SNVSBZ3 June 2021 LM5168-Q1
The switching frequency of the LM516x-Q1 is set by the on-time programming resistor connected to the RT pin. Equation 14 is used to calculate RT based on the desired switching frequency. For this example of 750 kHz, 33.2 kΩ is used.
Note that at very low duty cycles, the 50-ns minimum controllable on time of the high-side MOSFET, tON(min), limits the maximum switching frequency. In CCM, tON(min) limits the voltage conversion step-down ratio for a given switching frequency. Use Equation 15 to calculate the minimum controllable duty cycle.
Ultimately, the choice of switching frequency for a given output voltage affects the available input voltage range, solution size, and efficiency. Use Equation 16 to calculate the maximum supply voltage for a given tON(min) to maintain the full switching frequency.