Patent ID: 8772966

Claim:
A power supply source selection circuit comprising: a comparator comprising an input to accept a first reference voltage directly proportional to a bandgap reference voltage, an input to accept a second reference voltage directly proportional to a first supply voltage, and an output to supply a switch signal in response to comparing the second reference voltage to the first reference voltage; and, a switch comprising an input to accept the first supply voltage, an input to accept a second supply voltage, an input to accept the switch signal, and an output to supply a third supply voltage to a regulator, the third voltage comprising a voltage potential less than or equal to a maximum voltage value, the switch selecting between the first supply voltage and the second supply voltage in response to the switch signal, the switch comprising a first current path and a second current path comprising at least a first field effect transistor (FET) and a second FET, the first FET comprising a source connected to the second supply voltage and a gate connected to the switch signal, and the second FET comprising a source connected to the first FET and a gate biased to a portion of the second supply voltage, wherein the second current path further comprises: a third FET comprising a drain connected to a drain of the second FET, a gate, and a source; a fourth FET comprising a drain connected to the source of the third FET, a gate biased to the bandgap reference voltage, and a source connected to the switch output.