I am designing a battery based USB host circuit uses the AT90USB647-AU. Only USB flash memory sticks can be applied at the USB A connector. The supply of the citcuit is a battery (6.4Vmax, 5Vmin).
Looking at the datasheet of the uC:
Chapter 21/Host Mode Figures 21.7 and 21.8 (pages 249-250), there are 2 suggested circuits to use. The first one is used for Vcc=3V, with an On/Off controled DCDC converter, to apply 5V at VBus and the second one is used for Vcc=5V, with just a switch to upply 5V at VBus.
In my case the things are a little bit different. According to the USB2 specs. the VBus cannot be less than 4.85V. Because of the supply max. voltage which is 6.4V a diode is used to make a voltage drop of ~0.6V, in order to feed the uC Vcc. An very low dropout LDO is also used to apply 4.85V at VBus, which is input is connected directly to Vbatt.
There is no need of OTG so UID is not connected.
One important question is that if I have to connect the UVCC pin or not???
Furthermore, do you agree with the VBUS pin connectivity?
Also if you are able, could you please critisize the whole circuit?
Thanks in advance.