In an example, an apparatus includes a level-shifting circuit and a ramp detector. The level-shifting circuit has a current choke and a transistor coupled across the current choke, the level-shifting circuit adapted to be coupled to a first voltage source. The ramp detector has a ramp detector input adapted to be coupled to the first voltage source and a ramp detector output coupled to the transistor, the ramp detector adapted to be coupled to a second voltage source.