Is this a viable voltage translator ? 3.3v to 5v

Go To Last Post
13 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I'm trying to make a discrete fast (kind of) voltage translator, from 3.3v to 5v (AVR, Vih=3v).

I know I can usually connect them directly, but I'm assuming the 3.3v output drops below 3v, where the avr won't necessarily see it as logic high.

I have access to a 2N7002 mosfet, will it work with a 1.5-2MHz signal ?

I played around with it in LTspice and it seems to be ok, I don't have an oscilloscope to actually check the waveforms.

Attachment(s): 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

113 posts and you thought Tutorial was the right forum for this? :?

Moving to General Electronics.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Oops, I meant to put it in general electronics.
:oops:
My bad.

Can any mod move it please ?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

ItsMike wrote:
I'm trying to make a discrete fast (kind of) voltage translator, from 3.3v to 5v (AVR, Vih=3v).
No need to make since that method is in ICs from a number of manufacturers. It'll contain a pass FET and active pull-ups.

"Dare to be naïve." - Buckminster Fuller

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

See this post in the Sticky Electronics Interfacing 101, a.k.a. Tips & Tricks

A GIF is worth a thousend words   She is called Sylvia (2018), lives at Mint18.3 https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I know there are plenty of ICs that can do that, but they either have too many channels, where I only need one, or not available to me (especially those tiny 1 gate packages).

Last Edited: Mon. Jul 2, 2012 - 03:32 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Check out the last link in the topic I refer to.

A GIF is worth a thousend words   She is called Sylvia (2018), lives at Mint18.3 https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

My circuit is essentially the same as the one on that link, with the discrete mosfet.

The main question is, can this circuit (with the 2n7002) handle 1.5MHz-2MHz signal conversion ?
According to my simulation yes, but is there anything I'm missing ?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

It seems rather slow, so if you need faithful reproduction I guess it won't work; like a clock input that requires 50% duty cycle.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

This will be connected to a UART asynchronous mode.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

The issue with this FET circuit is that while it drives low, it floats high so it has to charge the input capacitance through the pullup resistor. I found that even with 2k7 or so, I was finding rise times no better than 300-400ns. As a result the edge-driven input I was driving was out of its specification, and about one time in half a million or so I would miss clock pulses.

The only effective way I found was to use active drivers.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

What if I used 2 rf npn transistors, both of them in inverter configuration one driving the other ?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

HI Ltsmike:

NPN translator are better for translation as they doesn't have the high capacitance of the 2N7000. For UART level translation they can work decently, depending on the speed your'e operating (preferably at low baudrates).

But if you have many signals to deal with, and/or the speed you are to drive it is too high, I'd suggest a chip like the 74LCX07

(http://www.fairchildsemi.com/ds/...)

A dirt cheap X6 buffers designed to be used as translators that are 5V tolerant (if you decide to power them up with the 3.3V bus) and have a threshold low enuf to accept 3.3V signals should you decide to power them with the 5V bus. Open drain and can tolerate being driven while powered off.

There are versions in single package also.

Cheerz
Nachus