The TPS54020 operates in pulse skip mode (see Figure 8-9) at light-load currents to improve efficiency by reducing switching, gate drive, and circulating current losses. When the output voltage is in regulation and the peak switch current at the end of any switching cycle remains below the pulse skipping current threshold, the device enters pulse skip mode. This current threshold is the current level corresponding to a nominal COMP voltage of 270 mV.

When in pulse skip mode, the device clamps the COMP pin voltage to 270 mV and inhibits the high-side MOSFET. Further decreases in load current cannot drive the COMP pin below this clamp voltage level.

When the device is not switching while in pulse skip mode, the output voltage tends to decay. As the voltage control loop compensates for the falling output voltage, the COMP pin voltage begins to rise. At this time, the device enables the high-side MOSFET, and a switching pulse initiates on the next clock cycle. The COMP pin voltage sets the peak switch current. The output voltage re-charges to the regulation set point value, and then the demand for peak switch current will decrease. Eventually, the COMP pin voltage once again falls below the pulse skip mode threshold, at which time the device again enters pulse skip mode.

Bias circuits in the BOOT regulator and high-side MOSFET gate drive both return bias current out from the PH pin. While this current is small and in the range of 150 µA (nominal), during very light load conditions, it is possible that the output voltage rises above the desired output voltage setpoint due to this current. If the application design anticipates that system loads can fall below this current level, it is recommended to add a fixed resistor load to the design that dissipates this current. An easy implementation of this fixed load can be achieved with the feedback voltage divider resistors. The recommendation is to use a lower divider resistor value of 2.5 kΩ or lower in this case, and this lower divider resistor should be installed even when the output voltage setpoint is 0.6 V.

Figure 8-7 TPS54020 in Continuous Conduction Mode

Figure 8-9 TPS54020 in Pulse Skipping Mode

Figure 8-8 TPS54020 in Discontinuous Conduction Mode