I'm modifying an existing design that has 3 different power inputs:
Ext power - ~12V
Batteries - 6 x AA
USB - 5V
The design currently just diode 'OR's' the supplies together before the main switching regulator.
The problem is that when there is no external power, but there are batteries and USB, the current is drawn from the batteries as they are a higher voltage. I'd really like to be able to isolate the batteries completely when on USB power.
I've been looking at high side and low side switches etc, but have yet to come up with a good solution.
I don't want to redesign the power supply - too much risk, and I'm limited for board space. I also don't want to introduce any significant losses that will limit run time on the batteries.
Any good ideas?
The schematics as published in the article on EDN shows the power connection of the AD8618 at the source of the MOSFETs (output ORed node). The AD8619 is a quad op-amp, so only 1 power connection is shown.
In the case you want to use 4 single op-amps then each one is powered by the same node (the output of the ORing function) which as Bob described always has a voltage on it as long as one of the inputs is plugged in.