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LM2651 fixed vs adjustable output

Are there more differences between the fixed-output and adjustable-output LM2651-s than the voltage divider at the FB input in the fixed-output versions? How high will the FB input  current to a LM2651-2.5 be? Could I put a resistor in series with the FB input, of a LM2651-2.5, shorting the resistor when I want 2.5V of output, leaving it un-shorted when I want 3.3V? The FB input current will define the value of this resistor.

/Staffan Cronstrom

  • With an adjustable part you can control the output voltage to be what you wish within the recommended operating values. With a fixed part, the highest Vout is the fixed Vout, adding any additional feedback dividers will help provide lower Vouts. Size your resistors keeping Rfb top constant and then deciding Rfb bottom based on Vfb of 1.238V and Vout of 2.5V.

    Let me re-direct your question to the appropriate forum so an apps expert can assist you.

    Thanks,
    Anston
  • Anston is correct,

    In general, we do not recommend connecting an external resistor divider to parts with fixed output voltage. There could be interactions with the circuits inside the FB pin. Anston's suggestion would also be easier to implement.

    Best Regards.

    Florent