High voltage charge pump using low voltage type transistors

Abstract

An integrated circuit includes an N isolation buried layer underlying high density and low voltage type P channel and N channel transistors to define islands of arbitrary voltage on the substrate. Thus such transistors, which otherwise are capable only of low voltage operation, become capable of operating at high voltage relative to the substrate. This allows integration, on a single chip, of high voltage circuit elements with low voltage and high density transistors all formed by the same fabrication process sequence. In one example this allows creation of an 18 volt range charge pump using a CMOS process which normally provides only 3 volt operating range transistors. This then allows integration on a single integrated circuit chip of a complex digital logic function such as a UART (universal asynchronous receiver and transmitter) with a high voltage function such as an RS-232 interface, including integrated capacitors for the RS-232 interface charge pump.