8.2.2.1 Selecting the Switching Frequency

The first step is to decide on a switching frequency for the regulator. Typically, the user will want to choose the highest switching frequency possible because this will produce the smallest solution size. The high switching frequency allows for lower valued inductors and smaller output capacitors compared to a power supply that switches at a lower frequency. The switching frequency that can be selected is limited by the minimum on-time of the internal power switch, the input voltage and the output voltage and the frequency shift limitation.

Equation 8 and Equation 9 must be used to find the maximum switching frequency for the regulator, choose the lower value of the two equations. Switching frequencies higher than these values will result in pulse skipping or the lack of overcurrent protection during a short circuit.

The typical minimum on time, t_onmin, is 130 ns for the TPS57060-Q1. For this example, the output voltage is 3.3 V and the maximum input voltage is 48 V, which allows for a maximum switch frequency up to 616 kHz when including the inductor resistance, on resistance and diode voltage in Equation 8. To ensure overcurrent runaway is not a cwhenrn during short circuits in your design use Equation 9 or the solid curve in Figure 33 to determine the maximum switching frequency. With a maximum input voltage of 48 V, assuming a diode voltage of 0.5 V, inductor resistance of 130 mΩ, switch resistance of 400 mΩ, a current limit value of 0.94 A and a short circuit output voltage of 0.1 V. The maximum switching frequency is approximately 923 kHz.

Choosing the lower of the two values and adding some margin a switching frequency of 500 kHz is used. To determine the timing resistance for a given switching frequency, use Equation 7 or the curve in Figure 31.

The switching frequency is set by resistor R₃ shown in Figure 51.