[volt-nuts] Trimming the LTZ1000 tempco

[Bruce Griffiths]

A tempco of +50ppm/K with a 7V output is equivalent to +350uV/K or about 
0.18 x Vbe tempco (~ -2mV/K) which can be achieved by using a series 
resistor of about 22 ohms or so.
The resultant 110mV or so increase in the voltage across the 70k 
collector resistor will only have a relateively minor effect on the Vbe 
tempco as The Vbe tempco is relatively insensitive to small (0.14%) 
changes in Ic.

Increasing the Vbe tempco by 18% will (if achievable at all) require a 
large decrease in Ic.


Bruce

Will wrote:
> Will wrote:
> Datasheet page 6 shows a way to "null" the overall tempco. As far as I
> understand (most likely I don't) adding the R1 doesn't much change the
> zener current. But the op-amp output voltage goes higher to maintain
> the balance which increases the transistor Ic which slightly changes
> the VBE tempco. But could we get the same result by adjusting the 70k
> resistor instead?
>
> Andreas wrote:
> I tried the influence of the 70k Resistor on tempco since I had to
> build mine of a 50K + a 20K precision WW resistor and wanted to save
> the 20K resistor. The change in tempco on my LTZ1000A #1 is from about
> 48ppm/K with 70kOhms to 52ppm/K with 50kOhms.
>
> Will wrote:
> 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.
>
>    
Not true, how on earth did you arrive at this erroneous conclusion?
>
> It seems that the page 6 trick slightly reduces the tempco but it
> still stays in the 50ppm/C area. Or I have forgot something in my
> calculations. I don't have a spare LTZ1000 available at the moment to
> run the tests but maybe Andreas has time in the future.
>
>    
Your analysis is faulty.
> The original unheated tempco is surprisingly high when compared to
> ordinary temperature stabilised zeners. Better stability at the
> expense of tempco?
>
> Another alternative is to adjust the 120 ohm resistor which changes
> the zener current. It could be more effective way to alter the tempco
> but there can be some side effercts?
>
>    
It may not be possible to achieve this without exceeding the maximum 
specified zener current.
> Will
>
>
>    
Bruce