The switching frequency of the TPS2586x-Q1 can be programmed by the resistor R_FREQ from the FREQ/SYNC pin and AGND pin. To determine the FREQ resistance, for a given switching frequency, use Equation 4:

Equation 4.

Figure 10-3 FREQ Set Resistor vs Switching Frequency

The normal method of setting the buck regulator switching frequency is by selecting an appropriate value FREQ resistor as shown in Table 10-1.

The FREQ/SYNC pin can be used to synchronize the internal oscillator to an external clock. The internal oscillator can be synchronized by AC coupling a positive edge into the FREQ/SYNC pin. When using a low impedance signal source, the frequency setting resistor FREQ 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. The AC coupled peak-to-peak voltage at the FREQ/SYNC pin must exceed the SYNC amplitude threshold of 1.2 V (typical) to trip the internal synchronization pulse detector, and the minimum SYNC clock HIGH and LOW time must be longer than 100 ns (typical). A 2.5 V or higher amplitude pulse signal coupled through a 1-nF capacitor, C_SYNC, is a good starting point. Figure 10-4 shows the device synchronized to an external system clock. The external clock must be off before start-up to allow proper start-up sequencing.

Figure 10-4 Synchronize to External Clock

The TPS2586x-Q1 switching action can be synchronized to an external clock from 200 KHz to 800 KHz.