In described examples, a transistor (320) has: a source and a drain coupled between a supply voltage and an output terminal; and a gate terminal. A charge pump (314) has: an output node (VGATE) coupled to the gate terminal; and a clock input. An oscillator (310) is coupled to generate a clock signal. A clock enable circuit (312) is coupled to: receive the clock signal; and selectively output the clock signal to the clock input, responsive to an enable signal. A comparator (360) is coupled to output the enable signal in response to a comparison between a reference current and a current through a series resistor (R1). The series resistor (R1) is coupled to the gate terminal.