The ROSC pin can be used to set the switching frequency by connecting a resistor to GND. The switching frequency of the device is adjustable from 250 kHz to 2.3 MHz.

To determine the switching frequency for a given ROSC resistance, use Equation 8 or the curve in Figure 8-10. To reduce the solution size one must set the switching frequency as high as possible, but tradeoffs of the supply efficiency and minimum controllable on time must be considered.

Figure 8-10 Switching Frequency versus ROSC

When an external clock applies to ROSC pin, the internal phase locked loop (PLL) has been implemented to allow internal clock synchronizing to an external clock between 250 kHz and 2.3 MHz. To implement the clock synchronization feature, connect a square wave clock signal to the ROSC pin with a duty cycle between 20% to 80%. The clock signal amplitude must transition lower than 0.4 V and higher than 2.0 V. The start of the switching cycle is synchronized to the falling edge of ROSC pin.

In applications where both resistor mode and synchronization mode are needed, the device can be configured as shown in Figure 8-11. Before an external clock is present, the device works in resistor mode and ROSC resistor sets the switching frequency. When an external clock is present, the synchronization mode overrides the resistor mode. The first time the ROSC pin is pulled above the ROSC high threshold (2.0 V), the device switches from the resistor mode to the Synchronization mode and the ROSC pin becomes high impedance as the PLL starts to lock onto the frequency of the external clock. TI does not recommend to switch from the synchronization mode back to the resistor mode because the internal switching frequency drops to 100 kHz first before returning to the switching frequency set by ROSC resistor.

Figure 8-11 Works With Resistor Mode and Synchronization Mode