Basically I'm looking to use a simple transistor circuit to diconnect one V+ (or ground) when another is connected and automatically switch back when the switching source is removed.
Like having a small lithium batt in a device with charge circuit external. When the charge supply is connected I'd like to run the device from the external supply while charging the battery and automatically swap back to battery supply when the charger is removed.
Planning to work with 5VDC.
Basically looking for pointers on how to setup such a switch?
already have the external charger worked out.
For clarity's sake:
I'd like to charge the battery while still in circuit with the load device while powering the device.
Wouldn't the batt need to be removed from load to do this safely? Could this be achieved by detecting the charger input with transistor switching and how?
Best Answer
This is often called "Power Path" and you use FETs connected as ideal diodes. Many chargers have this functionality built in, like the LTC4160 from Linear. TI also have a number of battery chargers with Power Path. (No direct link, you have to search the TI site).
Microchip has an app note about load sharing if you want to roll your own simple version.
You can also get ideal diodes nicely packaged.