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Original question:

TPS7A49 Vfb versus Vref

TPS7A49: Should we use VFB or VREF of TPS7A4901 to calculate Vout

BX27
Genius 3870 points
Part Number: TPS7A49

Hi there,

We use TPS7A4901DRBR to convert 5V-->3.3V , 5V-->2.5V, 5V-->1.8V, and 5V.3-->5V.

Just to make sure: VREF is voltage measured at NR/SS pin, and it is 1.188V, with 1.176V~1.212V range.  And VFB is 1.185V±2.5%. Is this understanding correct?

Why bother to give out these two value ?  VREF seems to be useless in designing process.

I thought we all we need is to use VFB=1.185V±2.5% to calculate Vout's nominal and max, min value,  and consider tolerance of R1 and R2 at the same time.

If I was wrong, pls point out. Thanks.

  • 0
    TI__Guru 55940 points

    Hi Yi,

    Correct the NR/SS pin is directly connected to the internal reference.  As such, if you are attempting to measure the internal reference, you would use the NR/SS pin.

    You are also correct that when selecting your resistors, you should use Vfb in your calculation.  Vref is given for information purposes only.

    As you mention, the accuracy stated in the datasheet includes all internal sources inaccuracy; however, it does not include the external resistor divider tolerance.  In order to calculate the impact of the external resistors on your overall rail accuracy, please refer to the following Application Report:

    http://www.ti.com/lit/an/slva450a/slva450a.pdf

    Very Respectfully,

    Ryan