I am currently working on a low power battery backed data logger. An Intersil ICL7673 backup switch is used to switch between the battery and external power. Because the circuit draws more than 30mA I have to use two mosfets (instead of transistors) as suggested in the datasheet:
One of the problems I ran into is that the mosfets let current flow into both directions. This makes it difficult to check if the supply is turned on. Two addional diodes could be a solution for this, but do I even need the intersil backup switch then? Wouldn't a simple diode-or be a more efficient solution?
Because the battery input already has a diode I am thinking about just putting another diode in series with the power supply. That would make the whole circuit cheaper and consume less power.
EDIT: Just for clarification: The circuit draws a bit more than 30mA once every few minutes for a few milliseconds. The rest of the time it will be at under 100uA. I tried the setup with resistors as described in the datasheet but it seemed to draw around 500uA (not mA, sorry for confusing that in my first comment). If I am using a 10k and 1k resistor souldn't that alone use around 0.3mA?
EDIT 2: After trying the solution with two mosfets, I took out the ICL7673 and put a schottky diode between power supply and vcc. It works perfectly. As long as the supply voltage is greater than the battery voltage, the circuit will only draw current from the power supply. On the battery side I needed a diode for polarity protection anyway, so the voltage drop is unavoidable.