A startup circuit delivers regulated startup current to a control IC in a switched-mode power supply (SMPS) system, and automatically disconnects the startup current when the SMPS Control IC starts switching the transformer or inductor used as the energy storage element in the SMPS system. Disconnection of the startup current may be triggered by detecting a time-varying voltage waveform on an accessible node in the SMPS system, or by detecting an increased current consumption by the SMPS Control IC, without requiring any ground reference to the SMPS Control IC, nor without requiring any logic signals generated by the SMPS Control IC. This provides for rapid and predictable startup of an SMPS Control IC and reduced power loss once the SMPS Control IC is operational, and is independent of the operating voltages of the SMPS control IC, and independent of the particular control scheme and switch topology of the SMPS system.