The MAX33250E and MAX33251E are 1Mbps, 600VRMS isolated RS-232 transceivers. The MAX33250E has 2 transmitters and 2 receivers (2Tx/2Rx), and the MAX33251E has 1 transmitter and 1 receiver (1Tx/1Rx). The isolation is provided by Maxim’s proprietary insulation material that can withstand 600VRMS for 60 seconds. The MAX33250E and MAX33251E conform to the EIA/TIA-232 standard and operates at data rates up to 1Mbps over the temperature range of -40ºC to 85ºC.
The MAX33250E and MAX33251E provide galvanic isolation and protection for digital signals from the local microcontroller’s logic UART port (primary side) to the field lines (secondary side). A capacitive design is utilized where the insulation material for the isolation barrier is rated for 600VRMS withstand voltage (VISO) for 60 seconds. The same material can also be exposed to a differential of 200VRMS of working voltage (VIOWM) for more than 50 years, providing longevity for many different types of end equipment. The isolation barrier also breaks ground loops and level translation for two different systems where it could potentially create inadvertent or misinterpret data signals.
Both parts have internal RS-232 power supplies that consist of a regulated dual charge pump that provides output voltages of +5.5V (doubling charge pump) and -5.5V (inverting charge pump), over the +3.0V to +5.5V range. Each charge pump is internally connected to a pair of flying capacitors and a pair of reservoir capacitors to generate the internal V+ and V- supplies, as shown in Typical Application Diagram.
The VCCA and VCCB supplies are both internally monitored for undervoltage conditions. Undervoltage events can occur during power-up, power-down, or during normal operation due to a dip in either power supply line. When an undervoltage event is detected on either of the supplies, all outputs on both sides are automatically controlled, regardless of the status of the inputs.
| INPUTS | VCCA | VCCB | RxOUT | TxOUT |
| RxIN = 1 | Undervoltage | Powered | High | — |
| RxIN = 0 | Undervoltage | Powered | Follows VCCA | — |
| TxIN = 1 | Undervoltage | Powered | — | Low |
| TxIN = 0 | Undervoltage | Powered | — | Low |
| RxIN = 1 | Powered | Undervoltage | High | — |
| RxIN = 0 | Powered | Undervoltage | High | — |
| TxIN = 1 | Powered | Undervoltage | — | *Low |
| TxIN = 0 | Powered | Undervoltage | — | *Low |
*TxOUT will be out of compliance with the RS-232 specification as VCCB falls below 2.9V.
The transmitters are inverting level translators that convert CMOS-logic levels from the UART or equivalent output port to +5V EIA/TIA-232 levels. The two devices guarantee 1Mbps with worst-case loads of 3kΩ in parallel with 1000pF, providing compatibility with PC-to-PC communication software. Transmitters can be paralleled to drive multiple receivers.
The receivers convert RS-232 signals to CMOS-logic output levels to the UART or equivalent input port. The devices feature inverting outputs that always remain active.
To reduce ripple and the chance of introducing data errors, bypass VCCA and VCCB with 0.1μF ceramic capacitors to GNDA and GNDB, respectively. Place the bypass capacitors as close to the power-supply input pins as possible.