7.3.10 Constant Switching Frequency and Timing Resistor (RT/CLK Pin)

The switching frequency of the TPS54678 is adjustable over a wide range from approximately 200 kHz to
2000 kHz by placing a maximum of 210 kΩ and minimum of 18 kΩ, respectively, on the RT/CLK pin. The RT/CLK is typically 0.5 V. To determine the timing resistance for a given switching frequency, use the curve in Figure 24. To reduce the solution size one would typically set the switching frequency as high as possible, but tradeoffs of the supply efficiency, maximum input voltage and minimum controllable on time should be considered.

The minimum controllable on time is typically 85 ns at 3-A current load and 100 ns at no load, and will limit the maximum operating input voltage or output voltage.

Equation 5.

Figure 24. Switching Frequency vs RT Set Resistor