The switching frequency of the LM76005-Q1 can be programmed by the impedance R_T from the RT pin to ground. The frequency is inversely proportional to the R_T resistance. The RT pin can be left floating, and the LM76005-Q1 operates at 400-kHz default switching frequency. The RT pin is not designed to be shorted to ground.

For an desired frequency, R_T can be found by:

Equation 6.

The LM76005-Q1 switching action can also be synchronized to an external clock from 200 kHz to 500 kHz. TI recommends connecting an external clock to the SYNC pin with an appropriate termination resistor. Ground the SYNC pin if not used.

Figure 7-7 Frequency Synchronization

The recommendations for the external clock include high level no lower than 2 V, low level no higher than 0.4 V, duty cycle between 10% and 90%, and both positive and negative pulse width no shorter than 80 ns. When the external clock fails at logic high or low, the LM76005-Q1 switches at the frequency programmed by the R_T resistor after a time-out period. TI recommends connecting a resistor R_T to the RT pin so that the internal oscillator frequency is the same as the target clock frequency when the LM76005-Q1 is synchronized to an external clock. This allows the regulator to continue operating at approximately the same switching frequency if the external clock fails.

The choice of switching frequency is usually a compromise between conversion efficiency and the size of the circuit. Lower switching frequency implies reduced switching losses (including gate charge losses, switch transition losses, and so forth) and usually results in higher overall efficiency. However, higher switching frequency allows use of smaller LC output filters and hence a more compact design. Lower inductance also helps transient response (higher large signal slew rate of inductor current), and reduces the DCR loss. 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. It is related to the following:

• Input voltage
• Output voltage
• Most frequent load current level or levels
• External component choices
• Circuit size requirement

The choice of switching frequency can also be limited if an operating condition triggers t_ON-MIN or t_OFF-MIN.