The TPS6521905-Q1 supports low power modes through the I2C-control or through the MODE/STBY pin. The configuration of the pin is selected by MODE_STBY_CONFIG in MFP_2_CONFIG register. The polarity of this pin can be configured by writing to MODE_STBY_POLARITY in MFP_1_CONFIG register. The polarity-configuration must not change after power-up. Only either MODE/RESET or MODE/STBY must be configured as MODE. If both are configured as MODE, MODE/RESET takes priority and MODE/STBY is ignored.

MODE/STBY configured as 'MODE':

• If configured as 'MODE', the pin-status determines the switching-mode of the buck-converters.
• Forcing this pin for longer than t_{DEGLITCH_MFP} forces the buck-regulators into PWM-mode (irrespective of load current). De-asserting this pin low allows the buck regulators to enter PFM-mode. The entry into PFM and exit from PFM is governed by the load current. Only one pin, either MODE/STBY or MODE/RESET must be configured as 'MODE'.
• The selection of auto-PFM/forced-PWM can also be controlled by writing to the bit MODE_I2C_CTRL in MFP_1_CONFIG register.
• A change of the MODE does not cause a state-transition.
• During power-up of any one of the three bucks, a MODE change is blanked on this rail and only takes effect after the ramp completed.

MODE/STBY configured as 'STBY':

• Forcing this pin for longer than t_{DEGLITCH_MFP} sequences down the rails selected to turn off in the STBY_1_CONFIG respectively the STBY_2_CONFIG register. De-asserting this pin sequences the selected rails on again.
• A transition into and out of STBY state can also be controlled by writing to the bit STBY_I2C_CTRL in MFP_CTRL register, provided I2C communication is supported during STBY state.
• A change of the MODE/STBY pin configured as 'STBY' does cause a state-transition by definition.
• Regardless of the pin-setting, the device always powers up into ACTIVE state. The device reacts to the STBY-pin-state or I2C-commands only after entering ACTIVE state.

MODE/STBY configured as 'MODE & STBY':

• The pin can be configured to perform both functions, MODE and STBY simultaneously
• Forcing this pin for longer than t_{DEGLITCH_MFP} sequences down the rails selected to turn off in the STBY_1_CONFIG respectively the STBY_2_CONFIG register and allows auto-PFM entry (only applicable in quasi-fixed-frequency mode). De-asserting this pin sequences the selected rails on again and forces the buck-regulators to forced-PWM. Polarity settings need to be harmonized for this configuration.
• If a transition into and out of STBY state is commanded by writing to the bit STBY_I2C_CTRL in MFP_CTRL register (provided I2C communication is supported during STBY state), a separate command for the MODE-change is required by writing to the bit MODE_I2C_CTRL in MFP_1_CONFIG register.
• A change of the MODE/STBY pin configured as 'MODE&STBY' does cause a state-transition by definition.
• By default STBY is deasserted and the pin is ignored until the device completed the power-up-sequence. During power-up of any one of the three bucks, a MODE-change is blanked on this rail and only takes effect after the ramp completed. A state-change commanded by STBY-pin is reacted to even during the ramp of rails (except during INITIALIZE-to-ACTIVE transition).

Please see below truth-table for pin- and I2C-commands.