Temperature sensing protection circuit for ic power chip having thermal hysteresis characteristic

Claim 1

Thermal protection circuit for an operating circuit, comprising: (a) A Schmitt trigger circuit including a first transistor having a temperature dependent base-emitter voltage, a second transistor operatively coupled to and conductively controlled by the output of the first transistor, and a feedback path from the second to the first transistor, (b) Means for applying a fixed biasing potential to the base of the first transistor of such magnitude to maintain the first transistor in a non-conductive state under normal operating conditions of said operating circuit, (c) Means thermally coupling the first transistor with the operating circuit, and (d) Means coupled between the trigger circuit and the operating circuit for disabling the latter in response to a reversal in the conductive states of the first and second transistors, whereby the first and second transistors become conductive and non-conductive, respectively, when the temperature of the operating circuit rises to a first predetermined level at which the base-emitter voltage of the first transistor drops below its effective biasing potential, and wherein the first transistor is again cutoff when the temperature of the operating circuit drops to a second predetermined level.