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Maxim > Design Support > Technical Documents > Application Notes > Power-Supply Circuits > APP 4417
Keywords: RS-232 transceiver, bipolar supply rails, charge pumps, flip-flops
APPLICATION NOTE 4417
Transceiver IC Generates ±30V
By: Nick Allen-Rowlandson
Zeeshawn Shameem, Member of the Technical Staff, Applications
Aug 06, 2009
Abstract: This application note explains how an RS-232 transceiver and a few external components can
be combined to generate supply rails up to ±30V. The MAX202 transceiver is featured in the design.
This design idea appeared in the July 20, 2006 issue of Electronic Design magazine.
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.
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