Selecting input power - external, usb or batteries

Question

Hi, 
 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? 
 G

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ADIApproved · Answer

Hi, Northy 
 I am the author of the ORing power selector article. Please see the attached which predates any editing by other parties. 
 1) I think that with 2 back-back FETs (adding one reversed polarity in series) in the battery fed input leg (gates tied together) and with some simple detection of USB power, the battery leg could be shut down when USB is present. I am thinking of no more than perhaps 2 transistors to implement this part of the "fix". If 12V power supply is connected then the circuit should work fine as originally intended without modification. 
 2) However my "article" design relies on MOSFET drain body diodes for startup. In other words you have diode oring before turning on any FET so that the op amp can initially get power. Now we are adding a reversed FET in series in the battery leg, so with battery only then the op amp will not get power and the whole shabang might not work. The solution would be to bridge the two back-back FETs with a 4V7 or 5V1 zener so that the op amp will get 4V during battery only startup. 
 3) You would need to have an op amp which can work from 4V0 to 12V0 and probably Pch MOSFETS which can take a similar range of gate drive. Alternatively you could add zener clamps to the MOSFETs in case they are limited to 8V max Vgs for example. 
 Please respond in this forum if you have any questions.