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LM2670: LM2760-3.3/5.0/12 fix output setting to 15V?

Florian Luetkebohle
Prodigy 40 points
Part Number: LM2670

Hi, 

we would like to replace LM2670-ADJ (Vout=15V) bye fixed Output alternatives from the LM2670. 

IIf this is possilbe, which resistor setting would you recommended?

Me examples are calculated for the LM2670-12 to set the output voltage to 15V. 

Option 1: 

Adding a resistor between Output and feedback with 6,1kΩ.

Option 2: 

Adding following voltage devider:

Rhigh:10kΩ

Rlow:39k2Ω

BR, 

Florian

  • +1
    TI__Guru 57555 points

    Hello

    What you propose is theoretically possible.

    However, the absolute values of the internal feedback resistors will vary over temperature and from one device to another.

    Of course, the ratio is accurate and stable; providing an accurate output voltage for a "fixed" output regulator.

    Any external resistor, or divider, will not track with the internal divider over temperature and from one device to another.

    The absolute values of the internal divider resistors may vary as much as +/- 30%.

    Using a single 6.1k resistor, and the above assumed variation, we get a variation of about 14.3V to 16.2V

    If you can tolerate that much variation, then that would be the simplest solution.

    However, a better choice is to use a voltage divider.  In this case you need to design the divider to draw more

    current than the internal divider in order to preserve accuracy.  Your proposed values are too large.

    Using a value of 1.05k for the top and 4.99k for the bottom, we find a variation of 14.9V to 15.3V.

    Thanks

  • 0 in reply to Frank De Stasi
    Prodigy 40 points

    Hi Frank, 

    thanks a lot for your great support.

    Could you als do a recommendation for the LM2670-5.0 to set the output to 6V?

    BR

    Florian

  • 0 in reply to Florian Luetkebohle
    TI__Guru 57555 points

    Hello

    I will look at this and provide feedback by EOB on Thur. 2/23/2023 PST.

    Thanks

  • 0 in reply to Frank De Stasi
    TI__Guru 57555 points

    Hello

    Using a top resistor of 681 Ohms and a bottom resistor of 5k, the approximate variation would be from 6.15V to 5.94V

    Using a single 2k series resistor, the approximate variation would be 6.4V to 5.7V.

    Thanks