The switching frequency of the TPS2585x-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, the typical FREQ resistors value are listed 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 (typ.) to trip the internal synchronization pulse detector, and the minimum SYNC clock HIGH and LOW time must be longer than 100 ns (typ). 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 TPS2585x-Q1 switching action can be synchronized to an external clock from 200 KHz to 3 MHz. Even the switching frequency can be set to higher than 2.4 MHz, but TI recommends to set the switching frequency below 2.4 MHz due to the power dissipation, the higher switching frequency results in more power loss on IC, cause the junction temperature and also the board temperature rising, then the device can enter load shedding under high ambient temperature.

Figure 10-5, Figure 10-6 and Figure 10-7 show the device switching frequency and behavior under different VIN voltage and R_FREQ = 8.45 kΩ.

Figure 10-8, Figure 10-9 and Figure 10-10 show the device switching frequency and behavior under different VIN voltage and synchronized to an external 2.2-MHz system clock.

VIN = 8 V

L = 0.68 uH

IPA_BUS = 3 A

IPB_BUS = 3 A

Figure 10-5 Switching Frequency When R_FREQ = 8.45 kΩ

VIN = 18 V

L = 0.68 uH

IPA_BUS = 3 A

IPB_BUS = 3 A

Figure 10-7 Switching Frequency When R_FREQ = 8.45 kΩ

VIN = 13.5 V

L = 0.68 uH

IPA_BUS = 3 A

IPB_BUS = 3 A

Figure 10-9 Synchronizing to External 2.2-MHz Clock

VIN = 13.5 V

L = 0.68 uH

IPA_BUS = 3 A

IPB_BUS = 3 A

Figure 10-6 Switching Frequency When R_FREQ = 8.45 kΩ

VIN = 8 V

L = 0.68 uH

IPA_BUS = 3 A

IPB_BUS = 3 A

Figure 10-8 Synchronizing to External 2.2-MHz Clock

VIN = 18 V

L = 0.68 uH

IPA_BUS = 3 A

IPB_BUS = 3 A

Figure 10-10 Synchronizing to External 2.2-MHz Clock