7.3.7 Switching Frequency (RT) and Synchronization (SYNC)

The switching frequency of the LM46000 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 LM46000 operates at 500-kHz default switching frequency. The RT pin is not designed to be shorted to ground.

For a desired frequency, typical R_T resistance can be found by Equation 3.

Figure 40 shows R_T resistance vs switching frequency F_S curve.

Figure 40. R_T Resistance vs Switching Frequency

Table 1 provides typical R_T values for a given F_S.

The LM46000 switching action can also be synchronized to an external clock from 200 kHz to 2.2 MHz. Connect an external clock to the SYNC pin, with proper high speed termination, to avoid ringing. Ground the SYNC pin if not used.

Figure 41. 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 LM46000 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 such that the internal oscillator frequency is the same as the target clock frequency when the LM46000 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, etc.) 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 input voltage, output voltage, most frequent load current level(s), external component choices, and circuit size requirement. The choice of switching frequency may also be limited if an operating condition triggers T_ON-MIN or T_OFF-MIN.