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LM22678: Output Divider

Josh Tombs
Prodigy 90 points
Part Number: LM22678


Hello,

The datasheet for the LM22678 recommends to use the -ADJ version for voltages <5V, but to use the 5V for output voltages of 5V and greater. On page 14 it provides information about selecting proper resistances for the feedback node, and it says that "a maximum of 2k is recommended for the -5.0 option". This leads me to believe that Rfbt + Rfbb should equal 2k.

Then I was looking at the SIMPLE SWITCHER Design guide on page 12 (labelled 21) and it recommends values 1k and 8.45k for and output voltage of 12V.

Which of these conditions holds more true? Should I use the resistors suggested in the design guide with the -ADJ version? With the -5.0 version? Or should I use the -ADJ version and use resistors that sum to 2k?

Thanks,

Josh

  • 0
    TI__Mastermind 25590 points
    Josh,

    The datasheet, "maximum of 2k" will give you the best results.

    More detail:
    Total feedback resistance of 10k strikes a good balance. Too much resistance and it's more susceptible to noise. Too little resistance and it's dissipating too much power. 10k is just about right for this device.

    The -5.0 version already has about 10k internally so it's best to keep additional resistances low. 2k total is a good balance.

    The Design Guide is a good guide, and will get you results, but the datasheet is usually the best place for the most device-specific optimization.

    -Sam