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Maxim > Design Support > Technical Documents > Application Notes > Interface Circuits > APP 1846
Maxim > Design Support > Technical Documents > Application Notes > Protection and Isolation > APP 1846
Keywords: RS232, RS 232, RS-232, low power RS 232, low voltage, EIA/TIA-562, transmitter,
transceiver serial interface, 3 to 5.5V system power source, resistor isolated transceiver, isolated data
interface, external filter, power-transfer capacitors
APPLICATION NOTE 1846
Using the MAX3250 ±50V-Isolated RS-232
Transceivers
Dec 30, 2002
Abstract: This paper describes how resistor-isolated transceivers are able to handle ±50V ground-offset
signals, and indicates how and why the offset voltage is limited by the design. The method of transferring
power across the isolation barrier is explained.
The MAX3250 high-offset-voltage (high-V
OS
) rejection RS-232 transceiver provides a ±50V isolated data
interface and power for the line-side transceiver circuits. Unlike optically- or transformer-isolated RS-232
transceivers, its main features are low cost, an allowed ±50V resistive-isolation character in a single
small IC package, and capacitive power transfer from non-isolated to isolated-side circuitry. It operates
from a single +3.3 or +5V system-power source, and requires only external surface-mount capacitors for
operation. An overall low profile is maintained when compared to other isolation techniques requiring
opto-couplers and/or a transformer.
This paper describes how resistor-isolated transceivers are able to handle ±50V ground-offset signals,
and indicates how and why the offset voltage is limited by the design. The method of transferring power
across the isolation barrier is explained.
Pseudo-Isolated RS-232 Transceiver
The MAX3250 provides a ±50V isolated data interface while, in the same IC package, providing isolated
power for the line-side transceiver circuits from a single +3 to +5.5V system power source. The detailed
functional diagram in Figure 1 allows a clear visualization of the isolation, power-transfer, and protection
circuits. External filter, charge-pump, and power-transfer capacitors are utilized.
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