Soft-start circuit

An apparatus for transitioning power between a plurality of power sources to a load, the apparatus comprising:

• an amplifier that receives a feedback voltage corresponding to an output voltage supplied to the load and that receives a reference voltage;
• an input circuit that is coupled to a first source, wherein the input circuit is adapted to provide a first voltage and a first current to a load having a capacitance;
• a startup circuit that is coupled to the input circuit and to the first source, wherein the startup circuit includes a current source and a startup capacitor coupled in series with one another;
• and
• a current limiter having:
• a cascode circuit having a bias transistor and a power transistor coupled in series with one another, wherein the bias transistor is coupled to a second source and is coupled to the node between the current source and the startup capacitor, and wherein the cascode circuit is adapted to provide a second voltage and a second current to the load, and wherein the bias transistor generally operates as a source follower during startup;
• and
• a discharge circuit coupled to a node between the bias transistor and the power transistor of the cascode circuit and coupled to a node between the startup current source and the startup capacitor.