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

The switching frequency of the TPS57140-EP is adjustable over a wide range from approximately 100-kHz to 2500-kHz by placing a resistor on the RT/CLK pin. The RT/CLK pin voltage is typically 0.5 V and must have a resistor to ground to set the switching frequency. To determine the timing resistance for a given switching frequency, use Equation 7 or the curves in Figure 32 or Figure 33. To reduce the solution size, the designer would typically set the switching frequency as high as possible, but consider tradeoffs of the supply efficiency, maximum input voltage, and minimum controllable on-time.

The minimum controllable on time is typically 130 ns, which limits the maximum operating input voltage.

The frequency-shift circuit also limits the maximum switching frequency. The following contains more discussion on the details of the maximum switching frequency.

Equation 7.

Figure 32. High-Range RT

Figure 33. Low-Range RT