[volt-nuts] Trimming the LTZ1000 tempco

[Bruce Griffiths]

In the standard positive reference circuit depicted in the latest datasheet

Vref  = Vz +Vbe
Transistor collector current, Ic = Vz/R2 (This increases slightly with 
temperature due to the positive tempco of Vz)

Zener current, Iz = Vbe/R1 (This decreases with temperature due to the 
negative tempco of Vbe.)

where Vz is the zener voltage, Vbe is the base to emitter voltage of the 
temperature compensation transistor.

Vz has a tempco of approximately +2mV/K and Vbe has an approximate 
tempco of -2mV/K
Thus Vref is nominally compensated for temperature changes.
However the tempco matching isnt perfect with a "typical" (note sample 
size = 1!!!!) residual of +50ppm/K (+350uV/K)
This can be reduced by adding a resistor (Rt) in series with the zener.

In this case (to a first order approximation)

Vref = Vz + (1+ Rt/R1)*Vbe
Ic = Vz/R2 +((Rt/R1)*Vbe)/R2
Iz = Vbe/R1
If the value of Rt is suitably selected the tempco can be adjusted to zero.
However the required value will vary for each LTZ1000 and the circuit 
only works if the tempco with Rt =0 is positive.
The magnitude of the Vbe tempco can be increased by reducing the 
collector current (requires increasing R2), however the required 
collector current to achieve zero Tc will probably be 1uA or even less, 
this increases the transistor noise and makes the circuit more 
susceptible to leakage shunting R2.
The compensating transistor is operated at zero collector base voltage 
which eliminates the variations in Vbe due to the Early effect.



Bruce

Will wrote:
> Thanks Bruce.
>
> Will wrote:
>    
>> Or did I misunderstand something again?
>>      
> Yes I did. And I had the same mistake with my previous calculations,
> which explains the strange results.
>
> Since you are an expert, an "LTZ1000 circuit description for dummies"
> would be great.
>
> The heater control is straightforward, but all the interactions in the
> reference circuit seem not to be clear to me. And most likely there
> are other dummies too.
>
> Will
>
>
>
> 2011/10/7, Bruce Griffiths<bruce.griffiths at xtra.co.nz>:
>    
>> There's little point in adding the tempco compensation resistor if you
>> do that as your proposed connection essentially eliminates the
>> additional correction provided by the resistor.
>> There's no way of avoiding the sensitivity to the value of this resistor
>> if this method of tempco trimming is used.
>> However if a +50ppm/K LTZ1000 residual tempco (with the tempco trim
>> resistor = 0) is typical, then the sensitivity will only be about 10% or
>> so of that esstimated by Adreas.
>>
>> Bruce
>>
>>      
>
>