SLUU180B January   2022  – February 2022 TPS40070 , TPS40071

 

  1.   Trademarks
  2. 1Introduction
  3. 2Description
  4. 3Schematic
    1. 3.1 Output Filter Components
    2. 3.2 MOSFET Selection
    3. 3.3 Frequency Feedforward Resistor Selection
    4. 3.4 Output Voltage Setpoint
    5. 3.5 Short Circuit Protection Resistor Selection
    6. 3.6 Miscellaneous Parts
    7. 3.7 Control Loop Compensation
  5. 4Test Setup
  6. 5Results
    1. 5.1 Control Loop Characteristics
  7. 6Assembly Drawing and PCB Layout
  8. 7List of Materials
  9. 8Reference
  10. 9Revision History

3.3 Frequency Feedforward Resistor Selection

To program the switching frequency of 300 kHz, R2 is selected according to the equation from the TPS40070/1/2Midrange Input Synchronous Buck Controller data sheet:

Equation 3. Rt=R= 1FSW×17.82 × 10-6-23=164 kΩ

A standard 1% value of 165 kΩ is selected.

After the switching frequency is selected, the value of Rkff would normally be selected to program the minimum desired start-up voltage by rearranging the equation for VUVLO_ON. However, the UVLO threshold is not a tightly controlled specification, so a low value start-up voltage cannot be accurately programmed. In this case, the converter will be allowed to start at the fixed UVLO threshold of 4.5 V. This requires that the value of Rkff should be selected to be less than the minimum value on the programmable UVLO VON and VOFF versus Rkff graph in the TPS40070/1/2Midrange Input Synchronous Buck Controller data sheet. In this converter, Rkff is selected to be 75 kΩ.