Circuit Design Guide for DC/DC Converters(6/7)

Selecting the RFB1 and RFB2

With an FB (feedback) model, RFB1 and RFB2 are used to determine output voltage. If a wide variety of combinations of RFB1 and RFB2 are available for an identical output voltage, the sum of RFB1 and RFB2 is recommended to be in the range of 150kΩ to 500kΩ. In this case, the efficiency at the light-load condition and the output stability at the heavy-load condition need to be considered. The currents flowing through RFB1 and RFB2 are not used for the output power and regarded as loss of the DC/DC converter. Therefore, to improve the efficiency at the light-load condition, larger values (RFB1+RFB2<1MΩ) should be selected.