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Original question:

LM2611: Rfb value calculations

LM2611: I you want -9V out of this dev. board...

Chris Vandewinckel
Prodigy 10 points
Part Number: LM2611

...solder an 8.2K ohm resistor across Rfb2 (10K) and then a 560K ohm across Rfb1 (29.4k) ohm resistor.  Rfb2 actually has a few thru-holes in parallel with it, so it's novice-easy, but Rfb2 has none so it'll be a little trickier.  It'll yield -9.04V on Vout.  I think a single 9.1K ohm resistor across Rfb2 (10K) would also yield close results, but I didn't have one of those.  Give it a shot and report, please.

Also, since Cin is only a 6.3V cap, you'll want to keep the soldering iron hot and replace that one with a higher voltage cap.  With these hacks you can connect +9V, say from a 9V alkaline battery, and you are good to go with a nice clean +/-9V supply.

And yes, it is a glaring omission on the datasheet that there isn't an easy formula, or chart, given to help designers achieve other negative voltages.  Someone on another thread pointed an inquiring designer to TI's Power Stage Design Tool, but it also comes up short on this area.  I tried it.  It'll only help with selecting inductors and caps.  Important?  Yes, but not fully helpful.

  • 0
    TI__Guru 68595 points

    Hi Chris,

    Thank you for posting and commenting. TI Power Stage Design Tool is continuing to expand the coverage to support online designs for more parts, and it takes time given so many devices in the portfolio. 

    You comments on the datasheet is well received.  I believe the original writer of the datasheet assumed the user would be aware of setting the output voltage through the feedback resistor divider.  If it is not clear, the resistors in of the feedback divider should satisfy the following equation, in order to set the output voltage to the targeted VOUT: 

    VOUT * Rfb2 / (Rfb1 + Rfb2) = -1.23V.

    To adjust the output voltage, we usually change Rfb2 and leave Rfb1 untouched, because changing Rfb2 would not affect the loop gain, and hence the stability.  In your case, you did right to replace the Cin.  For Vout adjustment, you may just replace Rfb2 with a 4.64k resistor, or add a parallel 8.66k, to modify the board for -9Vout. 

    Hope this clarifies.

    Thanks,

    Youhao