[sdiy] Any LM331 experts around?

[Neil Johnson]

Hi Ian,

Ian Fritz <ijfritz at comcast.net> wrote:
> At 07:36 AM 4/21/2008, Ian Fritz wrote:
> >At 03:03 AM 4/21/2008, Neil Johnson wrote:
> >
> >> > Mystery revealed.
> >>
> >>And described in:
> >>
> >>http://www.stanford.edu/class/ee122/Parts_Info/datasheets/LM331-AN-C.pdf
> >
> >
> >Mmmmm ... sorta. :-)
> >
> >He's talking there about compensating the timing-cap tempco in the 
> >low-freq circuit.  In the high-freq circuit the drift is much larger and I 
> >think due to something else.  Look carefully at the amount of extra 
> >current being supplied and I think you will see what I mean.
> 
> To clarify further ... the double-diode compensator Neil pointed to 
> *increases* the reference current as temperature increases.  This 
> compensates for the decrease in capacitance of the polystyrene timing cap, 
> which causes the output frequency to increase with temperature..
> 
> The transistor compensator, OTOH, *decreases* the reference current as 
> temperature increases.  This compensates the large tempco of the chip in 
> the high-frequency circuit.  This drift causes the output frequency to 
> decrease with increasing temperature.  I verified that the chip was the 
> source of the drift by individually heating each component in the 
> circuit.  Except for the cryptic transistor compensator, there seems to be 
> no explicit mention by National of this much larger (internal) drift in the 
> high-frequency circuit.

One difference is that the diode compensated circuit is a F/V converter, while the transistor compensated circuit is V/F.  Assuming I haven't confused my temp.coeffs (TC) and assuming a polystyrene capacitor with, say, -100 ppm/'C TC, the F/V converter output voltage would *decrease* with increasing temperature, so exhibiting the same TC sign as the capacitor.  Whereas in the V/F converter the output frequency would *increase* with temperature, so exhibiting the opposite TC sign as the capacitor.

(I think - I may be wrong :)

You're right about the magnitude of the compensating current being quite a bit higher by a factor of about 10 (hand-waving quickie SPICE simulation of the 2N2222 compensation circuit).  Of course, the *change* (i.e. the TC) in the compensating current is the same, about 3200ppm/'C.

This'll give me something to think about on holiday!

Cheers,
Neil
--


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