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MAX3232E-Q1: application circuit question

Dongbao Zhou
Expert 4130 points
Part Number: MAX3232E-Q1

Hello team,

Could you kindly help review below application circuit from my customer? 

  • Do we need to add pull-up resistors to input/output pins? If yes, how to choose?
  • How to choose the series resistors in data line R1/R2/R16/R12? Or we can just remove those resistors?
  • For 220pf capacitor, do you think it's ok? Why we need this capacitor?
  • Is it necessary to add bidirectional zener?

Thanks. 

Dongbao

  • 0
    TI__Mastermind 25705 points
    Dongbao,
    You don't need to add pull up/pull down resistors for output pins. RX input has 5kOhm resistor to GND, while TX input internally has pull down resistor as well. Theoratically you don't need to add any external bias for these pins.
    To let the device function, you don't need any series resistors. Sometimes small resistors (like 10Ohm) are added to limit the current during overstress event.
    I don't know the reason of adding 220pF cap. They might be applied to help ESD performance. Please make sure the voltage rating of the capacitor are at least above 10V.
    The zener should be bidirectional, since RS232 signaling is from -5V to +5V.
    Please let me know if you have any questions.
    Regards,
    Hao
  • 0
    TI__Expert 4130 points
    Since MAX3232E-Q1 has integrated +-15KV ESD protection, besides, RIIN and T1out could stand up to +-25V voltage, external bidirectional zerner are not needed.
    Another thing is R16/R22 must be reduced to 10ohm/33ohm/51ohm, because 1kohm will significantly damp signal.
  • 0 in reply to Dongbao Zhou
    TI__Mastermind 25705 points

    Dongbao,

    Although the device can stand 15kV IEC ESD, it would be still helpful to add external TVS diodes.

    I agree your comments about the resistor values. 

    Regards,

  • 0 in reply to Hao L
    TI__Expert 4130 points
    Hi Hao,
    Could you help further clarify the ESD rateing?
    Which IEC standard? What 's the Contact discharge and air-gap discharge rating ?
    Best regards,
    Dongbao
  • 0 in reply to Dongbao Zhou
    TI__Mastermind 25705 points

    Dongbao,

    It's IEC 61000-4-2 standard. The contact discharge is 8kV and air-gap discharge is 15kV.

    Regards,

    Hao