This is the scheme design of -15V / 1A Regulated DC Power Supply. It used uA723 precision voltage regulator and TIP105 PNP Epitaxial Silicon Darlington transistor. The supply receives -20V from the rectifier/filted which is fed to the collector of the Darlington PNP pass transistor, a TIP105. The base drive to the TIP105 is supplied through resistor R5. The base of the TIP105 is driven from Vz terminal at pin 9, which is the anode of a 6.2V zener diode that connects to the emitter of the uA723 output control transistor.
The method of providing the positive feedback required for foldback action is shown. This technique introduced positive feedback by increased current flow through resistor R1 and R2 under short-circuit conditions. This forward biased the emitter junction of the 2N2907 sensing transistor, which reduces base driver to the TIP105.