SLVSFM1 March 2021 TPS62902
When the MODE/S-CONF pin is configured for power save mode (auto PFM/PWM). The device operates in PWM mode as long the output current is higher than half of the ripple current of the inductor. To maintain high efficiency at light loads, the device enters power save mode at the boundary to discontinuous conduction mode (DCM). This happens if the output current becomes smaller than half of the ripple current of the inductor. The power save mode is entered seamlessly when the load current decreases. This ensures a high efficiency in light load operation. The device remains in power save mode as long as the inductor current is discontinuous.
In power save mode, the switching frequency decreases linearly with the load current maintaining high efficiency. The transition in and out of power save mode is seamless in both directions.
In addition to adjusting the switching, the TPS62902 adjusts the on-time (TON) in power save mode, depending on the input voltage and the output voltage to maintain highest efficiency using the AEE function when 2.5 MHz is selected as described in Section 7.4.2.
For very small output voltages, an absolute minimum on-time of about 50 ns is kept to limit switching losses. The operating frequency is thereby reduced from its nominal value, which keeps efficiency high. Using TON, the typical peak inductor current in power save mode is approximated by Equation 7:
There is a minimum off-time which limits the duty cycle of the TPS62902. When VIN decreases to typically 15% above VOUT, the TPS62902 does not enter power save mode, regardless of the load current. The device maintains output regulation in PWM mode.
The output voltage ripple in power save mode is given by Equation 8: