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Diodes instead of an OR gate

Discussion in 'Electronic Design' started by Matt Jenkins, May 23, 2013.

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  1. Matt Jenkins

    Matt Jenkins Guest


    I've recently designed a circuit in 74LS (that I plan to redo in 74HCT) that requires just a single OR gate. Rather than haul in an entire IC I was intending to replace this with Schottky diodes on each line.

    I couldn't find a lot of information online about this being done, so I thought I would ask the good people here if there's any unwanted side-effects (beside the forward voltage drop) that I should be aware of.


  2. Matt Jenkins

    Matt Jenkins Guest

    Thanks. I opted for:

    A--->|---+---- A_OR_B

    Cheaper and more convenient for board layout than using one quarter of an 'LS32 was my thinking.

  3. Tauno Voipio

    Tauno Voipio Guest

    With 74LS circuits, your diode gate is a bad idea: The TTL logic
    is designed to sink current from the inputs into the outputs, and
    your gate is going to do just the opposite.

    With CMOS, the situation is easier, but the gate will be sloow.
  4. On the bright side, it does guarantee a proper LS TTL or CMOS level.

    Instead of using diodes, you can use a Diodes Inc. 74AHCT1G32SE-7

    The diodes could work okay (assuming HCMOS), but there will be a time
    constant on turn-off related to the resistor value and input+stray
    capacitance, and, of course, there will be static power dissipation
    due to the resistor if either input is high, and you have to make the
    resistor high enough to guarantee enough noise immunity at the output
    taking into account both the diode drop and the output voltage under
  5. whit3rd

    whit3rd Guest

    Don't. The low threshold for TTL is VERY close to the highest LS-series output-low specification.
    The best way to do an OR with LS TTL is with open-collector gates (in negative logic, two
    open collectors wired together with a pullup resistor, is an OR function).

    If this was CMOS and high voltage (5V or more), the diodes would be a good option.

    74LS00 needs 0.8V to reliably indicate LOW, and drives only to 0.4V (loaded). There's
    not any margin left if you add a diode drop.
  6. Haha, yes, try the 74LVC245, 8 outputs switching simultaneously. My
    introduction to shoot-through currents.
    In my case I was able to substitute a milder LVX part.
  7. rickman

    rickman Guest

    I've never tried to quantify noise from digital logic. I assume a
    direct decoupling cap of very low inductance didn't help? A 3 ns
    shoot-through short is just the sort of thing the power distribution
    system (PDS) is supposed to keep out of the other logic and analog.

    Did you ever figure out exactly how it was getting into the rest of the
    board? Common resistance/inductance in the power/ground traces/planes
    maybe? A better topology for your PDS might have fixed this.
  8. rickman

    rickman Guest

    TO-220 is relatively huge. Diodes are common in DIP packages which
    would be much smaller.
  9. Matt Jenkins

    Matt Jenkins Guest

    Perhaps you should consider the problem I'm trying to solve before posting. Clearly, this will not work.
  10. Matt Jenkins

    Matt Jenkins Guest

    And at $0.41 in single unit quantities, they're cheaper and simpler to implement than the SOT dual schottky I was going to use instead.

    Thanks very much, Rich. Very cool little chips.

  11. Matt Jenkins

    Matt Jenkins Guest

    This would also do the trick!
    Yeah. In the circuit in question, both inputs to the OR are normally high, and one occasionally strobes low, and I'm wanting to propagate the strobe only when the other input is also low. Speed isn't a terrible concern since the strobes would be at worst < 300Khz, but the static power dissipation isprobably worth avoiding since the rest of the circuit (if implemented in HCT) has tiny power consumption so even though it doesn't matter a lot it's probably aesthetically poor form to waste current to save a small space.
  12. whit3rd

    whit3rd Guest

    That Rds is for the OUTPUT stage; there's also internal stages
    in most CMOS, and they add to the shoot-through currents.
    Look up 'exploding wires'; I've seen pass transistors feeding
    CMOS circuits with entirely missing bond wires, after failing
    under a load that my VOM claimed was sub-milliamp. Plastic-case
    transistors work better, in this kind of surge situation.
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