The TPS61088 improves the efficiency at light load with PFM mode. When the converter operates in light load condition, the output of the internal error amplifier decreases to make the inductor peak current down, delivering less power to the load. When the output current further reduces, the current through the inductor decrease to zero during the off-time. Once the current through the high side N-MOSFET is zero, the high-side MOSFET is turned off until the beginning of the next switching cycle. When the output of the error amplifier continuously goes down and reaches a threshold with respect to the peak current of I_LIM / 12, the output of the error amplifier is clamped at this value and does not decrease any more. If the load current is smaller than what the TPS61088 delivers, the output voltage increases above the nominal setting output voltage. The TPS61088 extends its off-time of the switching period to deliver less energy to the output and regulate the output voltage to 0.7% higher than the nominal setting voltage. With PFM operation mode, the TPS61088 keeps the efficiency above 80% even when the load current decreases to 1 mA. In addition, the output voltage ripple is much smaller at light load due to low peak current. Refer to Figure 7-2.

Figure 7-2 PFM Mode Diagram