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LMR10515X Rfb1 & Rfb2 value

Atthen
Prodigy 60 points
Other Parts Discussed in Thread: LMR10515, TPS62063, TPS62751

Hi,

I'm looking for smallest power supply solution. My design requirements are: VinMin = 4.0V, VinMax = 5.0V.

Output requirement is Vout = 3.3V, Iout = 1.5A.

Using TI Webench, it proposed me to use LMR10515X together with others components below: 

1. Cin : 4.7uF

2. Renable : 10K ohms

3. Rfb1 : 10K

4. Rfb2 : 45.3K

5. L1 : 3.9uH

6. Cout : 10uF

7. D1 : MR05030T1G

But, when i read at LMR10515 datasheet at page 16. I'm confused with Design Example 2.

I would like to know should i use R2 or Rfb1 at 10K or 100K ohms? 

The Webench proposed 10K ohms. But, datasheet example use 100K ohms.

If i use the formula where R1 = [ Vout / Vref) - 1 ] x R2, i found that R1 is 10K ohms.

Can help me to confirm the components value generated from Webench?

Thanks..

Regards,

Atthen

  • 0
    TI__Expert 4365 points

    Hello Atthen,

    Actually the magnitude of the divider resistors is generally not critical.  It is the ratio of the top to bottom resistor that sets output voltage. One value depends on the other. Some people pick 10K and others pick 100K. I usually pick the bottom feedback resistor based on the closest value to the feedback voltage. So if the error amp has a 1.24V reference I use a 1.24 kohm or 12.4kohm  or maybe a 124 kohm resistor.  With this method the other resistor can be found through subtraction rather than the algebra equation given in the data sheet.

    The resistor divider is part of the supply solution and the current through the divider is a load on Vout that impacts the ultimate efficiency.  Sometimes the divider values are scaled downward to place a minimum load on the output. For micropower stages that need the highest possible efficiency the values tend to be scaled up to preserve light load efficiency.

    The best choice of resistor values are determined by computer programs that have a table of all of the EIA96 1% resistors that are commonly available. (96 values per decade - 1.00K 1.02K 1.05K etc. By the way this table can easily be generated in a spreadsheet by recognizing that the 96 values are evenly spaced on a logrithmic scale. The 96th root of 10 gives the ratio from one EIA96 value to the next. The program will find the best choice of resistor pair to establish the target output voltage with the least error.  Many of these programs are available on the internet.

    But I digress.   ------------------------

    The TI and NSC devices have not all been merged into a single selector system.  Webench has all the historical NSC deivces and is rapidly having the TI library added.

    For the application parameters that you have listed, you should really use a synchronously rectified buck. 

    Using the TI selector tool I'd suggest TPS62751 or TPS62063 over the non-sychronously rectified device you are looking at.

    Alan Martin TI / SVA