A controller for a secondary side of a switched mode power supply. A thermistor and an LED of an optocoupler are connected in parallel with each other between a voltage-supply-pin and a STOP pin of the controller. A reference-source provides a reference-signal between the STOP pin and the voltage-supply-pin. The STOP pin receives a temperature-measurement-signal from the thermistor, wherein the temperature-measurement-signal is representative of the resistance of the thermistor. The controller also includes an OTP-comparator that compares: (i) the temperature-measurement-signal; with (ii) a threshold-level, and provides an OTP-signal that is representative of whether or not the temperature-measurement-signal at the STOP pin crosses the threshold-level; and a switchable-current-source that selectively provides a bias-current to the STOP pin based on the OTP-signal, wherein the bias-current causes the LED to emit a light-signal that is representative of a fault to an associated photo-detector.