[Re: LM13700 and 1shot555 (or VCO)]

[INDRID COLD]

ok just to make things a little more confusing; there is another 13600 clone
called the NE5517. what is the diff between this and the 13600/700?
its too much i will crawl back under the blanky now. mike.
http://www.indridcold.conk.com

Harry Bissell <harrybissell at netscape.net> wrote:
Michael:

The LM13700 (data sheet) is just exactly the same as the LM13600, so much so
that there are errors (Q3 in text) where it was copied verbatim from the
LM13600. THE only difference is that there is a variable bias current that is
set from the Gm input in the 13600. This makes the (i think) input impedance
of the darlington buffer higher when the Gm is very low. (output impedance is
higher at low Gm bias.) This makes the buffer perform better in cases like a
VCO, but may cause control voltage feedthrough in Audio apps with rapidly
varying Gm bias. Moot point, we're not using the buffer anyway.

The NE555 one shot is adapted from a circuit by Dr. Scott Wetencamp from EDN
march 5, 1978. I never throw away a good app. note. "Minor changes turn VCO
into voltage controlled one shot". But I found out it wasn't retriggerable. It
is now...  My changes were to add the exponential converter lifted from TomG's
VCO4 design (I think). You need to add an inverting op-amp in front of the
resistor feeding the base of the NPN. Right now, lower voltages give higher
frequencies. The voltage range starts a few millivolts positive, lower than
that and the oscillator will go to a minimum frequency. To make this a full
VCO, run the trigger input to the threshold pin like any normal 555, and
delete the PNP-NPN circuit.
    The PNP-NPN circuit on the input does two things. The input pulse is
differentiated to drive the trigger low. And the time constant on this is
longer than that on the PNP-NPN. So a negative going trigger will discharge
the timing cap (the .001uF), and then the trigger will stay low a little
longer, to begin the timing cycle. Any additional triggers will repeat this
process. You can't use the reset pin, because it also resets the 555
flip-flop. (YOu could use a dual 555 to accomplish this same function...).
    The timing cap is driven by a constant current source. I forget where I
heard this, but the PNP and two diode drop circuit is fairly temp stable (do
you care?) If linearity is not critical, a simple resistor to +15 could
replace it. The NPN from the cap to ground "robs" some of the current, and
slows down the timing. The original circuit had a Darlington connected NPN
pair, but one seemed to work fine for me. The Diode beneath the cap is to feed
reset current to it when the discharge pin pulls low. The 4pF cap is parasitic
to make the sim work... delete it. The 20ohm probably isn't needed in the real
world either.
    I've used the linear ramp part of this as a timebase for a sonar system
(in air) it works fine. The 555 output goes high with a negative going
trigger, and stays high during trigger or retrigger. Remove the PNP-NPN
circuit and its a NON-retriggerable one shot.
   If temp performance is critical, you could eaasily do the heated 3046 trick
for the expo current source. The expo converter op-amp needs a negative
supply. I used it because (a) voltage input direct to the NPN on the timing
cap didn't work well, and a single NPN current source would be SOOO touchy
about input current. This converter will servo to the proper operating point.
    Hope the file ain't too big. E-mail me if you "got questions?" of give me
a fax number and I'll send a photocopy of "Dr. Scott's" original article with
all the math stuff....  :-) Harry

Oh hell, I'll send it to the list... somebody might want it.



 
Roy Tate set up a temporary URL where I could download the LM13600 datasheet, 
so I'm covered for now.  I still wish I could look at the real LM13700 
datasheet, though, just to learn more about any subtle differences between 
the two parts.

Also, I would very much like to see your voltage-controlled 555 one-shot 
circuit, but I don't have a fax machine.  I will ask a friend of mine if it's 
OK to recieve a fax on his machine, and get back to you with the number.

Best regards,
Michael Bacich





____________________________________________________________________
Get your own FREE, personal Netscape WebMail account today at
http://webmail.netscape.com.


> --------------------------------------------- 
>	Attachment: VC1shot.gif 
>	MIME Type: image/gif 
> --------------------------------------------- 

____________________________________________________________________
Get free e-mail and a permanent address at http://www.netaddress.com/?N=1