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LM2678: design help

Marvin Cantos
Expert 6875 points
Part Number: LM2678


Hi Team, 

Good day. I am posting this inquiry on behalf of the customer.

"I plan to use LM2678S-12 as 26-27V regulator driving feedback (FB pin) through resistor instead of direct voltage output, see below.

Used formula from datasheet: Vout=Vfb*(1+R1/R2)=1.21V*(1+(22k3+30k)/2k5)=26.5V, where 22k3 is value of internal resistor for 12V.

Please advise whether this approach is ok if all other recommendations for inductor, capacitors, and diode are met as if the regulator was LM2678S-ADJ type.

If it's ok, can the fixed LM267x family regulators be used as a replacement for ADJ type using the external resistor generally?"

Thank you.

Thank you for extending your help.

Kind regards, 

Marvin

  • 0
    TI__Guru 74055 points

    Hello

    Theoretically speaking, you can use a series resistor or external voltage divider to increase the output voltage above the "fixed" value.

     The "fixed" output voltage versions of our devices have an internal voltage divider connected between the FB pin

    and the high impedance input of the error amplifier (please see the block diagram in the data sheet).

    The presence of this internal divider represent a rather low impedance at the FB pin to ground. 

    In the LM2678 this resistance is about 24.8k for the 12V version.

    This resistance will load down any external voltage divider.

     In theory the values of the external resistances can be adjusted to compensate for the loading

    of the internal divider.

     Unfortunately, the actual absolute values of the resistors in the internal divider are not know accurately.

    Furthermore, the absolute value of these resistors may vary as much as +/-20% (or more) over process variations

    and temperature.

     Keep in mind that TI does not test nor ensure the absolute values of the internal voltage divider resistors.

     Of course, the ratio of these resistors is trimmed to provide an accurate output voltage that is stable within the

    accuracy limits given in the data sheet.

     It may be that the customer is not concerned with a large variation in the nominal 26V rail.

    In that case, the customer can use either a single external resistor or a divider.  The customer should estimate the variation in the output voltage based on the above assumed variation of the internal voltage divider resistances.

    Thanks