SLVSDV8 July 2017 TPS54424
The first step is to decide on a switching frequency for the converter. It is capable of running from 200 kHz to 1.6 MHz. Typically the highest switching frequency possible is desired because it will produce the smallest solution size. A high switching frequency allows for lower valued inductors and smaller output capacitors compared to a power supply that switches at a lower frequency. The main trade off made with selecting a higher switching frequency is extra switching power loss, which hurt the converter’s efficiency.
The maximum switching frequency for a given application is limited by the minimum on-time of the converter and is estimated with Equation 12. Using a maximum minimum on-time of 130 ns for the TPS54424 and 17 V maximum input voltage for this application, the maximum switching frequency is 814 kHz. Considering the 10% tolerance of the switching frequency, a switching frequency of 700 kHz was selected. Equation 13 calculates R7 to be 69.7 kΩ. A standard 1% 69.8 kΩ value was chosen in the design.