For applications requiring only a few milliamps of supply current, an RS-232
transceiver IC (MAX202) and a few external components can generate bipolar supply
rails up to ±30 V. Such capability is useful ±15-V op-amp circuits
that supply (for example) meter drive, LCD bias, and gasdetector bias.
In the application for which it was designed, the internal charge pump of this
IC generates ±10 V to drive an RS-232 line. Referring
to the figure, the IC's transmit section drives an external charge pump
that generates up to 30 V. Modifications allow the circuit to generate other
voltages, such as ±20 V. Also, by adding external linear regulators,
it can produce ±15 V for bipolar op-amp circuits.
The maximum operating frequency for the external charge pump is approximately
120 kHz. Therefore, a JK flip-flop (or other divide-by-two circuit connected
between +5 V and ground) is inserted in the oscillator line to lower the frequency
applied to the pump. Lower voltages can be obtained (±20 V, for instance)
by connecting the anode of D1 and the cathode of D2 to ground instead of ±10
V.
