Interfacing 5V -> 3.3V with just a serial resistor

JJ,

This idea is full of problems (in most cases). A better way is to interface the signals with 5 volt tolerant 3.3 volt logic device such as the LVX family. See link below.

Dave,

This can lead to all sorts of problems, the biggest being that the 3.3V device can be powered from the 5V supply, via the clamp diodes. Since most regulators can't sink current, the "3.3V" zooms up to about 4V, with deleterious effects.

Use a resistive divider (slow and current- hungry), resistor/zener (slow, expensive and current hungry), or a level- shifting IC like 74LVC series (fast and expensive).

Paul Burke

You need parts with '5V tolerant' inputs e.g.

Graham

Are they actually that expensive ?

I just checked 74LVC00 and 74HC00 on ti.com and there's only a couple of cents in it.

Graham

The method works fine, but there are a few things to watch out for.

With most devices, any attempts to drive more than Vdd +0.6 V will turn on a protection diode between the input pin and Vdd. This is fine, so long as the current is low (no more than a few mA) AND the power supply voltage of the low-voltage device will not be made to rise unacceptably by this extra current.

If too high a current (usually >100mA) is driven into the protection diode, the chip may latch up and self destruct.

All these problems are solved by using two resistors as a resistive divider.

The final thing to look out for is the effect on the speed of the circuit. It will be slowed down in proportion to the series resistance and the stray capacitance plus the device input capacitance.

John

All very true which is why an LVC part makes more sense.

Graham

You're interfacing a 5V microcontroller to a SD/MMC card?

Use a voltage divider. It makes your three resistors into six resistors, but that's not too arduous.

using Imax will cuase a lot of problems, use a lot lower current, definatly less than the minimum curent drawn by the

3.3 rail, otherwise it will push the 3.3 rail up if its not fast enough use a small parallel capacitor, so that it forms a capacitave divider with the input/stray capacitance.

Colin =^.^=

Thanks for the fast replies!

I want actually to interface a 5V quadrature encoder with a XOR gate. I was planing to get a 5V tolerant gate, but our supplieres Farnell

and RS in Austria don't have those parts!! Really sad...;-(((

snipped-for-privacy@gmail.com wrote in news:1158838538.163960.68360 @b28g2000cwb.googlegroups.com:

Since it is an encoder, and therefore probably slow, you can probably use the resistors.

One trick is to use a 4 element resistor network instead of 3 resistors. If you parallel two of them for the series R, you will get a 2/3 divider (you have a spare R). This is often cheaper and perhaps smaller than using two discrete resistors since the labor cost more than the parts

--
Al Clark
Danville Signal Processing, Inc.
--------------------------------------------------------------------
Purveyors of Fine DSP Hardware and other Cool Stuff
Available at http://www.danvillesignal.com

In my application, I used the 74LVC4245A. They are bidirectional 3.3V 5.0V level transcievers. I think they cost about 0.90 in qty 1 from digikey, and downwards of a 0.25 in typical production quantities.

I've used IDT Quick Switches tin high speed applications. They're

*FAST* (advertised as "zero delay").

Application note for the level translators:

It'll be slow, but it should work with CMOS.

--
  Keith

The NPN/2-resistor scheme can be rearranged to do +5V to +3.3V translation.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

The neat thing about the QuickSwitches is that they'll go both ways and *fast* (though from 3.3V->5V you may need a pullup, somewhat slowing things that direction).

--
  Keith