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may i use LM2733 to implement a power of negative -12v input,positive output +3.3v

Alan Jeck
Prodigy 80 points
Other Parts Discussed in Thread: LM2733

may i use LM2733 to implement a power of negative -12v input,positive output +3.3v?

if can,how to calculate the value of R18 & R19?

  • 0
    TI__Expert 3950 points

    Hello Alan,

    it could work but you have to use a zener diode in series with the VIN pin.That is needed since the maximum voltage on the VIN is 14V to GND.

     

    In order to calculate the feedback resistor values please consider that the BJT is used as current buffer:

    VFB / R19 = (Vo - Vbe) / R18

    hence:

    R18 = R19 x (Vo - Vbe) / VFB

    where:

    VFB is the feedback pin reference voltage, for the LM2733 is equal to 1.23V (typ)

    Vo is the output voltage

    Vbe is the base-emitter voltage of the BJT

     

    Best Regards,

    Giuseppe

     

     

  • 0 in reply to Giuseppe Pinto
    Prodigy 80 points

    thank you, Giuseppe.

    i don't know  whether Cf1 was connected correctly?(direct connected to FB Pin,or parallel connected with R18?).


    could you send me the full schematic?

  • 0 in reply to Alan Jeck
    TI__Expert 3950 points

    Hello Alan,

     

    your schematic is fine. I don't think it is going to make a big difference if you put Cf1 directly to the FB pin or in parallel with R18.

    You can try it and see how the output voltage ripple looks like during load transients then you can decide which one is better.

     

    Regards,

    Giuseppe