• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 62 subscribers
  • Views 567 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS7B86-Q1: feedback resistance selection

Amelie Zheng
Genius 10865 points

Part Number: TPS7B86-Q1

Hi team,


The feedback resistance selection of the power supply is not very clear; mainly for formula 2;

1. Is IFB calculated through I1-I2, or 10nA of the upper limit of the spec; if it is the former, this inequality is all true;

2. CFF selection calculation

  • 0
    TI__Guru 50025 points

    Hi Amelie,

    Equation 2 is used to set an upper limit on the total feedback resistance R1 + R2. To use it, take the IFB from the EC table (10nA for TPS7B86-Q1) and insert it into Equation 2, and insert the nominal Vout to get the limit for R1 + R2. The point here is to size the resistors such that the current into the feedback pin IFB is very small (by at least a factor of 100) compared to the current through the feedback resistors and the IFB term can be neglected when setting the output voltage. 

    For the CFF selection, in general the larger the CFF, the lower frequency noise from the reference voltage is prevented from being gained up by the feedback and also the lower frequency noise from the supply that is shunted directly to the FB pin (such that it also does not get gained up as much). These equations allow you to calculate the location of the pole and zero the CFF contributes.

    Regards,

    Nick