National or Linear (maybe TI) makes a SEPIC charger/supply that would 
work for this. It isn't an inexpensive part though.
-
Martin K

Mauricio Jancic wrote:
> Hi,
> 	I need to develop an application that will run on 4 AA Ni-Me
> batteries. It will have a PIC18F6xJ50 (3.3V) a RTC, external EEPROM and a
> graphic lcd, capable of working at 3.3V and at 5V.
> 
> 	The device is required to be able to charge the batteries from the
> USB port.
> 
> 	Assuming a battery voltage from 1.6V to 0.9V (the user would use
> alkaline batteries) the voltage range of the device will be from 3.6V to
> 6.4V. 
> 
> 	I can't find some elegant solution to solve both the supply to the
> electronics and charging the batteries. Would it be correct to use some step
> down switching to get 3.3V and a step-up to boot from the 5V on the USB to
> the 6.4V needed to charge the batteries.
> 
> 	Any comments will be really appreciated.
> 
> Regards,
> 
> Mauricio
> 
> 
> 

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