# Adjustable LDO's - Voltage Input, Output, & Potentiometer Use

I am looking to use an LDO with a negative adjustable output voltage. I have found two products sold by TI that have similar schemes for setting the output through external resistors.

I am slightly confused though, as one of the resistors in each scheme is illustrated as a potentiometer. Why is this so? Can a fixed value resistor be selected instead?

Also, the first regulator series (LM79XXAC) have fixed negative outputs but external circuitry can be configured to make it adjustable. For instance, take the -5-V fixed LDO in adjustable output configuration.

If this was instead the -12 V fixed output version, would the output equation change such that 12 V is substituted for 5 V in the above?

The external circuitry for the second LDO (LM337LM) is much the same as the other LDO series as shown below.

However, the output equation varies from that of the first LDO series, not including a current term. Is there a reason for this?

Finally, both datasheets do not seem to provide an input voltage range. Is there something I am missing? I need to figure out how to select a value of Vin for a given Vout.

Here are links to the Datasheets:

LM79XXAC

LM337LM

Thanks, everyone

• Iq should always be included, but sometimes it's so small compared to the current through the setting resistors it is ignored. Read the datasheet carefully to see what's guarranteed about Iq for any given regulator of this type. Jan 1, 2022 at 21:49
• both datasheets do not seem to provide an input voltage range Look at the circuits, do these regulators (they're not LDOs, they need a somewhat high (Vout-Vin) ) "see" the input voltage directly? What if you increase the input voltage by 10 V (while maintaining a 5 V output voltage), where does the extra 10 V "go"? Maybe (Vout - Vin)? Is something mentioned about a maximum for that in the datasheet? Jan 1, 2022 at 21:56
• They don't literally mean a potentiometer, just that you need to pick the value. Also consider something newer than the ancient 78xx regulators. The input voltage will specified, usually in the Electrical Characteristics section. Jan 1, 2022 at 21:58
• @user1850479 Do you mean 79xx regulators? What is wrong with them? Do you have experience with the LM337LM regulator? Jan 1, 2022 at 22:02
• To be pedantic, neither the LM337 nor the 790x is an LDO. They are linear regulators, but not low drop-out regulators. An LDO is a linear series regulator with less than 1 V $V_{do}$. Jan 1, 2022 at 22:11