Powering Atmega, DAC and Bluetooth in Car
I would use the standard lM2576-5V switching regulator circuit and a lM1117-3.3 to bring that 5V down to 3.3v. I've had good luck with using this approach in cars in the past. You can use the schematics they have given in the schematics

http://www.ti.com/lit/ds/symlink/lm2576.pdf
https://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2004/zz25/lm1117.pdf