For single-output, dual-phase operation, two LM708x0-Q1 devices are required. Additional phases cannot be added. Configure the first device as a primary and the second device as a secondary per Table 7-2. This action disables the feedback error amplifier of the secondary device and places it into a high-impedance state. Connect EXTCOMP pins of the primary and secondary devices together with minimal trace length. Add an external compensation network near the primary device. Internal compensation feature is not supported when operating in dual-phase configuration. The PG / SYNCOUT pin of the primary device must be connected to the PFM / SYNCIN pin of the secondary device. The SYNCOUT of the primary device is 180° out-of-phase and facilitates the interleaved operation. RT pin is not used for the oscillator when the LM708x0-Q1 is configured as a secondary device but instead is used for the slope compensation. RT resistance on the secondary device needs to be the same value as RT resistance on the primary device to make sure of correct operation. The oscillator frequency is derived from the primary device. When operating in Interleaved mode, both devices need to be enabled at the exact same time for start-up. After the regulator has started, pull the secondary EN pin low (< 0.8V) for phase shedding if needed at light load to increase the efficiency.

Configure PFM mode by connecting both the PFM/SYNC pin of the primary and the FB pin of the secondary to the VDDA pin as shown in Figure 7-6.

Figure 7-6 Simplified Schematic for Single-Output Dual-Phase Operation in PFM Mode

Configure FPWM mode by applying an external synchronization signal to the PFM/SYNCIN pin of the primary or connecting the pin to the AGND and connecting the FB pin of the secondary to the AGND pin as shown in Figure 7-7.

Figure 7-7 Simplified Schematic for Single-Output Dual-Phase Operation in FPWM Mode

In PFM mode, while the primary device is pulse skipping to reduce the I_Q-SLEEP current, the primary device disables the synchronization clock output, therefore the phase shedding is not supported. Phase shedding is only supported in FPWM.

When operating in PFM mode under light load conditions, either primary device or secondary device or both devices can switch.

In FPWM mode, the secondary device can be disabled to reduce the I_Q-SLEEP current, and then the device can be enabled to support higher load currents when needed. When the secondary device enable is recycled the internal soft-start is pulled low, and then the LM708x0-Q1 goes through a normal soft-start turn-on. During the secondary soft-start time (2.8ms typical) there is a phase current imbalance until the soft-start is done. The phase current imbalance is also possible when in dropout as there is no control over the current. Matching the impedance of the outputs of the primary and secondary devices minimizes the phase current imbalance.

For more information, see Benefits of a Multiphase Buck Converter and Multiphase Buck Design From Start to Finish.