TPS53681: Behavior in Light or No-Load condition

There are NVM settings in this device to operate (1) all phases, forced CCM mode, (2) dynamic phase shedding, CCM mode, phases will drop based on configured thresholds when the output current is low, and (3) auto-DCM mode, similar to dynamic phase shedding, but also enable diode emulation in 1 phase mode. 

You can refer to the description on pg 88 of the datasheet and the TRM document describes the register settings. 

Auto-DCM is a diode emulation mode. This means at light load below the critical conduction point, (e.g. when the inductor current would be going negative in FCCM mode), the controller will sense when the current is reaching zero and turn OFF the low-side FET, using the body diode of the FET as a diode rectifier. This increases light-load efficiency because turning off the low-side FET will prevent the power stages from sinking current, and keep the output voltage at the regulation point higher - So the converter will need to send pulses less often to keep the output voltage in regulation - hence reducing the switching loss. There is an OK description in this document from TI: https://www.ti.com/seclit/ug/slyu036/slyu036.pdf around pgs 16-18 showing power component waveforms. 

To try it with TPS53681:

• MFR_SPECIFIC_15[PAGE = 0x00 or 0x01][15] bit to 1b to enable DCM mode in 1 phase operation
• MFR_SPECIFIC_06[PAGE = 0x00 or 0x01][15] bit to 1b to enable DCM through tri-state PWM operation.
• MFR_SPECIFIC_14[PAGE = 0x00 or 0x01][7] bit to 1b to enable dynamic phase shedding. 

These should all be accessible through the Fusion Digital Power Designer GUI.