A power transistor and a cascode transistor are connected in series. A driver circuit has an output driving a control terminal of the power transistor. The driver circuit has a first power supply node coupled to receive a floating voltage that is also applied to a control terminal of the cascode transistor. A variable voltage generator generates the floating voltage. The floating voltage track either a power supply voltage or a reference voltage over a first range of voltage levels for the power supply voltage. The floating voltage further satisfies a ratio metric relationship dependent on the power supply voltage and reference voltage over a second range of voltage levels for said power supply voltage.