A buck switching voltage regulator, with high side and low side switching transistors, includes mode control circuitry for switching between PWM and PFM modes based on sensing inductor current through the low side switch during switching cycle OFF times (inductor discharge). Mode switching is based on comparing a an integrated inductor current sense signal with an integrated reference signal corresponding to a predefined average inductor current IAVE. In one embodiment, a mode switching condition is based in part on [IVALLEY=2IAVE−IPEAK], where IPEAK is a detected peak inductor current at the beginning of an OFF time, and IVALLEY is an inductor current value determined by IAVE and IPEAK.