MAX232

The MAX232 is the original single-supply RS-232 transceiver family from Maxim Integrated (now Analog Devices): two line drivers and two line receivers that produce compliant EIA/TIA-232 voltage levels from an ordinary 5 V rail. The trick is the on-chip capacitive voltage generator — a charge pump that first inverts the supply and then doubles it, using four external capacitors C1 to C4 plus the two reservoir pins VS+ and VS−. From those rails the drivers swing at least ±5 V and typically ±7.3 V unloaded into a 3 kΩ load, meeting TIA/EIA-232-F and ITU V.28 without a ±12 V supply.

Capacitor value depends on which version you buy. The classic MAX232 uses 1 µF charge-pump capacitors (Maxim's selection guide lists both 1.0 µF and 0.1 µF options) and is rated to 120 kbps; the MAX232A moves to 0.1 µF capacitors with a higher slew rate and 200 kbps; TI's own MAX232 specifies 1 µF and 120 kbps, and points to the MAX202 for a 0.1 µF design with ±15 kV ESD. Supply current is 5 mA typical and 10 mA maximum for Maxim's MAX232, while TI's version draws 8 mA typical — worth checking if your 5 V rail is tight.

On the line side, the receivers accept ±30 V inputs, which means the part survives being connected to a live RS-232 port with the wrong polarity or an over-driven cable. Receiver thresholds are 1.7 V to 2.4 V positive-going and 0.8 V to 1.2 V negative-going with about 0.5 V of hysteresis, receiver input resistance is 3 kΩ to 7 kΩ, and propagation delay is 500 ns maximum; the drivers are slew-rate limited to 30 V/µs to reduce radiated emissions. On the logic side, inputs are standard TTL/CMOS levels (VIH 2.0 V, VIL 0.8 V) and receiver outputs are push-pull CMOS able to source 1 mA and sink 3.2 mA. The device runs from 4.5 V to 5.5 V, comes in 16-pin PDIP, SOIC, wide SOIC and SOP packages, and is available in commercial (0 °C to +70 °C) and industrial or extended (−40 °C to +85 °C) grades with 2 kV HBM ESD on the base part.

Design practice: use low-ESR X7R ceramic capacitors placed close to the charge-pump pins, do not share them with other circuitry, add a 0.1 µF decoupling capacitor on VCC, and remember that cable capacitance plus the receiver's input resistance set the practical baud limit. For 3.3 V systems the correct part is the MAX3232 (3.0 V to 5.5 V, 0.1 µF, 250 kbps), and for exposed connectors choose an ESD-rated variant such as the MAX3232E or MAX202 rather than the base MAX232. Procurement: MAX232 is one of the most remarked interface ICs in Asia, so buy from the ADI/Maxim or TI authorized channel. Test a sample lot by loading both drivers with 3 kΩ and measuring the output swing — clones frequently produce only ±3 V and fail above 19.2 kbps — and verify laser marking, date codes and package weight. MOQ is 1 reel (2,500 pcs SOIC-16) or 100 pcs loose for prototyping, with 1-2 day dispatch from Shenzhen stock and 4-8 weeks through the authorized channel.