Again, playing the newbie card here… but the adjustable voltage regulator calculator is really tricking me!

So I’m trying to calculate the R2 value using a ST-Micro LD1117, and the result I get is the opposite to the data sheet formula and other online calculators. Am I being slow?

Here’s what I put in to try to calc value of R2 in order to get a 3.3V output:

R1 = 120ohm per datasheet recommendation

Vref=1.25 per datasheet

Vout = 3.3

Select the option button next to R2, then click Calculate, and it spits out 73ohm. All other calcs show a value of 200ohm for R2.

Please can someone tell me if I’m mad?

The calculator output looks right to me. Your reference is 1.25V which is dropped over R2, and your output is 3.3V which means 2.05 is dropped over R1. Since the same current has to go through both R1 and R2, you would expect R1 to be larger than R2. Calculating the current shows 1.25V/73 = 2.05V/120 = 0.017A.

1 Like

I doubt you would have made this mistake, *but* if the roles of R1 and R2 were reversed in the other formula you tried, then you would get a value of 200 for R1: 1.25V/120 = 2.05V/200 = 0.010A.

Ah, thanks for your help - makes sense now. I assumed that VRef was consistently applied, but if I look at the LD1117 datasheet, they indicate VRef is across R1, not R2. So yes, I did get the R1 and R2 switched - or the calculator and datasheet define them differently.