The switching frequency of the LMR14030 can be programmed by the resistor R_T 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 5 or the curve in Figure 6-4. Table 6-1 gives typical R_T values for a given f_SW.

Equation 5.

Figure 6-4 R_T vs Frequency Curve

The LMR14030 switching action can also be synchronized to an external clock from 250 kHz to 2.3 MHz. Connect a square wave to the RT/SYNC pin through either circuit network shown in Figure 6-5. Internal oscillator is synchronized by the falling edge of external clock. The recommendations for the external clock include: high level no lower than 1.7 V, low level no higher than 0.5 V and have a pulse width greater than 30 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 (that is, 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 6-6, Figure 6-7 and Figure 6-8 show the device synchronized to an external system clock.

Figure 6-5 Synchronizing to an External Clock

Figure 6-6 Synchronizing in CCM

Figure 6-8 Synchronizing in Sleep-mode Mode

Figure 6-7 Synchronizing in DCM

Equation 6 calculates the maximum switching frequency limitation set by the minimum controllable on time and the input to output step down ratio. Setting the switching frequency above this value causes the regulator to skip switching pulses to achieve the low duty cycle required at maximum input voltage.

Equation 6.

where

• I_OUT = Output current
• R_IND = Inductor series resistance
• V_{IN_MAX} = Maximum input voltage
• V_OUT = Output voltage
• V_D = Diode voltage drop
• R_{DS_ON} = High-side MOSFET switch on resistance
• t_ON = Minimum on time