SNVSAN3F August 2017 – November 2020 LMR33630
The output voltage of the LMR33630is externally adjustable using a resistor divider network. The range of recommended output voltage is found in Section 7.3. The divider network is comprised of RFBT and RFBB, and closes the loop between the output voltage and the converter. The converter regulates the output voltage by holding the voltage on the FB pin equal to the internal reference voltage, VREF. The resistance of the divider is a compromise between excessive noise pick-up and excessive loading of the output. Smaller values of resistance reduce noise sensitivity but also reduce the light-load efficiency. The recommended value for RFBT is 100 kΩ; with a maximum value of 1 MΩ. If a 1 MΩ is selected for RFBT, then a feedforward capacitor must be used across this resistor to provide adequate loop phase margin (see Section 188.8.131.52). Once RFBT is selected, Equation 3 is used to select RFBB. VREF is nominally 1 V (see Section 7.5 for limits).
For this 5-V example, RFBT = 100 kΩ and RFBB = 24.9 kΩ are chosen.