Patent ID: 7541872

Claim:
An apparatus for providing a startup voltage for a first subcircuit, comprising: a first subcircuit having a current sink for sinking a predetermined constant current during normal operation of said first subcircuit, and adapted to conduct an operating current drawn from a power supply terminal thereof, a second subcircuit having a control node, an output terminal coupled to the power supply terminal of said first subcircuit, an input terminal coupled to an output terminal of said first subcircuit, and having an output voltage responsive to the second subcircuit input signal; a comparison circuit responsive to the level of said operating current, having an input node coupled to said current sink and having an output node coupled to said control node, said comparison circuit having a first output state if the level of said operating current is below the level of said predetermined constant current, and having a second state if the level of said operating current is above the level of said predetermined constant current; whereby, in response to said first output state of said comparison circuit, the output voltage of said second subcircuit is driven to rise to a level high enough to provide an operating voltage for said first subcircuit, and whereby, in response to said second output state of said comparison circuit, the output voltage of said second subcircuit is independent of the output state of said comparison circuit, thus allowing normal operation of the overall circuit beyond the startup time period.