Need help understanding how to calculate the R value for LD1117

Question

I am really trying to understand how to use adjustable voltage regulators such as the LD1117 sold by sparkfun. I had a look at the datasheet they provide in the product page but it points me to a Fairchild semi conductor datasheet which has a typical application note that looks like this:

All fair and well so I went over to this online calculator and tried to see how these values work. This is where I suspect I am making a mistake or misunderstand something. See below on how I calculated the values:

That did not make sense. So I realized that Vr is not related to Vin. I redid the calculation and I think I understand it now.

This looked much better and more inline with what I was excepting. My first challenge was to do a simple 5V to 3.3V conversion as I have done this before with a fixed output regulator.

Then I had a look at the IC close up and well confusion struck me. The IC is marked a ST Electronics LD1117 E439. Now this IC has a datasheet here. This datasheet is a lot more detailed and on page 24 there is an application note for the adjustable variant.

Which one should I use? I did a calculation based on the sparkfun datasheet and I got a value for R2 of 0.174KOhm/174Ohm using the ST datasheet I got very close to this 0.168KOhm/168Ohm.

I am pretty sure I am correct at this point but I hate the blue smoke monster and I just needed a sanity check from a copilot.

One other question with the adjustable voltage regulators the input voltage can range from 15V downwards? I know at some point when the Vin drops too low there will be now conversion but using these values above would I be able to convert say 9V,12V or even 15V to 3.3V?

UPDATE:

So last night I built a PCB using the LD1117. Initially I got an output of 2V... I was stumped. Then I realized I had the two resistor swapped around...duh... facepalm...

Long Story short. I can confirm this works. With the experiment I have just done I am pretty sure I understand this now.

Answer 1

These regulators try to maintain their reference voltage (your Vr)(usually about 1.25 volts) between the output and adjust terminals, so your second calculation is correct.

However, in this world, nothing is perfect. On Page 14 of the ST datasheet, the typical reference voltage is given as 1.25 volts, but it can vary between 1.238 volts and 1.262 volts (+/-1%), so, for any pair of resistors, the oubput voltage can vary +/- 1% - but the resistors also have a tolerance - often 5% (but 1% is quite common these days), so that will cause a further variation in output voltage. You will want to use standard resistor values, which will also adjust the output voltage a bit...

The Electrical Characteristics table lists a "Dropout voltage" (sometimes called "headroom") - this is the minimum diference between input and output voltages that will allow the regulaltor to operate. In this case, if the regulator is supplying 100 mA, the input voltage should be at least 1.1 volts higher than the desired output voltage. If the input voltage is too low, the device will be unable to regulate the output, and the output voltage will drop, probably keeping 1.1 volts across the regulator.