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TPS74801: Datasheet Clarification

Part Number: TPS74801
Other Parts Discussed in Thread: TPS7A89, TPS7A90, TPS7A85, TPS7A39

Hello,

I have a question regarding component selection of the feedback resistors in LDO datasheets in general, the TPS748 being an example.

In the TPS748 datasheet, under the "Application Information" section, there is a table, listing different output voltages and corresponding configuration resistor values. However - in some cases, the theoretical value of the output voltage one gets from calculating using the 0.8*(1+(R1/R2)) formula deviates from the stated output voltage in the table. This deviation is, however, very small. What is puzzling though, is that "perfect" voltage is achievable using common resistors. For example:

  •  The table states, for example, that the resistor combination for 1.2V output is R1=2.49K R2=4.99K. Calculating using the formula yields 1.1975....V output. ~2.5mV deviation.
  • But, it is possible, while using "Standard 1% Resistor Values", to get exactly 1.2V using R1=1.1K and R2=2.2K. These even are more "common" resistors, much more available especially if you are a hobbyist. These resistors are still fulfilling the R2 smaller than 4.99K suggestion in the datasheet.

This phenomenon is seen across many datasheets in similar sections, across different manufacturers and different parts. It seem that there is some advantage in using E96 series resistors even if the configuration is slightly off, while the E24 series gives perfect results(and are widely available in 1% tolerances).

Now, I am a novice, and it seems I am missing something that is perhaps obvious to anyone in the industry dealing with such components - why many datasheets recommend using values that yield an imprecise voltage, when more common resistors that produce more precise voltages are available? is there an advantage I am failing to see?

Thank you,

Shay

  • Hi Shay,

    A lot of that goes down to finding the right values for a lot of voltages - it turns into an optimization problem. Webbench now tries to solve that problem, and we have taken it upon ourselves that in newer devices that we have more accurate resistor choices and show the calculated output voltage with those resistors (such as the TPS7A39, TPS7A85, TPS7A89, TPS7A90, etc).

    Regards,
    David