An LDO regulator includes two linear regulator circuits and an internal
priority logic scheme that favors generating a regulated output voltage
using a regulated supply voltage over an unregulated supply voltage. The
unregulated supply voltage is applied to a first input terminal from a
raw voltage source. The regulated supply voltage is applied to a second
input terminal from, for example, a switching (e.g., BUCK) regulator. Two
output devices are respectively connected between the first and second
input terminals and the LDO output terminal. The first regulator circuit
causes the first output device to supply the desired regulated output
voltage while the switching regulator ramps up. Once the regulated supply
voltage is high enough to allow regulation, the internal priority logic
scheme disables the first regulator circuit, whereby the desired
regulated output voltage is generated solely by the second regulator
circuit through the second output device.