# Adjustable LDO feedback resistor values

I'm designing a circuit using an LDO voltage regulator; specifically the LD39130SPUR from ST. This variant of the part allows for an adjustable output voltage via a potential divider as seen on page 8 of the datasheet. Usually, when using adjustable LDOs and other similar parts, a formula is provided to calculate the output voltage.

My issue is that I can't seem to find any such formula or other information that will allow me to calculate the output voltage. Am I missing something? Or, is there a 'standard' formula used to calculate this sort of thing given specific input parameters?

I'm looking to achieve a voltage of ~3.5V.

Thanks.

## 2 Answers

You need to lookup the latest version of the datasheet on the ST website, where it shows

VO=VADJ(1+R1/R2)

Later on it gives you the specs for Vadj, typically 800mV.

• Thanks; seems like a bit of an oversight on the earlier revisions of the datasheet. I made the mistake of assuming that RS would have an up-to-date version. Sep 9, 2019 at 15:48
• If you want to find the best standard value resistors for a particular output voltage you can go to www.syncbuck.com now that you know the reference voltage. Sep 9, 2019 at 16:01
• @JohnD Thank you - that's such a useful tool! Very much appreciate you sharing that. Sep 9, 2019 at 16:04
• Take the adj pin current into account -- even at "just" $1\mu\mathrm{A}$, your enthusiasm only has to take you into dividers in the tens of $\mathrm{k}\Omega$ to start impacting the output voltage. Sep 9, 2019 at 17:15

It is standard. The regulator adjusts the output until the voltage at the feedback input pin matches the internal voltage reference. Therefore, you just make a divider that steps down your desired output voltage to match the internal reference voltage and feed that to the feedback pin.

• And what is the reference voltage? I can't find it. Sep 9, 2019 at 15:41
• @Oldfart Table 6 Vadj = 800mV Sep 9, 2019 at 15:42
• Ah! that is where it is. I also have missed it. Sep 9, 2019 at 15:44
• @Oldfart It's poorly labelled. Sep 9, 2019 at 15:44