The rapid, widespread use of 3.3V logic circuits complicates the selection of circuits. In these systems, an chip which uses the 5V supply for charge pump and driver operation and the 3V supply for receiver output levels, provides the best performance. Systems with only a 3V power supply are unable to use 5V powered circuits. Charge pump triplers (or quadruplers) have losses too great for generating RS232 voltage and current levels from a 3.3V supply. Battery-powered 3V systems can use the RS232 transceiver’s SHUTDOWN and Driver Disable controls to maximize battery charge life.

VPP Supply and RS232 Interface Circuit diagram 

Get more information about for 3.3V System by Gary Maulding in the following article. (source: linear.com)



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