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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.

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2 Answers 2

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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.

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  • \$\begingroup\$ 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. \$\endgroup\$
    – amitchone
    Sep 9, 2019 at 15:48
  • \$\begingroup\$ 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. \$\endgroup\$
    – John D
    Sep 9, 2019 at 16:01
  • \$\begingroup\$ @JohnD Thank you - that's such a useful tool! Very much appreciate you sharing that. \$\endgroup\$
    – amitchone
    Sep 9, 2019 at 16:04
  • \$\begingroup\$ 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. \$\endgroup\$
    – TimWescott
    Sep 9, 2019 at 17:15
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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.

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

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