# Help - Hard time understanding logic of this simple circuit

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

1. ### news.verizon.netGuest

Hi guys,

I'm a newbie in electronics and I am having a hard time understanding how
this simple circuit works (http://www.mmendes.com/odb_driver.jpg).

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 http://www.mmendes.com/odb_driver.jpg.

Thanks.

Captain.

PS -> To reply via email, please remove the upper case letters from the
email address. <- "poor man's anti spam"

2. ### John PopelishGuest

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.

http://www.microchip.com/1000/pline/analog/anicateg/power/pwrmang/mosfet/index.htm

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. ### Captain BrazilGuest

Thanks for the quick replies, John.