Electronic SPDT switch for power source?

[cosmicboy]

Hi folks,

I have a battery powered circuit that also has a USB connection and I am looking for a way to disconnect the battery and power the board from USB when the USB is plugged in.

The simplest way I can think of accomplishing this is by using a SPDT switch (something like a sn74lvc1g3157 perhaps?) but one problem of this approach is, how do I power the switch?

This is a little block diagram of what I am trying to achieve:
**broken link removed**

So when there is USB Power the circuit is powered from the USB bus and when it is unplugged the circuit is powered from the battery.


I am sure someone must have thought about this before so could you give me some pointers?


Thanks!

 

[FvM]

First point to consider is minimum VBUS voltage of 4.35 V, another is maximum allowed current consumption in device suspended state. You may base your design on typical USB host implementations, that connect VBUS to a (possibly unfused) 5V supply, but it can't be guaranteed to work with any USB host then. Furthermore, the circuit isn't allowed to drive out voltage to VBUS.

On the internal side, the 3.3V current consumption and regulator dropout voltage have to be defined.

A threshold comparator determining sufficient VBUS voltage would be needed to my opinion. It can also be used as a VBUS voltage monitor for the USB device controller to control attachment.

VBUS may be connected to regulator through a schottky diode and 9V through a transistor switch.