Switching power sources for USB PIC device

Need some advice:
I have PIC18F2450 based USB device that has dual power: USB and external power with TL2575-05 regulator.
Device should be able to run if at least one source present (USB is not plugged in all the time).
For now I use two diodes to prevent USB backfeeding, but forward voltage drop 0.8V over diode brings PIC to the lower spec margin of 4.2V.

I am exploring the idea to use Enable signal on TL2575 to shut it down when USB is active, but not sure if this is a good idea.

So, any ideas how to handle it?
Needs to be something simple and cheap...

Use a 1N5817 diode not a 1N4001.

Yes, Schottky diodes have much lower forward voltage drop, but they have significant leakage current and this may create problems with beckfeeding PC USB port that is not allowed under USB specification

For External PS I can boost voltage by adding resistor to the feedback circuit of TL2575. But USB voltage still will be low.

Edit - I added resistor - now external voltage is OK, still looking for a diode with low voltage drop and low leakage current for USB...