The switching frequency of the LMR38020 can be programmed by the resistor RT from the RT/SYNC pin and GND pin. The RT/SYNC pin cannot be left floating or shorted to ground. To determine the timing resistance for a given switching frequency, use Equation 2 or the curve in Figure 8-4. Table 8-1 gives typical R_T values for a given f_SW.

Figure 8-4 R_T Versus Frequency Curve

The LMR38020 switching action can also be synchronized to an external clock from 300 kHz to 2.1 MHz. Connect a square wave to the RT/SYNC pin through either circuit network shown in Figure 8-5. The internal oscillator is synchronized by the falling edge of external clock. The recommendations for the external clock include: high level no lower than 2.0 V, low level no higher than 0.6 V, and have a pulse width greater than 50 ns. When using a low impedance signal source, the frequency setting resistor R_T is connected in parallel with an AC coupling capacitor, C_COUP, to a termination resistor, R_TERM (for example, 50 Ω). The two resistors in series provide the default frequency setting resistance when the signal source is turned off. A 10-pF ceramic capacitor can be used for C_COUP.

Figure 8-5 Synchronizing to an External Clock