An integrated circuit for use in a switching voltage regulator circuit, the switching voltage regulator circuit providing a regulated voltage output at an output terminal,the integrated circuit including internal drive circuitry, a power switching transistor and control circuitry for varying the switching duty cycle of the switching transistor, the integrated circuit having at most five terminals including an inputterminal, an output terminal a ground terminal and first and second function terminals for connection to discrete external components to implement the switching voltage regulator circuit, the integrated circuit comprising first means connected to one of the function terminals for accepting a feedback signal from the output of the switching voltage regulator circuit and for enabling the integrated circuit to operate in a first mode to regulate the output of theswitching voltage regulator by varying the duty cycle of the switching transistor as a function of the magnitude of the feedback signal second means connected to the input and output terminals for enabling the integrated circuit to operate in an isolated flyback mode to regulate the output of the switching voltage regulator circuit as a function of a feedback voltage developedacross a primary winding of a discrete external transformer and mode select means connected to one of the function terminals and to said first and second means to disable the first means and to enable the second means in response to a disable signal applied to that function terminal by the discretecomponents.