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TPS7A49: Inconsistency in the datasheets

Nicolas Christophel
Prodigy 20 points
Part Number: TPS7A49
Other Parts Discussed in Thread: TPS7A30

Hello im a beginner at designing electronic circuits and the following questions might have an obvious answer i did not see.

While designing a low noise filter for an op amp i think i came across an inconsistency in your data sheet for the TPS7A49 and im not sure how to proceed now with my design. In  the datasheet on page 14 under adjustable configuration equation 2 writes: V(FB nom)/R2  > 5 μA

But when looking at the Userguide on page 3 the R2 changes to (R1 + R2) for the TPS7A49. Also for the TPS7A30 datasheet the equation is with (R1+R2).

Which one is it?

Another thing that confuses me in the data sheet for the TPS7A49 is on page 17 equation 5 where this is used to calculate the resistors. For R1 we calculate it to 1.265 kΩ but in the next sentence R1 is now set at 68.1 kΩ?

kind regards

Nicolas

  • +1
    TI__Genius 12511 points

    Hi Nicolas,

    So the 2 equations are not identical, but both are sound mathematically. The datasheet is defining a minimum current through R2 by utilizing the fact that the LDO will regulate to get the FB pin to Vref, labeled Vfb(nom), during normal operation. So the voltage differential across R2 is [Vfb(nom)-GND], and dividing by R2 gives the current through R2 by using ohm's law.

    The user's guide is using Vout as the voltage. Vout is the voltage potential across the equivalent resistance of (R1+R2) so to get the current in this case you would divide Vout by the sum of the resistances. The math is correct in both instances, it just uses two methods to compute the current going through the ground in the feedback network, both the voltages used and resistances used change to reflect this.

    If you compute out the Vref value, you will find that 1.256k is the correct value to get an output of 1.2V that the example was using. The 68.1k will give you an output voltage of 2.0V which is also a standard rail, it just seems that a miscommunication occurred when finishing that section.

    Regards,

    John