I am trying to figure out the correct value of a potentiomenter for trimming Vo in a step-down DC/DC converter. I am using a 150W 12V output DC/DC from murata. From the datasheet, equations are given to build a resistive network to adjust the DC/DC. Also from the datasheet, option to use a potentiometer is suggested. The problem is that it is not clear if the equations for the resistive network are also valid when using a potentiometer.
These equations can be seen in page 33 of the datasheet:
http://power.murata.com/datasheet?/data/power/uee150w.pdf
From the equations, I have calculated that a 20% trim down can be achieved with a resistor of 15k33 Ohm, and a 10% trim up with a resistor of 488K Ohm.
Many manufacturers offer the same equations for resistive networks for different step-down converters, though none specify an appropiate value for a potentiometer.
I thought of using a 2M potentiometer, but I suspect a much lower value could be used. Am I right?
Thank you very much.
Best Answer
The values for individual trim up/down resistors assume a known resistive loading at trim input pin. They shift the voltage divider ratio that is present between the sense and trim pins.
The fact that no value is given on p33 of that data sheet for the potentiometer method implies that the value of the pot is not critical. This pot will adjust the voltage divider ratio.
It should be possible to compute what the reference voltage is, and the resistor ratio behind the trim pin, from the two calculations. However, that's not necessary.
I would use a 10k pot, as the value is much less the existing behind the trim terminal appear to be, given the values calculated. A 2Mohm pot would also work, but I suspect you would find it did little in the mid range, then got a bit vicious towards either end.