Hi.
I'm developing USB device which should use AC power when unplugged from USB. Here is simplified schematic: http://img159.imageshack.us/my.php?image=test04ra0.gif
What values of diodes and resistors should I use there? Should I compile the driver as USB powered or externally supplied? I'm forced to use LE33 instead of LE35, will 3.3V be fine for data lines?
USB and AC power
The LE33 is even better from the USB perspective. We decided for the LE35 because the higher supply voltage allows higher clock rates. This is not so much of a problem with modern AVRs.
You should compile the driver for bus-power because the device CAN draw power from the bus.
In my opinion, you can save the extra 78L05 and send the rectified AC directly into the LE33 (through the schottky diodes, as you suggested), provided that the regulator's power rating is not exceeded. If it is, you might choose a low drop regulator with higher power rating.
For the schottky diodes, look at the catalog of your favorite supplier. Use whatever is cheap and can stand the voltage and current.
You should compile the driver for bus-power because the device CAN draw power from the bus.
In my opinion, you can save the extra 78L05 and send the rectified AC directly into the LE33 (through the schottky diodes, as you suggested), provided that the regulator's power rating is not exceeded. If it is, you might choose a low drop regulator with higher power rating.
For the schottky diodes, look at the catalog of your favorite supplier. Use whatever is cheap and can stand the voltage and current.