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

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

  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
     
    JJ, Sep 21, 2006
    #1
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  2. JJ

    Dave Boland Guest

    JJ wrote:
    > 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,
     
    Dave Boland, Sep 21, 2006
    #2
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  3. JJ

    Paul Burke Guest

    JJ wrote:
    > 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
     
    Paul Burke, Sep 21, 2006
    #3
  4. JJ

    Eeyore Guest

    JJ wrote:

    > 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, Sep 21, 2006
    #4
  5. JJ

    Eeyore Guest

    Paul Burke wrote:

    > JJ wrote:
    > > 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).


    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
     
    Eeyore, Sep 21, 2006
    #5
  6. JJ

    Guest

    JJ wrote:
    > 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
     
    , Sep 21, 2006
    #6
  7. JJ

    Eeyore Guest

    wrote:

    > JJ wrote:
    > > 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?

    >
    > 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
     
    Eeyore, Sep 21, 2006
    #7
  8. JJ

    Guest

    JJ wrote:
    > 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.
     
    , Sep 21, 2006
    #8
  9. JJ

    colin Guest

    "JJ" <> wrote in message
    news:...
    > 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 =^.^=
     
    colin, Sep 21, 2006
    #9
  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...;-(((

    > JJ wrote:
    > > 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
     
    JJ, Sep 21, 2006
    #10
  11. JJ

    Al Clark Guest

    wrote in news:1158838538.163960.68360
    @b28g2000cwb.googlegroups.com:

    >
    > JJ wrote:
    >> 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.
    >
    >


    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
     
    Al Clark, Sep 21, 2006
    #11
  12. JJ

    Noway2 Guest

    Eeyore wrote:
    > Paul Burke wrote:
    >
    > > JJ wrote:
    > > > 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).

    >
    > 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.
     
    Noway2, Sep 21, 2006
    #12
  13. JJ

    krw Guest

    In article <>,
    says...
    > 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.

    --
    Keith
     
    krw, Sep 22, 2006
    #13
  14. JJ

    Jim Thompson Guest

    On Thu, 21 Sep 2006 22:22:57 -0400, krw <> wrote:

    >In article <>,
    > says...
    >> 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.
    >


    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.
     
    Jim Thompson, Sep 22, 2006
    #14
  15. JJ

    Keith Guest

    In article <>, To-Email-
    says...
    > On Thu, 21 Sep 2006 22:22:57 -0400, krw <> wrote:
    >
    > >In article <>,
    > > says...
    > >> 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.
    > >

    >
    > 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).

    --
    Keith
     
    Keith, Sep 22, 2006
    #15
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