[volt-nuts] Trimming the LTZ1000 tempco

[John Devereux]

Will <willvolts at gmail.com> writes:

>> The change in tempco on my LTZ1000A #1 is from about 48ppm/K
>> with 70kOhms to 52ppm/K with 50kOhms.
>
> A rough estimation:
> Change in Ic when R1 is set from 0 to max value 200 ohms is equivalent
> to changing the 70k resistor to 60k. I haven't prototyped or
> simulated, so there can be more complex interactions that I haven't
> included.
>
> Maybe it is not even possible to "null" the tempco in a way simple
> temperature compensated zeners allow to do that? The trick in the data
> sheet seems not to be very effective.
>
> Another alternative is to adjust the 120 ohm resistor which changes
> the zener current.
>
> It is surprising how slavishly all equipment manufacturers have copied
> the original datasheet circuit during the last 20+ years.

Hi Will,

I was wondering about this too, it is not clear to me how important the
actual circuit values are. In the canonical "7V positive reference
circuit" we have R2=70k, R1=120 ohms.

OK, R1=120 ohms gets you 5mA zener current at the operating point (when
it starts to turn on the reference transistor Q1 at ~0.6V Vbe).

And R2=70k gets you a 100uA Q1 collector current, when Q1 is turned on
such that its collector voltage equals its base.

But these are awkward values when shopping for precision
resistors. R1=100 ohms and R2=50k would be easier. Yielding 6mA diode
current and 130uA. Is there anything wrong with that? Or are the
5mA/100uA magic values in some way? Perhaps they have been designed for
zero TC at 5mA - like a reference diode is selected for 7.5mA.

The (unstabilised?) "low noise reference" on page 1 has a 30k collector
resistor. And the "averaging" dual reference on P6 has 150ohms zener
resistor and a 5mA collector current!


-- 

John Devereux