Mode control circuitry is disclosed for use in a buck switching voltage regulator capable of operating in a pulse width modulation (PWM) mode and a pulse frequency modulation (PFM) mode, with the regulator including an inductor having first and second opposite inductor terminals, a first transistor switch connected between the first inductor terminal and a power input terminal and a second transistor switch connected between the first inductor terminal and a circuit common. Current sensing circuitry is provided to sense inductor current through the second switching transistor when the second switching transistor is switched to an ON state and to produce a current sense signal which is integrated over time starting when the second switching transistor is switched to an ON state and to produce a sense signal. The mode switching circuitry switches between the PWM and PFM modes in response to the sense signal.