Patent ID: 8476965

Claim:
A circuit for limiting an operating area of a transistor in a constant current source, the circuit comprising: a driver comprising a variable voltage source coupled to a first input of a first comparator included in the constant current source, the variable voltage source configured for providing a reference voltage to the constant current source, wherein a second input of the first comparator is directly coupled to a source of the transistor in the constant current source and an output of the first comparator is coupled to a gate of the transistor, the driver adapted to be controlled to decrease drain current at the transistor by decreasing the reference voltage provided to the constant current source; and a detector coupled to the constant current source and to the driver, the detector comprising: a constant voltage source coupled to a first input of a second comparator included in the detector and configured for providing a set voltage based on determining a safe operating area of the transistor; and the second comparator including the first input, a second input connected to a drain of the transistor of the constant current source, and an output coupled to the variable voltage source included in the driver, the second comparator being adapted to: compare the set voltage output from the constant voltage source with voltage at the drain of the transistor; and output a signal to control the variable voltage source to decrease the drain current at the transistor and, by controlling the variable voltage source to decrease the drain current, cause the transistor to operate in the safe operating area for the transistor, wherein the comparator is adapted to output the signal to control the variable voltage source in response to the voltage at the drain of the transistor exceeding the set voltage output from the constant voltage source.