[sdiy] Any LM331 experts around?

[Ian Fritz]

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.

Old cartoon:  Two rocket scientists are looking at a missle with its nose 
buried straght into the ground.  One turns to the other and says: "Sign error".

Ian