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# TPS799: How to calculate Vn as per equation in the datasheet

Part Number: TPS799

Hi ,

I have found the noise voltage equation in the TPS799xx series datasheet, but I'm not sure what are the parameter those mentioned were.

How to calculate Vn? Is V (in the denominator) is input voltage ?

Is the below my assembtion is correct??

Vn= 10.5 x Vout/V

=10.5 x 2.8/3.3

=12.375uVrms

• Hello,

I read equation (1) as placing a unit in the denominator.  The "V" is a unit which is then canceled out by Vout to give you the final value of Vn.  The units of Vn should be uVrms. From the Electrical Characteristics table, the measurement of 10.5*Vout = Vn is for a noise reduction capacitor of 10nF.  For no external Cnr capacitor, the only capacitance would be the very small internal capacitor, and the value would be 94 * Vout = Vn.

Two plots are provided in the datasheet which give further insight into the noise of the system.
Keep in mind that figure 19 uses units of nanofarads (nF) in the x-axis, while figure 20 uses microfarads (uF) in the x-axis.
All of this (figure 19, figure 20, the EC table and the equation (1)) give the exact same answer.

Thanks,

- Stephen