UCC14241-Q1: RFBVEE difference between datasheet(10kohm) and spread sheet(50kohm), which is correct?

(1) According to the UCC14241-Q1 calculator tool in the product folder, RFBVEE_COM is 50kΩ at cell of C24 .

• The default value in the Excel Design Tool is populated as 50kΩ but you can change it to be something else. For the purpose of BOM component similarity, I try and start with the lower FB resistors as 10kΩ for both FBVDD and FBVEE but 50kΩ is ok.

On the other hand, page 39 of the data sheet states that if VEE-COM=2.5V, RFBVEE_COM should be set to 10kΩ.
Which is correct?

• Same answer as above - both can be correct. The resistor dividers for FBVDD and FBVEE are set so that the ratio of the upper to lower feedback results in 2.5V at the FBVxx pin, according to the desired regulation set voltage.

(2) Also, please tell me the calculation process for RFBVEE_COM on the calculation sheet.

• It's a simple resistor divider calculation where the resistors are selected according to the desired regulation voltage, VEE for example:
  • VEE=2.5V*[R_FB(top)+R_FB(bottom)]/R_FB(bottom)
  • or if you want to select the bottom resistor, R_FB(bottom), (10kΩ or 50kΩ or something else?) and calculate the top resistor for a desired VEE: 
    • R_FB(top)=[R_FB(bottom)*(VEE-2.5V)]/2.5V

(3) Changing the value in this section in the spreadsheet did not have any particular effect on the calculation results.

• Changing the value of cell C24 will result in a different calculated result for the upper FBVEE, feedback resistor shown in cell C46. The spreadsheet formula will be like:
  • C46=[C24*(C14-2.5V)]/2.5V

Could you please let us know if there are any limits on the resistance value of RFVEE-COM.

• There is no imposed limit on the FB resistor values except that large values will result in less load on VDD, VEE and smaller resistor values will place more load (efficiency impact) on VDD, VEE.

 Steve