I bought a couple of cheap Leonardo boards , named pro micro. They where suposed to be able to operate on 3.3V, and could solve the problem of controlling ADF4153 PLL SPI bus working on 3.3V . I used to have the arduino nano and a voltage divider/level shifter in between, but it quickly becomes messy.
But the new boards did not operate on 3.3v but on 5V. After some investigation i found out that the voltage regulator on board, marked S8PC, gave 5V output. A search on the internet was not able to find the component, so i decided to replace it with a TPS73601 , found in my component stock, an adjustable LDO with 0.4A and in Sot23-5 package. I requires two external resistors R1=51K and R2=30K to get 3.3v.
You will be able to see R2 mounted on top of the package and R1 between pin 4 and pin5 . Now i had 3.3Vcc and alo on io pins.
One open question is if it will work with 16MHz clock frequency, according to the datasheet it should be arround 8MHz, when using 3.3 V. I did somme test with SPI interaface and other programs and did not found any problems. So maybee a conservative spec for the full temperature range and not 25+-5degrees.
So it is working, very small- but no mounting holes…..