The internal oscillator frequency is controlled by the impedance on the RT pin. If the RT pin is open circuit, the LM73605-Q1/6-Q1 at default switching frequency, 500 kHz. The RT pin is not designed to be connected directly to ground. To program the switching frequency by R_T resistor, Equation 13, or Figure 8-12, or Table 8-1 can be used to find the resistance value.

Equation 13.

Figure 8-12 R_T Resistance versus Switching Frequency

The choice of switching frequency is usually a compromise between conversion efficiency and the size of the solution. Lower switching frequency has lower switching losses (including gate charge losses, switch transition losses, and so forth) and usually results in higher overall efficiency. However, higher switching frequency allows the use of smaller power inductor and output capacitors, hence a more compact design. Lower inductance also helps transient response (higher large signal slew rate of inductor current), and has lower DCR. The optimal switching frequency is usually a trade-off in a given application and thus needs to be determined on a case-by-case basis. The following are factors that need to be taken into account:

• Input voltage range
• Output voltage
• Most frequent load current level or levels
• External component choices
• Solution size/cost requirements
• Efficiency
• Thermal management requirements

The choice of switching frequency can also be limited whether an operating condition triggers t_ON-MIN or t_OFF-MIN. Minimum on-time, t_ON-MIN, is the smallest time that the HS switch can be on. Minimum off-time, t_OFF-MIN, is the smallest duration that the HS switch can be off.

In CCM operation, t_ON-MIN and t_{OFF_MIN} limit the voltage conversion range given a selected switching frequency, f_SW. The minimum duty cycle allowed is:

The maximum duty cycle allowed is:

Given an output voltage, the choice of the switching frequency affects the allowed input voltage range, solution size and efficiency. The maximum operational supply voltage can be found by:

At lower supply voltage, the switching frequency decreases once t_OFF-MIN is tripped. The minimum V_IN without frequency foldback can be approximated by:

With a desired V_OUT, the range of allowed V_IN is narrower with higher switching frequency.

The LM73605-Q1/6-Q1 an advanced frequency foldback algorithm under both t_{ON_MIN} and t_{OFF_MIN} conditions. With frequency foldback, stable output voltage regulation is extended to wider range of supply voltages.

At very high V_IN conditions where t_ON-MIN limitation is met, the switching frequency reduces to allow higher V_IN while maintaining V_OUT regulation. Note that the peak-to-peak inductor current ripple will increase with higher V_IN and lower frequency. TI does not recommend designing the circuit to operate with t_{ON_MIN} under typical conditions.

At very low V_IN conditions, where t_OFF-MIN limitation is met, the switching frequency decreases until t_ON-MAX condition is met. Such frequency foldback mechanism allows the LM73605-Q1/6-Q1 to have very low dropout voltage regardless of frequency setting.