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

Question

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

Answer 1

Of course you can select fixed resistors instead of a potentiometer to make it non-adjustable.

The 79xx regulators are intended to be used as fixed regulators, but can be made adjustable, and the actual voltage of the regulator just sets the minimum voltage, it can be adjusted higher.

The output voltage term always includes the ground or adjust pin current, the LM337 output voltage also has the pin current in the formula - the LM337 just has about 10x smaller current, and due to the LM337 already typically has an adjustment divider current of 5mA, the adjust pin current can be ignored because it does not have much effect on the output voltage.

The datasheets do contain the formula for the resistors. For 7905 the minimum voltage will be -5V and it needs at least -2 volts more than the output voltage to work properly. The LM337 can go down to the -1.25V reference voltage, and it needs -3 volts more on the input than on the output.