Almost all of the SILabs / Cygnal evaluation boards that I have purchased use an RS-232 converter chip from Sipex called the SP3223. This part is indeed specified at the common 3 to 3.3 volts usage range. These parts use a different voltage booster topology so that the VCC gets trippled to the V+ voltage. This tripled V+ level is then inverted to produce the V- voltage.

This is in stark contrast to the old standard 5V specified MAX232 chip which uses a voltage booster in a doubler configuration.

Note that TI, MAXIM, and others have parts equivalent to the SP3223.

Michael Karas