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MAX3232E: application schematic design review

Lilian Chang
Intellectual 1555 points
Part Number: MAX3232E

Hi Team,

Could you please help to double review the schematic design of MAX3232E as below?  Left pins connected to on board MCU, and right pins connected to the RS232.  And customer has few questions about the schematic design：

1, If the series resistors(R1221~R1224, R1217~R1220) on the input/output pin are needed?  If yes, what's the recommend value?   If not, the series resistance like below will worsen the signal performance?

2. If the pull up resistors on the R/1R2OUT pin are needed? 

3. Customer found when add a pull up resistor at R1OUT pin connected to the other power rail,  even device is powered off(VCC 0V), they measured flow backward voltage at VCC pin and RIN1 pin around 1V, like below picture 2.   Can we explain the reason?

Thank you.

  • 0
    TI__Guru 62920 points
    Hi Lilian,

    Series resistances are not required on the IOs. Sometimes these can be useful in limiting the currents flowing into and out of the device during fault conditions (like a short-circuit) or transient stresses (like ESD). If they are used, though, the value needs to be small enough to not interfere with communication. In most cases 100 Ohms would be OK. The factors to consider are the amplitude attenuation that would result (since the resistance would form a voltage divider with the receiver input resistance, which could be as low as 3 kOhm) and the bandwidth limitation (since the resistance would create a low-pass filter with the parasitic capacitance of the cable used).

    The ROUT pins use a push-pull output driver and so external pull-up resistances are not required. If pull-up resistors are used and if they are powered on without the transceiver receiving power then it is possible that ROUT pins will consume some leakage current. This could mean that other pin voltages would measure > 0 V.

    Since it is not recommended for the ROUT voltage to exceed VCC + 0.3 V (see Absolute Maximum Ratings table), I would recommend either removing these pull-up resistances or tying them to the transceiver VCC rather than a different power supply. (If you are shutting off power to the transceiver in order to reduce quiescent current, you might instead want to use an RS-232 device with shut-down controls.)

    Regards,
    Max