The TPS54561 operates in a pulse skipping Eco-mode at light load currents to improve efficiency by reducing switching and gate drive losses. If the output voltage is within regulation and the peak switch current at the end of any switching cycle is below the pulse skipping current threshold, the device enters Eco-mode. The pulse skipping current threshold is the peak switch current level corresponding to a nominal COMP voltage of 600 mV.

When in Eco-mode, the COMP pin voltage is clamped at 600 mV and the high side MOSFET is inhibited. Since the device is not switching, the output voltage begins to decay. The voltage control loop responds to the falling output voltage by increasing the COMP pin voltage. The high side MOSFET is enabled and switching resumes when the error amplifier lifts COMP above the pulse skipping threshold. The output voltage recovers to the regulated value, and COMP eventually falls below the Eco-mode pulse skipping threshold at which time the device again enters Eco-mode. The internal PLL remains operational when in Eco-mode. When operating at light load currents in Eco-mode, the switching transitions occur synchronously with the external clock signal.

During Eco-mode operation, the TPS54561 senses and controls peak switch current, not the average load current. Therefore the load current at which the device enters Eco-mode is dependent on the output inductor value. As the load current approaches zero, the device enters a pulse skip mode during which it draws only 152-μA input quiescent current. The circuit in Figure 8-1 enters Eco-mode at 25-mA output current and with no external load has an average input current of 280 µA.