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

Discussion in 'Electronic Design' started by JJ, Sep 21, 2006.

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  1. JJ

    JJ Guest

    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
     
  2. Dave Boland

    Dave Boland Guest

    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,
     
  3. Paul Burke

    Paul Burke Guest

    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
     
  4. Eeyore

    Eeyore Guest

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

    Graham
     
  5. Eeyore

    Eeyore Guest

    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
     
  6. Guest

    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
     
  7. Eeyore

    Eeyore Guest

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

    Graham
     
  8. Guest

    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.
     
  9. colin

    colin Guest

    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 =^.^=
     
  10. JJ

    JJ Guest

    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...;-(((
     
  11. Al Clark

    Al Clark Guest

    wrote in @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
     
  12. Noway2

    Noway2 Guest

    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.
     
  13. krw

    krw Guest

    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.
     
  14. Jim Thompson

    Jim Thompson Guest

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

    ...Jim Thompson
     
  15. Keith

    Keith Guest

    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).
     
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