This design idea appeared in the July 20, 2006 issue of Electronic Design
When applications require only a few millamps of supply current, an RS-232 transceiver IC (here, the MAX202
) and a few external components can generate bipolar supply rails up to ±30V. This capability is useful in ±15V op-amp circuits that provide, for example, meter drive, LCD bias, and gas-detector bias.
In the standard application for which it was designed, the internal charge pump of the MAX202 generates ±10V for driving an RS-232 line. In Figure 1
the IC's transmit section drives an external charge pump that generates up to ±30V. Modifications allow the circuit to generate other voltages, such as ±20V. With the addition of external linear regulators it can produce ±15V for bipolar op-amp circuits.
Figure 1. This RS-232 transceiver drives an external charge pump, which provides ±30V supply voltages at a few milliamps.
The maximum operating frequency for the external charge pump is approximately 120kHz, so a JK flip-flop (or other divide-by-2 circuit connected between +5V and ground) is inserted in the oscillator line to lower the frequency applied to the pump. Lower voltages can be obtained (±20V, for example) by connecting the anode of D1 and the cathode of D2 to ground instead of ±10V.