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Help - Hard time understanding logic of this simple circuit

Discussion in 'Electronic Basics' started by, Sep 3, 2003.

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  1. Hi guys,

    I'm a newbie in electronics and I am having a hard time understanding how
    this simple circuit works (

    The description of the circuit says:

    "CLR is active high while AB is active low. These two pulses generate the
    timing for ODB."

    I am correct to think that the following truth table is correct?

    if CLR = 0 and AB = 0 then ODB = ~4.8v
    if CLR = 0 and AB = 1 then ODB = ~6.3v
    if CLR = 1 and AB = 0 then ODB = ~6.3v
    if CLR =1 and AB = 1 then ODB = ~ 12v

    Please take a look at the circuit diagram and let me know if I am correct/in
    the right way but not quite there/wrong. Again, you can check out the
    diagram at



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  2. Sorry, I can hardly make sense of this data sheet. There is neither a
    capacitance, rise time or input current spec for OBD input, as far as
    I can see. Without these, it is very hard for me to understand
    exactly how their driver produces the voltage waveform hinted at in
    figure 1. (It would take some digging through the sheet to define all
    the time intervals and rise times shown in that figure.)

    I think I might try to work out a DC design for the ODB driver,
    perhaps based on the TTL input gate drivers from Microchip Technology.

    Though the 4 amp with 2 ohm output are hard to find. But it looks
    like most of that 4 amps goes into shunt capacitors in their design,
    anyway. I especially like the note that says that decoupling and
    bypass capacitors are not shown on the schematic. Good luck keeping
    the supply lines clean with 4 amp drivers hitting .22 uf capacitors
    with 15 volt pulses.

    I have loathed TI data sheets for a long time. Lots of typical
    values, lots of missing information and crude suggestions for
    application circuits. Yep, this looks very familiar. I always
    suspected that they really didn't want ot tell designers enough to
    actually use their chips. I wonder if they intend to use all of them,
    in house.
  3. Thanks for the quick replies, John.
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