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intentional Slew rate limiting

Discussion in 'Electronic Design' started by Mook Johnson, Mar 16, 2007.

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  1. Mook Johnson

    Mook Johnson Guest

    I need to make a high curent driver (say 500mA peak) that I can adjust the
    slew from say 50nS to 500nS in both directings. The input is 3.3V digital
    and the ouput is a 0- 10V square wave driving about 200ft of wire. I figure
    the lump capacitance is about 1000pF. I need to adjust he slew rate to
    reduce the amount of cross coupling from this wire to its parellel neighbor
    but I need the edges as sharp as I an get them.

    I'm looking at lower end FET drivers with resistors between the output and
    the line. Is there a way to make these have a linear slew from 0 - 10Vover
    the 25 - 250nS range mentioned?

    Any other circuit ideas for intentionally limiting the slew?
  2. I've made multiple similar circuits requiring precision slewing
    ramp rates, using high-speed amplifiers and programmable opamp
    slew-rate circuits. I doubt you need a high level of accuracy,
    so a more simple scheme could work for you. There's a concept
    called a Miller integrator, which you can implement by adding a
    feedback capacitor from the output MOSFET's drain to gate, plus
    use a series gate resistor to set the dV/dt integration rate,
    where Rgate = Vin-Vgs / dVo/dt determines the gate resistor.
  3. Lionel

    Lionel Guest

    Maybe a second FET with the drain & source between the driver output &
    a cap to ground?
  4. Phil Allison

    Phil Allison Guest

    "Mook Johnson"
    ** Why ?

    ** That would be a massive source a radio interference on AM and SW bands.

    ** 5 pF per ft - so it sure ain't co-ax.

    ** The solutions to crosstalk are well known.

    Co-axial cable, metal shielding, twisted pairs etc.

    Use one of them.

    ...... Phil
  5. Fred Bloggs

    Fred Bloggs Guest

    View in a fixed-width font such as Courier.
  6. Below is the downmarket, belt 'n braces suggestion.

    | |0.1? |
    | === \220
    \_|_ | /
    Zener,5V1 /_\ | |
    | | |/e
    23mA\|/ _|_ |\
    | /_\ |
    \ | | Linear
    430 / '-----+ +1 amp.
    \ _|_
    | \_/ -+- +15V
    0V-+- | _ |
    20mA\|/Constant. | \|
    38mA\|/Switched. | |_/|
    3.3V-+-----+------. _|_ | |
    | | | \_/ C=== -+- -5V
    |\| |/ | | |
    ---| >---|npn | +--------. | Tr/f = 10*C/19mA
    |/| |\e | | _|_ |
    | | |/ \| /_\ |
    0V--+- +----| npn |--. | |
    | |\e e/| | | |
    | | | '---+---+-0V
    | '-+-'
    | | Diodes are Schottky.
    \ \
    680/ /110
    \ \
    | |
    -5V --+--------+-----

    If C = Cstray = 50pF then Tr/f(min) = 26nS.

    If C = (470+50)pF then Tr/f = 274nS.
  7. whit3rd

    whit3rd Guest

    You ask about 'other' ideas.

    Well, there's an OLD idea that might work: an increductor.
    The inductance of a simple choke will be the slew rate limiter,
    and the inductance responds to the field in the core, so you
    make a high-impedance winding with some DC current in
    it and at high DC current it has low inductance (50 nS slew into
    load) and at low DC current it has high inductance (resulting in
    500 nS slew into load). The signal is sent through a low-impedance
    winding in the same core.

    Because it's current-programmed, it's easy to adjust. Capacitors
    with voltage-adjust are generally not of such size as to work
    for 500 nS delay at 500 mA drive.
  8. Ken Smith

    Ken Smith Guest

    Its too bad the CA3080 isn't with us today. There was a nifty circuit
    that used one of them to make slew rate limiting.

    But here's an idea

    ASCII art:

    ! !
    ! r-r opamp ---!!---+
    ---!+\ ! !
    ! >-+-/\/\----+--!-\ !
    ---/\/\--+-!-/ ! ! >--+------- Output
    ! ! GND-----!+/
    --!!-- Power op-amp

    The r-r opamp limits the slew rate by hitting its power supply rails.
    Finding a r-r opamp that recovers well from this may be the biggest
    problem with this.
  9. John  Larkin

    John Larkin Guest

    The classic slew limiter is a diode bridge...

    / \
    a a
    k k
    / \
    in---------+ +-------+------out
    \ / |
    a a |
    k k c
    \ / |
    + |
    | gnd

    where current i into the cap sets the slew rate.

  10. Fred Bartoli

    Fred Bartoli Guest

    Ken Smith a écrit :
  11. mook Johnson

    mook Johnson Guest

    The input is 3.3V digital and the ouput is a 0- 10V square wave driving

    You are correct the capacitance is actually 10,000pF or 50pf/foot. The
    cable is twisted shielded quad (4 wires inside a single shield). Wire to
    wire coupling capacitance is about 10p/ft.

    My opinion is that the situatuin is near hopeless for high speed
    communication. As soon as you get the edges steep enough to look like
    digital its going to crosstalk like a SOB.
  12. Fred Bartoli

    Fred Bartoli Guest

    mook Johnson a écrit :
    Is it quad or dual pairs?
    Do you have a ref?

    For long cables, quad is worse than twisted pairs since it's coupling
    the same way all along the path.
    Twisted pairs cancel much of this.
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