The switching frequency of the LV14540 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 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 versus Frequency Curve

The LV14540 switching action can also be synchronized to an external clock from 250 kHz to 2 MHz. Connect a square wave to the RT/SYNC pin through either circuit network shown in Figure 6-5. The internal oscillator is synchronized by the falling edge of the external clock. The recommendations for the external clock include a high level no lower than 1.7 V, a low level no higher than 0.5 V, and 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 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 470 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 PSM

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