TPS745-Q1: is the I_FB input or output, if its input, should I use I_FB_max for "R1 + R2 ≤ VOUT / (IFB × 100)" calculation?

thanks Jorge. 

if its input current (biasing?), and we use resistor < 75KR (lower the impedance path to FB), the biasing current might higher than IFB_max specification (probably not since its high impedance but since TI defined it in specification ) , would it be a problem? 

Hey Tommy, 

It shouldn't be a problem as the current would be sourced to ground because of your R2 resistor. For a larger and larger impedance rating on R2, the FB current decreases and increases voltage output accuracy. 

Regards,

Jorge 

Hi Jorge, 

I thought increase the R2 impedance might increase the leakage IFB that leads output to not that accurate?  

Hi Tommy, 

Actually, increasing R2 would decrease the amount of FB current and increase VOUT accuracy. Now you have to remember that having to large of a resistor network could affect your outputs stability and accuracy. To help with accuracy and stability issues we typically recommend:

1) R2 > R1

2) R1 + R2 ≤ VOUT / (IFB × 100)

Following these two would reduce the FB current and increase VOUT accuracy. 

For an better explanation, you can read more of this on Design Flexibility of TPS7A1408 to Variable Vout and Stability Check. 

Hopefully its of good use!

Kind regards,

Jorge

thanks Jorge. check if my understanding correct below: 

• Make (R1 + R2) smaller to reduce FB leakage that affects V_OUT sensing accuracy 
  • Small R1 can make IFB_leakage only makes small difference on V_OUT sensing  
• Make R2 bigger for V_OUT to better latch at FB 

Yes this is correct!