A switch mode power supply and its start-up circuit produce a controlled output
voltage from an input voltage. A power supply control circuit is powered by the
output voltage and is responsive to the output voltage to control the power supply.
The control circuit responds to differences between a fraction of the output voltage
and a reference voltage to control the start-up circuit. This control is inhibited
at low values of the output voltage to ensure that the power supply starts up correctly
despite the reference voltage being indefinite or unstable at such low values of
the output voltage.
