Controllable on-time reduction for switching voltage regulators operating in pulse frequency modulation mode

1. A method of controlling switching voltage regulator that includes a power stage for delivering output current to a load through an inductor, the method comprising:

setting the power stage in a PFM (pulse frequency modulation) switching mode if the output current decreases below a first threshold, each period of the PFM switching mode including an on-time during which a high-side transistor of the power stage is on and a low-side transistor of the power stage is off, an off-time during which the low-side transistor is on and the high-side transistor is off and a HiZ-time during which the high-side transistor and the low-side transistor are both off; and

varying the on-time of the PFM switching mode responsive to a change in the output current, by:

increasing the on-time of a PFM pulse from the preceding PFM pulse if the time between the PFM pulses is shorter than a first expected period: and

decreasing the on-time of the PFM pulse from the preceding PFM pulse if the time between the PFM pulses is longer than a second expected period.

2. The method of claim 1 , wherein the on-time of the PFM switching mode is varied so as to maintain, in the PFM switching mode, a fixed ratio between a peak current of the inductor and the output current without measuring the actual output current or inductor current.

3. The method of claim 1 , further comprising:

fixing the on-time of the PFIVI switching mode responsive to the HiZ-time, frequency or period of the PFM switching mode increasing to a maximum threshold above which the on-tune remains constant.

4. A switching voltage regulator, comprising:

a power stage configured to deliver output current to a load through an inductor, the power stage comprising a high-side transistor and a low-side transistor; and

a controller operable to:

set the power stage in a PM (pulse frequency modulation) switching mode if the output current decreases below a first threshold, each period of the PFM switching mode including an on-time during which the high-side transistor is on and the low-side transistor is off, an off-time during which the low-side transistor is on and the high-side transistor is off and a HiZ-time during which the high-side transistor and the low-side transistor are both off; and

vary the on-time of the PFM switching mode responsive to a change in the output current, by increasing the on-time of a PFM pulse from the preceding PFM pulse if the time between the PFM pulses is shorter than a first expected period and decreasing the on-time of the PFM pulse from the preceding PFM pulse if the time between the PFM pulses is longer than a second expected period.

5. The switching voltage regulator of claim 4 , wherein the controller is operable to vary the on-time of the PFM switching mode so as to maintain, in the PFM switching mode, a fixed ratio between a peak current of the inductor and the output current without measuring the actual output current or inductor current.

6. The switching voltage regulator of claim 4 , wherein the controller is operable to fix the on-time of the PFM switching mode responsive to the HiZ-time, frequency or period of the PM switching mode increasing to a maximum threshold above which the on-time remains constant.

7. The switching voltage regulator of claim 4 , wherein the switching voltage regulator is a multi-phase switching voltage regulator and the power stage is one phase of the multi-phase switching voltage regulator.