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About FB resistor values

GARAM AN
Intellectual 1990 points
Other Parts Discussed in Thread: TLV767

Hi all,

I read "the reason that suggestion having a larger current draw on the feedback resistors is to reduce any amount of noise that could get coupled into the FB pin. Normally this is specified to be around 1mA, but this can be lower."

I could not understand why the larger current on the FB path can be reduce amount of noise, could you explain this?

Thank you

  • 0
    TI__Expert 3611 points

    Hi,

    Lowering the current through the feedback resistors means that if noise currents get induced into that trace, then they will be a larger part of the total current through the resistor (percentage-wise). As a result, the voltage on the FB pin will have more fluctuations. 

    Another way to look at it: If we have the same amplitude of noise currents flowing through an impedance, the noise voltage that those currents will generate across the impedance is going to increase linearly as the said impedance increases (because of V = I*Z).

    Hope this clears it up for you.

    Regards,

    Davor

  • 0 in reply to Davor Pavlovski
    TI__Intellectual 1990 points

    Thank you Davor,

    I have one more quesion that what is the different bewteen current from voltage divider and Ifb on the datasheet.

    TLV767 device, the Ifb 10nA is written on the datasheet, but I could calculate the I_FB_divider(I_FB_divider=Vout/(R1+R2), and it's over than that.

    What is the different between them?

  • 0 in reply to GARAM AN
    TI__Expert 3611 points

    Hi,

    Ifb is the leakage current into the FB pin.

    Whereas the current labeled as I_FB_divider is the current flowing through R1, which can be calculated with the following equation:

    I_FB_divider = Vout/(R1+R2) + Ifb.

    Regards,

    Davor

  • 0 in reply to Davor Pavlovski
    TI__Intellectual 1990 points

    Hi Davor,

    Thank you, I could understand this.

    A customer wanted to choose fixed output voltage type, because the Ifb(leakage current) brings some error in the past.

    Ifb(leakage current) is super small like nA, Do you think or have you seen it brings some issues?

    I could understand that if I_FB_divider is bigger, than the error in the current might be small part.

    If the I_FB_divider is larger too much, what effect come out?

    Thank you.

  • 0 in reply to GARAM AN
    TI__Expert 3611 points

    Hi,

    Choosing to have a large current through the feedback divider decreases the efficiency of the design.

    More details about designing the feedback divider can be found in the following app note:

    Design considerations for a resistive feedback divider in a DC/DC converter.

    Regards,

    Davor