The TPS55289 improves the efficiency at light load condition with PFM mode. By enabling the PFM function in the internal register, the TPS55289 can work in PFM mode at light load condition. When the TPS55289 operates at light load condition, the output of the internal error amplifier decreases to make the inductor peak current down to deliver less power to the load. When the output current further reduces, the current through the inductor will decrease to zero during the switch-off time. When the TPS55289 works in buck mode, once the inductor current becomes zero, the low-side switch of the buck side is turned off to prevent the reverse current from output to ground. When the TPS55289 works in boost mode, once the inductor current becomes zero, the high side-switch of the boost side is turned off to prevent the reverse current from output to input. The TPS55289 resumes switching until the output voltage drops, so PFM mode reduces switching cycles and eliminates the power loss by the reverse inductor current to get high efficiency in light load condition.