I have some different projects going on, and on one i have a microchip and a ultrasonic range detector. The chip takes 3v and the sensor 5. I want to power the entire circuit with a 3v lipo ion battery. Small and powerfull.
So how can i step up the 3v to a 5v for the sensor?
Pololu has a selection of step-up voltage regulators, here is one example http://www.pololu.com/product/2562
Note that if you directly connect a 5V sensor to a 3V microcontroller, you may destroy it. You will need one or more level converters.
I hear the SR-04's actually need 5v, so you can't do the "first resort" of just driving everything off the battery and praying it works.
Use a boost converter. DC-DC voltage converters are amazing now, and cheap as dirt.
You can buy boost converters of all sizes and descriptions on ebay, starting as low as ~$2. Buy a couple, so when you accidentally short the output and let the smoke out, you can just grab a new one. You can also get some from hobby circles - they call them "voltage boosters" and sell them in with their UBECs (which are just fixed-voltage-output dc-dc converters.
If the chip is spec'ed to run at 5v, i'd simplify the system by running the chip and sensor at 5v - that'd save you from any sort of level shifting (though you could probably eliminate the need for a proper level shifter - you can handle the output with a couple of dividing resistors, and often you can get away with using a 3.3v signal. Not sure if that's the case with your ultrasonic sensor).
Also, if the chip has to run at a lower voltage, you're aware that a Li-ion battery at full charge puts out 4.2V, right?