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LM2595-Adj Inverting buck boost feedback question.

martin b
Intellectual 560 points
Other Parts Discussed in Thread: LM2595

Hi everyone.

Following up on my last power supply question, I want to assemble the circuit of an inverting buck boost regulator which I came across from an old NS App note AN1118 ( see fig1), but I want to use an adjustable version of the LM2595, but am unsure of the correct feedback I should use. Fig2 is what I think it should be, but fig3 is one I came across for another one of TI's regulators that is very similar to my circuit. Can anyone help on this?

Thanks
Martin.

  • 0
    TI__Guru 64365 points

    Both ways will work.  Your figure 2 will regulate only the negative supply; like the original circuit.

    Your figure 3 will tend to regulate both pos and neg outputs; but I do not know how well they will regulate.

    I would try both ways and see what gives the best results for you application.

     

  • 0 in reply to Frank De Stasi
    Intellectual 560 points
    Thank you Frank.

    The LM2595 data sheet gives the method for programming the output voltage as:
    Vout=Vref(1+R2/R1), Vref=1.23V

    If R1 is known, then: R2=R1(Vout/Vref-1)

    Will this calculation also work if I use the cct in fig3 as the feedback method.

    The TPS54xx data sheet lists the calculation as:
    R1=R2[(Vout-0.8V)/0.8V]

    If I use the method shown in fig3 as the feedback for the LM2595 cct, can I use the calculation as listed in the TPS datasheet and replace the value of Vref from 0.8V to 1.23V?

    Also SLVA369A.pdf lists the output voltage calculation as R1=R2(Vopos – Voneg/Vref-1), these two are not the same, I assume they both will work just fine, but why the difference and which is preferred?

    Can you tell I’m a switcher newbie?

    Thanks
    Martin.
  • 0 in reply to martin b
    TI__Guru 64365 points

    The equation Vout=Vref*(1+Rfb2/Rfb1) will work with figure 2; not figure 3.

    For figure 3, with the LM2595, use the equation in SLVA369A and put Vref=1.23V:  R1=R2*[(Vpos-Vneg)/Vref-1].