TPS546C25 device uses a zero-crossing (ZC) circuit to perform the zero inductor current detection during skip-mode operation. The ZC threshold is set to a small negative value before the low-side MOSFET is turned off, entering discontinuous conduction mode (DCM) operation. After entering DCM, the ZC threshold hysteresis increases the threshold to a small positive value after entering DCM. As a result, the device delivers better light-load efficiency.

When the load current increases enough such that the device exits DCM, the ZC circuit must detect 16 consecutive cycles of negative inductor current below the ZC threshold before returning to DCM. Only one cycle without ZC detection is required to exit DCM.

When the output is enabled, the ZC circuit is also enabled during the first 32 switching cycles while the device is in soft start. If the MSEL resistor value is for FCCM, ZC is disabled and the device transitions to FCCM when soft-start is complete. If there are not at least 32 switching cycles before soft-start is done, such as during start up with a high output prebias, the ZC is not disabled until the first high-side MOSFET on-time after soft start is complete.