In described examples, in response to a voltage at an external power terminal falling below a safe limit: a charge pump is operated at a first frequency to produce a voltage at a charge pump node; and a first controlled current is coupled from the charge pump node to a control terminal of a power switch transistor. The power switch transistor has a conduction path coupled between the external power terminal and an internal power terminal at which an internal power source is connected. In response to the voltage at the external power terminal reaching a selected level: the charge pump is operated at a second frequency, lower than the first frequency; and a second controlled current, lower than the first controlled current, is coupled from the charge pump node to the control terminal of the power switch transistor.