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

[JJ]

I would like to interface an IC with CMOS 5V output to an CMOS 3.3V
input which can have max 3.3V+0.5V. The simplest method I can think of
is using a serial resistor with value 5V-3.3V/Imax. But I don't see
this method used a lot, specialy not with CMOS I/O. Can somebody please
tell me why?

Cheers,
JJ

 

[Dave Boland]

I would like to interface an IC with CMOS 5V output to an CMOS 3.3V
input which can have max 3.3V+0.5V. The simplest method I can think of
is using a serial resistor with value 5V-3.3V/Imax. But I don't see
this method used a lot, specialy not with CMOS I/O. Can somebody please
tell me why?

Cheers,
JJ

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.
http://www.fairchildsemi.com/products/logic/lowvolt/#lcx

Dave,

 

[Paul Burke]

I would like to interface an IC with CMOS 5V output to an CMOS 3.3V
input which can have max 3.3V+0.5V. The simplest method I can think of
is using a serial resistor with value 5V-3.3V/Imax. But I don't see
this method used a lot, specialy not with CMOS I/O. Can somebody please
tell me why?

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

 

[Eeyore]

I would like to interface an IC with CMOS 5V output to an CMOS 3.3V
input which can have max 3.3V+0.5V. The simplest method I can think of
is using a serial resistor with value 5V-3.3V/Imax. But I don't see
this method used a lot, specialy not with CMOS I/O. Can somebody please
tell me why?

You need parts with '5V tolerant' inputs e.g.
http://focus.ti.com/docs/prod/folders/print/sn54lvc541a.html

Graham

 

[Eeyore]

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).

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

 

[[email protected]]

I would like to interface an IC with CMOS 5V output to an CMOS 3.3V
input which can have max 3.3V+0.5V. The simplest method I can think of
is using a serial resistor with value 5V-3.3V/Imax. But I don't see
this method used a lot, specialy not with CMOS I/O. Can somebody please
tell me why?

Cheers,
JJ

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

 

[Eeyore]

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.

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

Graham

 

[[email protected]]

I would like to interface an IC with CMOS 5V output to an CMOS 3.3V
input which can have max 3.3V+0.5V. The simplest method I can think of

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.

 

[colin]

I would like to interface an IC with CMOS 5V output to an CMOS 3.3V
input which can have max 3.3V+0.5V. The simplest method I can think of
is using a serial resistor with value 5V-3.3V/Imax. But I don't see
this method used a lot, specialy not with CMOS I/O. Can somebody please
tell me why?

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 =^.^=

 

[JJ]

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
(http://at.farnell.com) and RS (http://www.rs-components.at) in Austria
don't have those parts!! Really sad...;-(((

 

[Al Clark]

[email protected] wrote in @b28g2000cwb.googlegroups.com:

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.

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

 

[Noway2]

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

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.

 

[krw]

I would like to interface an IC with CMOS 5V output to an CMOS 3.3V
input which can have max 3.3V+0.5V.

I've used IDT Quick Switches tin high speed applications. They're
*FAST* (advertised as "zero delay").

Application note for the level translators:
http://www.idt.com/products/files/7530/AN_11.pdf

The simplest method I can think of
is using a serial resistor with value 5V-3.3V/Imax. But I don't see
this method used a lot, specialy not with CMOS I/O. Can somebody please
tell me why?

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

 

[Jim Thompson]

I've used IDT Quick Switches tin high speed applications. They're
*FAST* (advertised as "zero delay").

Application note for the level translators:
http://www.idt.com/products/files/7530/AN_11.pdf


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

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

...Jim Thompson

 

[Keith]

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

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).