MAX3232-EP: Wrong oscillations on RS232

Part Number: MAX3232-EP

Hello,

It was noticed that “V+” was at a correct voltage level when idle, but starts oscillating when a message is
transmitted from the module. see following:

The same measurement was done on “V-“, which has a faulty and noisy voltage level when idle, and also
oscillation when a message is being transmitted as noticed on the following capture, furthermore the voltage
is incorrect, around -4.75V instead of -5.5V. Significant noise can also be noticed at all times, which is not
present on a unit with a functional RS-232 transceiver.

If we replace the part the problem disappears.

Has TI some experiences or similar cases already reported for such part, that can indicate some technical issues of the component or wrong usage?

Thank you.

pietro

5 Replies

  • Pietro,
    I would like to make sure I understand. You were seeing transmission errors on this device/board? The above results are from inspection of the failing device. After inspection, you replaced device and now do not see transmission errors nor noise on V+ or V-?

    Thanks,
    Wade
  • In reply to Wade VonBergen:

    Hi Wade,

    yes you understood the issue, the oscillations we see at the output of the components will disappear if we replace the part.

    We have many pieces with this issue, if we replace the part with a "good" one we will not see the problem anymore.

    Pietro

  • In reply to Pietro Zappia:

    Can you let me know voltage for VCC?
    Also capacitance values for V+, C1 to C1-, C2 to C2-, V-?

    Have you looked at V+ waveform on working board?
    Thanks,
    Wade
  • In reply to Wade VonBergen:

    Hi Wade,

    we had that problem on three different products.

    On old two projects:

    • Vcc = 3.3V
    • All capacitances are 470nF X7R 16V  (instead of 100nF as specified in datasheet)

     On recent project :

    • Vcc = 3.3V
    • All capacitances are 100nF (X7R 50V ; excepted between Vcc and GND which is a X7R 16V) ( see schematic here below)

    V+ during TX access (v+ oscillates between 4.31V and 5.54V) with 100nF (recent project) :


     

    V+ zoomed during TX access:


     

    Schematic of recent project:

    Has TI been notified of similar issues on that part by other customers?

    Thank you.

    Pietro

     

  • In reply to Pietro Zappia:

    Pietro,
    I was able to check with another engineer that is familiar with this device.
    Based on your scope shots, he suspects that there is ESD damage on the failing units.

    Regards,
    Wade