# Precision voltage reference (LT1236-5) trimming range

I am trying to use this voltage reference in a circuit. I am trying to understand the trimming part of the chip. On the datasheet, one care read the following:

The precise relation between the output voltage and R1/R2 however is not specified, and I am interested in the trimming range that I will have in this configuration to see if its compatible with my application.

Q: Does anyone know how to derive the R1/R2 relation to the output voltage?

We could estimate it manually, but it's easier and probably more accurate to just simulate it in LTspice:

With the pot at 0/0.5/1 rotation we get a voltage at the trim terminal of 0.5264/3.673/4.7845V

It is stated in the datasheet that the effect at the trim pin is reduced by 13:1 and the open-circuit voltage is 4V.

So we would expect a total range of 327mV and we would expect the voltage to be 25mV low with the pot at mid-scale and 60mV high with the pot all the way up.

So a range of +60mV to -267mV with -25mV at mid-scale.

As unsolicited advice, I would not attempt to trim the 5V on this part in that way. There's way too much trim range (and thus too much sensitivity to pot position) for a part that's +/-2.5mV or +/-5mV (depending on grade) and plenty of opportunity to screw up the tempco because of matching between temperature of the diode and IC die temperature etc. I'd use a different method or a different part.

• Thank you for the answer, do you have any suggestion as to how I would implement a stable adjustable current source with the same kind of trimming range ? The output of this part goes through a 40.2k resistor to generate an average current of 0.124.. mA. But this current needs to be adjusted slightly for my purpose thus the trimpot. Commented Sep 14, 2023 at 3:12
• Maybe you can put a (much smaller value) variable resistor in series with the fixed resistor. That will allow you to precisely control the current. Commented Sep 14, 2023 at 3:21