02:53:43 pm
content: Jsut looking through some old data books, Ur probably correct, 2.4V was Uoh.

author: Syntax Error
author_rank: Frequent Contributor
timestamp: November 17, 2020, 03:43:18 pm
content: FYI guys. Used as a 3 to 5 volt level shifter with 'arduino' speed controllers and neopixels, the 74HCT244 is an 8-bit buffer/line driver with 3-state outputs. I was in search of a compact single line device, but this baby would do on the breadboard.For all of you tech heads, an interesting application note from Texas Instruments about selecting level translators:QuoteSelecting the Right Level-Translation Solution [url=https://www.ti.com/lit/an/scea035a/scea035a.pdf?ts=1605605748578]https://www.ti.com/lit/an/scea035a/scea035a.pdf

author: JustMeHere
author_rank: Frequent Contributor
timestamp: November 18, 2020, 03:11:58 am
content: Quote from: Yansi on November 17, 2020, 10:04:14 amExcept I have a ESP32 driving a 5V level shifter at 20MHz.The pxmatrix project is proof.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 19, 2020, 10:42:00 pm
content: Are you not satisfied with a bipolar transistor connected to a common base circuit?

author: David Hess
author_rank: Super Contributor
timestamp: November 20, 2020, 12:31:25 am
content: Quote from: S. Petrukhin on November 19, 2020, 10:42:00 pmAnd if that is not good enough, follow it with complementary emitter followers for increased drive.Check out appendix E of Linear Technology application note 13 for ideas on fast discrete level shifters.Incidentally, 1 MHz is not particularly demanding.The simplest solution has already been given, HCT or ACT logic with parallel gates if necessary.