Came across this circuit, thought others may be interested in it for DIY designs.
FYI
Yes, nice function. One does have to understand the voltage/current limits for their specific version, only one polarity allowed, and it only works for DC not AC. Many firms now offer modules based on this, and many even allow an input pin so that the powered up microcontroller can turn itself off, say by measuring the battery voltage and turning the power off before the battery is approaching it's safe discharge voltage limit.