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LM22675-ADJ Feedback resistor problem

Other Parts Discussed in Thread: LM22675

Hello

I'm having some issues with feedback resistors when trying to get 9.0V out with a 12V input.

Webench suggests using Rfb1 = 430R and Rfb2 = 330R. However this only makes Vout around 2.3V with a 12V input. To verify my connections I have tried using Rfb1 = 1k and Rfb2 = 1.5k in order to get a Vout at 3.3V and that works fine and as expected. Using the equation found in datasheet Rfb2 = (Vout/1.285-1)*Rfb1 to set 9Vout would give 6k = (9.0/1.285-1)*1k and using those resistors values, Rfb1=1k Rfb2=6k, is also confirmed and works as supposed to.

In the datasheet of the LM22675 one can read: "A good value for RFBB is 1k Ω. This will help to provide some of the minimum load current requirement and reduce susceptibility to noise pick-up.".

What my actual question is pointing towards is wether the use of lower than 1k ohm resistor in Rfb1 should work or not? My tests so far shows, as stated in datasheet, that values above 1k ohm works fine but values suggested by Webench in this case do not work at all.

  • Hi Andreas,

    It sounds like you are using the LM22675-ADJ option.
    As per datasheet recommendation pg. 7, for output voltages over 5V, the LM22675-5.0 option has better internal compensation. WEBENCH is thus using the 5.0 version in its design. This would explain the difference in Feedback resistor selection. 
    As per your question about the low resistance values. They should be fine, it is the ratio difference that is causing the voltages you see. If you plug 430R and 330R into the ADJ version equation  you  get 2.3V ~ 1.285*(450+330)/450   but for the 5.0 version you get 9V = 5+330(5+430*5E-4)/430

    As per datasheet pg.14 though, this is not to say your ADJ design is bad. It is only that the 5.0 version can provide better loop bandwidth that may be needed in some applications, not all. If you tested your design and it works you are fine.

    Best regards,
    Tommy 

  • Ahh, I understand what you're saying. And you are correct, I use the -ADJ version of the LM22675. I must have missed the part in the datasheet where it mentions the use of 5.0 version for "higher" voltage out. And I also now see that the name of generated schematics in Webench is "LM22675MR-5.0/NOPB 12.0V-24.0V to 9.0V @ 1.0A". Sometimes one need a simply remainder that things are easier than they look.

    Thanks Tommy!